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SpaceX Vehicles and Missions => SpaceX Starship Program => Topic started by: Semmel on 10/14/2018 04:41 pm

Title: Starship Methox RCS Thrusters
Post by: Semmel on 10/14/2018 04:41 pm
I have not read or heard a lot about the steering thrusters of BFR. They are a new type of engine in their own right with its own development project, control and fuel supply. The list below is from memory and I would love to update them to something more substantial.

What we know:
* Methane/Oxygen fuel
* Pressure fed

What we assume:
* gas-gas combustion
* max thrust >50kN

What we dont know:
* min thrust
* ISP
* mass
* reaction time (important since they should be used for attitude control)
* control precision
* how the liquid CH4/O2 is converted to gas (preburner would not work)
* Number and location on BFR

This thread is intended to be an analogue to ITS Propulsion – The evolution of the SpaceX Raptor engine (https://forum.nasaspaceflight.com/index.php?topic=41363.0) and I would hope for a technical discussion on the engines and their plumbing.
Title: Re: BFR control thrusters
Post by: speedevil on 10/14/2018 05:00 pm
I have not read or heard a lot about the steering thrusters of BFR. They are a new type of engine in their own right with its own development project, control and fuel supply. The list below is from memory and I would love to update them to something more substantial.

What we know:
* Methane/Oxygen fuel

What we assume:
* Pressure fed
* gas-gas combustion

What we dont know:
* max/min thrust
* ISP
* mass
* reaction time (important since they should be used for attitude control)

We know some of these.
It's been stated landing in 60KM/H winds is OK. Elon on twitter (https://mobile.twitter.com/elonmusk/status/1015654917782978560?lang=en)

This allows you to come up with a figure close to 15 tons/axis for BFS, somewhat more for BFR (to cancel the wind velocity and land vertically with no gimbal)

This more or less works with the observed thruster diameters on IAC2017 model - I have not gone over the 2018 images looking for them.

Min thrust of a gas thruster can be very low, if you operate it in cold gas mode, plus, the whole reason for going gas/gas was 'minimum impulse bit'. Elon on Reddit (https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodfh6x/)

If the thrusters are a little more powerful than this, 30 tons - they enable lunar landing and takeoff with enough fuel to ascend (50t) and a minimal payload, and perhaps even land horizontally.

Have they thought of this and realised it would be handy - who knows.
Oversizing them this way from the 15 tons calculated above for 60km/h to 60MPH  winds would considerably improve ability to ignore most winds.
Title: Re: BFR control thrusters
Post by: Slarty1080 on 10/14/2018 08:27 pm

We know some of these.
It's been stated landing in 60KM/H winds is OK. Elon on twitter (https://mobile.twitter.com/elonmusk/status/1015654917782978560?lang=en)

This allows you to come up with a figure close to 15 tons/axis for BFS, somewhat more for BFR (to cancel the wind velocity and land vertically with no gimbal)

This more or less works with the observed thruster diameters on IAC2017 model - I have not gone over the 2018 images looking for them.

Min thrust of a gas thruster can be very low, if you operate it in cold gas mode, plus, the whole reason for going gas/gas was 'minimum impulse bit'. Elon on Reddit (https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodfh6x/)

If the thrusters are a little more powerful than this, 30 tons - they enable lunar landing and takeoff with enough fuel to ascend (50t) and a minimal payload, and perhaps even land horizontally.

Have they thought of this and realised it would be handy - who knows.
Oversizing them this way from the 15 tons calculated above for 60km/h to 60MPH  winds would considerably improve ability to ignore most winds.

Another benefit of more powerful thrusters higher up on the BFR would be to reduce the amount of debris that will be blasted into the surroundings and the engines during take-off and landing.
Title: Re: BFR control thrusters
Post by: Semmel on 10/15/2018 07:26 pm
I have not read or heard a lot about the steering thrusters of BFR. They are a new type of engine in their own right with its own development project, control and fuel supply. The list below is from memory and I would love to update them to something more substantial.

What we know:
* Methane/Oxygen fuel

What we assume:
* Pressure fed
* gas-gas combustion

What we dont know:
* max/min thrust
* ISP
* mass
* reaction time (important since they should be used for attitude control)

We know some of these.
It's been stated landing in 60KM/H winds is OK. Elon on twitter (https://mobile.twitter.com/elonmusk/status/1015654917782978560?lang=en)

This allows you to come up with a figure close to 15 tons/axis for BFS, somewhat more for BFR (to cancel the wind velocity and land vertically with no gimbal)

This more or less works with the observed thruster diameters on IAC2017 model - I have not gone over the 2018 images looking for them.

Min thrust of a gas thruster can be very low, if you operate it in cold gas mode, plus, the whole reason for going gas/gas was 'minimum impulse bit'. Elon on Reddit (https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodfh6x/)

If the thrusters are a little more powerful than this, 30 tons - they enable lunar landing and takeoff with enough fuel to ascend (50t) and a minimal payload, and perhaps even land horizontally.

Have they thought of this and realised it would be handy - who knows.
Oversizing them this way from the 15 tons calculated above for 60km/h to 60MPH  winds would considerably improve ability to ignore most winds.

Ahh nice piece of information. Assuming the 15 tons (say, 150kN) is emitted at both ends of the stage, which is logical to be able to allign the centre of thrust with the centre of mass. Then you need at least 2 thrusters to have a combined thrust of 150kN. They will not be exactly equal since running the system without contingency is dangerous. So I would guess that the system would be able to give at least 200kN and is distributed over 4 thrusters, two at the top and 2 at the bottom. Probably more but this gives an upper bound for the max thrust of 50 kN.
Title: Re: BFR control thrusters
Post by: DAZ on 10/18/2018 08:24 pm
Reading to this thread brings up a question. These gas- gas thrusters would seem to be relatively easy to build. So my question is, could you modify the Raptor engines to have a gas-gas mode and would it be useful? I know the efficiencies would be much lower, what I'm thinking of is giving you more finesse / control especially in lower gravity Landings.
Title: Re: BFR control thrusters
Post by: speedevil on 10/18/2018 10:14 pm
Reading to this thread brings up a question. These gas- gas thrusters would seem to be relatively easy to build. So my question is, could you modify the Raptor engines to have a gas-gas mode and would it be useful? I know the efficiencies would be much lower, what I'm thinking of is giving you more finesse / control especially in lower gravity Landings.

You would need additional gas injection systems into the raptors, valving, distribution pipework, redesigned cooling, more high pressure gas storage, and ...
It is not in principle impossible, but it would add considerable complexity and probably weight, for not very much obvious benefit.

If you really, really wanted to, you could shove a few RCS class thrust pods in the cargo bay area, but there is little point - as the existing ones all over the vehicle work pretty much the same.
Title: Re: BFR control thrusters
Post by: Semmel on 10/19/2018 05:42 am
As far as I know, raptor already operates in gas gas mode. But the problem is the turbo pumps and preburners, so they are made for high sustainable thrust. Not precise thrust control and little control puffs. Adding gas gas injection side tracking the turbo machinery has little benefit. For steering its useless, because it's at the end of the rocket with little to no Leaver arm. For acceleration its too small thrust. There is really no reason to do this.
Title: Re: BFR control thrusters
Post by: docmordrid on 10/19/2018 06:01 am
Elon Musk, Reddit AMA

https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/

Quote
The BFS will have methalox RCS thrusters for spaceship attitude control. (See the three dark dots at the bottom of the spaceship.)
>
The control thrusters will be closer in design to the Raptor main chamber than SuperDraco and will be pressure-fed to enable lowest possible impulse bit (no turbopump spin delay).
Title: Re: BFR control thrusters
Post by: Semmel on 10/19/2018 06:31 am
Elon Musk, Reddit AMA

https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/

Quote
The BFS will have methalox RCS thrusters for spaceship attitude control. (See the three dark dots at the bottom of the spaceship.)
>
The control thrusters will be closer in design to the Raptor main chamber than SuperDraco and will be pressure-fed to enable lowest possible impulse bit (no turbopump spin delay).

Thank you doc, I updated the opening post.
Title: Re: BFR control thrusters
Post by: DigitalMan on 10/19/2018 06:35 am
Since there is going to be another AMA in a week or two has anyone thought about getting questions together on these (and other) issues to see if some of them could be answered?

There are a lot of great questions posed by folks around here.
Title: Methalox gas RCS for Starship and other applications.
Post by: speedevil on 10/06/2019 07:40 pm
What can we say about the methalox RCS for SS (and possibly SH)?

At the beginning of time,Elon stated of the RCS (https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodfh6x/).
Quote from: Elon
The control thrusters will be closer in design to the Raptor main chamber than SuperDraco and will be pressure-fed to enable lowest possible impulse bit (no turbopump spin delay).

We can estimate the thrust with the further tweet (https://twitter.com/elonmusk/status/1015648140341403648)
Quote from: Elon
Yes. All-weather. ~300km/h high altitude winds. ~60km/h ground winds. It’s a beast.

Assuming the drag coefficient of SS worst-case sideways is 1, 60km/h, and 450m^2, this comes out to around 10 tons drag.

It seems likely that if you were to want to land in this crosswind, you would want a RCS capable of at least 20 tons per axis, or at most 10 tons per cluster of RCS jets.

At an exhaust expansion of around 2 bar, this puts the approximate diameter of a nozzle at 0.5m^2 - some 80cm. (seperate nozzles needed for the three or four axes per cluster)

Assuming you want to be able to brute force an unexpected wind throughout the last 5 seconds of landing and come to a pinpoint landing - with an ISP of 330, and 10000kg thrust, this comes out to 5s*30kg/s of propellant - 150kg.

The tank volume at 4500PSI would be around a cubic meter total, though this would need to probably be at each cluster of nozzles.
In addition, you would need a high pressure gas feed line (to slowly refill over minutes).

Not igniting the gas gives you an ISP of around 70, not 330, so you have a thrust of around 2 tons.
Allowing very small flows would be simple, even in the absence of vernier thrusters.

Composite pressure vessels would bring the mass of each cluster (with propellant) to at least 300kg.

How the pressure vessels are filled is an interesting question - in principle there are many ways:
Cryogenic liquid reservoir and a rapid heating to go from ~50PSI liquid to ~4000PSI gas - possibly in a multichamber device to eliminate liquid pumping.
Compressor running from the tank gas volume.
...
With the revelation this is coming pretty soon 'Mk4' or so - it will be interesting to see the development.
Title: Re: Methalox gas RCS for Starship and other applications.
Post by: sanman on 10/06/2019 09:06 pm
Doesn't RCS particularly need good response time and precision?

How does Methalox measure up for these, compared to cold gas thrusters?
Title: Re: Methalox gas RCS for Starship and other applications.
Post by: TheRadicalModerate on 10/06/2019 09:13 pm
Are you sure the low-altitude 60 km/h is the limiting factor, and not the high-altitude 300 km/h?

Just for some starting numbers, my cheesy model for the F9 gives:

Altitude: 16.7 km
Airspeed: 540 m/s
max-q: 23.4 kPa
max drag force: 165.6 kN

(Anybody know what the average numbers really are?)

A sudden perpendicular change of 300 km/h (83 m/s) generates an effective AoA of arctan(83/540) = 8.7 deg.

This is about where my patience (and competence) for working out even the induced drag, to say nothing of the lift, fails me.  But 8.7 deg seems like a lot--likely more than simple gimbaling is going to be able to correct for.  Seems like some thruster support will be necessary.
Title: Re: Starship Methalox RCS Thrusters
Post by: Norm38 on 10/06/2019 10:12 pm

There already is a gas generator system, autogenius pressurization if the main tanks.
What pressure is that, and can it be tapped to fuel the thrusters?
Title: Re: Starship Methalox RCS Thrusters
Post by: ThomasGadd on 10/06/2019 10:16 pm
One reason to consider the lower altitude a limiting is you have less time to react. 
Title: Re: Starship Methalox RCS Thrusters
Post by: speedevil on 10/06/2019 10:57 pm

There already is a gas generator system, autogenius pressurization if the main tanks.
What pressure is that, and can it be tapped to fuel the thrusters?
~3 bar - 45PSI.
So probably not very useful.
Title: Re: Starship Methalox RCS Thrusters
Post by: speedevil on 10/06/2019 11:00 pm
One reason to consider the lower altitude a limiting is you have less time to react.
I was assuming it in the original post, as there is a _long_ phase from 20km down where you can correct for errors over a long period, and then be prepared to light the main engine with some error that needs correcting.
In the last five seconds of landing, you have limited if any capability to cope with gusts without moving sideways or landing at an angle - without a very capable RCS.
Title: Re: Starship Methalox RCS Thrusters
Post by: gin455res on 10/06/2019 11:33 pm

There already is a gas generator system, autogenius pressurization if the main tanks.
What pressure is that, and can it be tapped to fuel the thrusters?
~3 bar - 45PSI.
So probably not very useful.
This is the tank pressure, what about the pressure in the system that is pressurizing the tank?
Might this be used to pressurize a seperate tank to a higher pressure?
Title: Re: Starship Methalox RCS Thrusters
Post by: livingjw on 10/06/2019 11:59 pm
Copied this from General Engineering thread:

- From inonepiece: Elon said that at around mark3/mark4 they'd likely replace starship cold gas thrusters with methalox methox driven from high pressure CH4/O2 cylinders.  Does "high pressure" mean COPVs?  I was looking forward to a non-COPV future for SpaceX since they've been responsible for several major problems...

My guess would be somewhere around 500-1000 psi. Reasonable pressure fed rocket pressure, but pressure used will depend on optimizing the RCS/OMS system including, in particular, its recharge subsystem.

For Moon and Mars launches, must have a recharge subsystem which takes cryo propellants from header tanks and pumps, gasifies and transfers propellants to high pressure gaseous RCS/OMS tanks. This will probably be done using electric pumps and heaters. You definitely want to pump liquids, not gases.

Years ago my team worked on a rechargeable gaseous RCS system. It had 1000 psi tanks of gaseous Lox GOX and CH4. As the gases were used the pressure and temperature of the gases go down.  When the tank pressure dropped below a set limit, an electric pump and heater were used to to pump liquid and gasify propellants from the main tanks into the RCS tanks. As long as you had liquid propellants and electrical power, you could maintain your high pressure gaseous system. It could even be completely offline for years and be recharged before launch from Mars or the Moon.

RCS and OMS are probably going to both be gaseous CH4/GOX because they need to both operate in zero g, and operate instantaneously. It will probably be an integrated RCS/OMS system.

I think they will make use of a lot of the tech they developed for draco/superdraco. Very similar requirements from a pressurization, plumbing perspective. Biggest difference is the need for fail safe ignition, since CH4/GOX is not hypergolic. I would expect to see redundant ignition systems in each rocket as well as redundant rockets. 2 OMS, ~12 RCS.

John
Title: Re: Starship Methalox RCS Thrusters
Post by: Barley on 10/07/2019 01:18 am
Is oxygen gas now considered lox? :'(
Title: Re: Starship Methalox RCS Thrusters
Post by: Lar on 10/07/2019 01:38 am
Is oxygen gas now considered lox? :'(
No, it's gox.
Title: Re: Starship Methalox RCS Thrusters
Post by: Llian Rhydderch on 10/07/2019 03:05 am
Thread title is wrong now.

They are methox (gas - gas) thusters, not methalox. 
Musk even called them methox in that interview with Tim Dodd.  And said they'd be of pressure fed type.

There is no LOX in the Starship thrusters.
Title: Re: BFR control thrusters
Post by: TheRadicalModerate on 10/07/2019 05:58 am
If the thrusters are a little more powerful than this, 30 tons - they enable lunar landing and takeoff with enough fuel to ascend (50t) and a minimal payload, and perhaps even land horizontally.

Have they thought of this and realised it would be handy - who knows.
Oversizing them this way from the 15 tons calculated above for 60km/h to 60MPH  winds would considerably improve ability to ignore most winds.

That'd be an awful lot of ascent gravity drag if you use the thrusters for more than the first couple of hundred meters of ascent.  Same thing on descent.  I'd assume that you'd use the Raptors the rest of the time.

Dry mass: 120 t
Prop for 20 t downmass (relative to Earth) to TEI/EDL = 200 t
Max payload upmass = 150 t
Total weight in lunar gravity = 761 kN = 77.6 terrestrial tf

If you have four pods, that's a 19.4 tf thruster.  Figure a 15 degree cosine loss and you're at 20.1 tf.  Call it 25 tf for safety.

But that brings up a question:  Assuming you're down around Isp=280, that'd be mdot = 89 kg/s.  Are there practical restrictions on the mass flow coming out of a single thruster?  If I SWAGged the Isp right, that's 2700 m/s of not-particularly-hot gas (although it's likely underexpanded), which is well within the mass flow and pressures that the SS would experience during reentry.  Anything else that might be a gotcha here?
Title: Re: Starship Methalox RCS Thrusters
Post by: Semmel on 10/07/2019 07:58 am
Thread title is wrong now.

They are methox (gas - gas) thusters, not methalox. 
Musk even called them methox in that interview with Tim Dodd.  And said they'd be of pressure fed type.

There is no LOX in the Starship thrusters.

I changed the thread title.
Title: Re: Starship Methalox RCS Thrusters
Post by: Semmel on 10/07/2019 08:51 am
Years ago my team worked on a rechargeable gaseous RCS system. It had 1000 psi tanks of gaseous Lox GOX and CH4. As the gases were used the pressure and temperature of the gases go down.  When the tank pressure dropped below a set limit, an electric pump and heater were used to to pump liquid and gasify propellants from the main tanks into the RCS tanks. As long as you had liquid propellants and electrical power, you could maintain your high pressure gaseous system. It could even be completely offline for years and be recharged before launch from Mars or the Moon.

Fantastic, someone who worked on a similar system! :) May I ask some questions?
* How powerful were your thrusters?
* How big was the reservoir/how long could you sustain using it? Say, in cumulative thruster firing seconds at full thrust?
* What was the responds time and thrust resolution?
* Did you go for throttle or pulse to control impulse?

RCS and OMS are probably going to both be gaseous CH4/GOX because they need to both operate in zero g, and operate instantaneously. It will probably be an integrated RCS/OMS system.

I think they will make use of a lot of the tech they developed for draco/superdraco. Very similar requirements from a pressurization, plumbing perspective. Biggest difference is the need for fail safe ignition, since CH4/GOX is not hypergolic. I would expect to see redundant ignition systems in each rocket as well as redundant rockets. 2 OMS, ~12 RCS.

I would guess, when they 'arm' the RCS system, a small sustained torch goes on in all the thrusters using spark ignition for a sustained flame. Then they only need to feed gaseous CH4 and O2 into the combustion chamber. The torches will use some small amount of fuel, but the hot RCS system only needs to be on for a few minutes during reentry, so its not a big loss in terms of mass.

I assume the same thruster could work as  a cold gas thruster for in-space maneuvering using CH4 only. This could be used to align the Starship engines with the sun or during precision maneuvering like docking. Not sure if the hot fire mode would be precise enough for these.

For docking you need pointing and translation, so I guess there must be at least 2 reservoirs each for CH4 and O2, one set in the nose of Starship and one at the bottom.
Title: Re: Starship Methalox RCS Thrusters
Post by: livingjw on 10/07/2019 01:52 pm

Fantastic, someone who worked on a similar system! :) May I ask some questions?
* How powerful were your thrusters?
* How big was the reservoir/how long could you sustain using it? Say, in cumulative thruster firing seconds at full thrust?
* What was the responds time and thrust resolution?
* Did you go for throttle or pulse to control impulse?

I would guess, when they 'arm' the RCS system, a small sustained torch goes on in all the thrusters using spark ignition for a sustained flame. Then they only need to feed gaseous CH4 and O2 into the combustion chamber. The torches will use some small amount of fuel, but the hot RCS system only needs to be on for a few minutes during reentry, so its not a big loss in terms of mass.

I assume the same thruster could work as  a cold gas thruster for in-space maneuvering using CH4 only. This could be used to align the Starship engines with the sun or during precision maneuvering like docking. Not sure if the hot fire mode would be precise enough for these.

For docking you need pointing and translation, so I guess there must be at least 2 reservoirs each for CH4 and O2, one set in the nose of Starship and one at the bottom.

- RCS/OMS modeling effort was parametric and not scaled to any set size. You entered the desired RCS and OMS thrusts and associated delta Vs. Model was developed to estimate size and weight of the system given different types of pressurant gases.

- Thrust and tank sizes depend on vehicle size. No thrust or tank size limits, from a physics standpoint. Everything is more or less linear with thrust and delta V required.

- Important thing is that it was able to be recharged to full capacity as long as you had liquid propellants and electricity available. This means that you can size the tanks for a particular part of the mission, for example launch or reentry, and not the whole mission since you can recharge during the mission.

- Response time was not modeled nor was ignition details. Your ideas sound reasonable. Other ignition options are also available and have been worked.

- Pulsed versus steady state was not considered. Our modeling work was just for sizing.

- Isps for GCH4/GOX will be in the 300 - 350 range depending on expansion ratio.

- I suspect they would have tanks fore and aft. OMS would be aft so would expect aft tanks to be larger. Tanks could also be placed on top of upper main tank and lines run fore and aft.

John
Title: Re: Starship Methalox RCS Thrusters
Post by: Norm38 on 10/12/2019 03:23 am

There already is a gas generator system, autogenius pressurization of the main tanks.
What pressure is that, and can it be tapped to fuel the thrusters?
~3 bar - 45PSI.
So probably not very useful.
This is the tank pressure, what about the pressure in the system that is pressurizing the tank?
Might this be used to pressurize a seperate tank to a higher pressure?


Years ago my team worked on a rechargeable gaseous RCS system. It had 1000 psi tanks of gaseous Lox GOX and CH4. As the gases were used the pressure and temperature of the gases go down.  When the tank pressure dropped below a set limit, an electric pump and heater were used to to pump liquid and gasify propellants from the main tanks into the RCS tanks. As long as you had liquid propellants and electrical power, you could maintain your high pressure gaseous system. It could even be completely offline for years and be recharged before launch from Mars or the Moon.


So do we have the full pressurization system between these?  The methox thrusters need pressure, the tanks need pressure.  Pressurization requires accumulator tanks, make them 1000psi for the thrusters.  A bleeder valve can then pressurize the main & header tanks to ~50 psi off the accumulator.  A 20-1 ratio between pressure and volume is manageable for one system.  It can be redundant.  Put a system in the nose and a backup astern.  Either can pressurize the tanks.
Issues, concerns?

This sounds like an organic evolution.  Methalox is chosen for main engines.  Autogenous pressurization is chosen. That requires a high pressure gas system to press the tanks, which enables the use of methox thrusters.  Simple.

Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 10/12/2019 04:53 pm
Here is a simplified schematic of a rechargeable RCS / OMS system:

- LCH4 and LOX comes from header tanks.

- electric pumps pump in liquids and heaters gasify to maintain pressures above set lower limit.

- electric heaters maintain desired temperature range.

- A coolant loop could be added to reduce electrical needs by transferring heat from thrusters back to the RCS tanks.

John
Title: Re: Starship Methox RCS Thrusters
Post by: Keldor on 10/12/2019 06:04 pm
The problem with any sort of pump from the main tanks into the RCS tanks is that most of the RCS usage will be in a short period during landing.  Out in space, they can afford to maneuver with a little puff of RCS and it's not a big deal if it takes 5 minutes for the vehicle to sloowly turn around to the direction they want it in.  This is even true with docking since you can just move very slowly.

However, this does not hold for atmospheric control.  In this case, you have to overcome aerodynamic resistance to maneuvering, meaning the thrusters have to exert a lot of force for a more sustained period, and thus you burn a lot of fuel.  Remember, this phase of flight is very short, with the bulk of the maneuvering needing RCS happening in the last seconds transitioning from belly flop to vertical landing.  In order for pumps to work, they would need to keep up with the thrusters, so you would have to vaporize on the order of 10s to 100s of kgs of fuel per second, which takes a huge amount of power.

To put things in perspective, 1 kW will vaporize 1 gram of methane every 5 seconds.  But if the assumption about the bulk of RCS fuel being used in less than 5 seconds to perform the horizontal-vertical transition we need many thousands of times higher rate if we want an underprovisioned pressurized tank!  Probably less that a gigawatt, but certainly quite a few megawatts...

It's very hard to imagine the components needed to produce and transfer this sort of wattage being lighter than just using a bigger pressure vessel.  I suppose you could get it through a preburner (though obviously we're not talking electrical any more), but now the entire RCS system is beginning to look like a strange full flow engine which can turn on and off very rapidly and has propellant storage in the middle.  Is this sort of complexity worth it?
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 10/12/2019 06:19 pm
The problem with any sort of pump from the main tanks into the RCS tanks is that most of the RCS usage will be in a short period during landing.  Out in space, they can afford to maneuver with a little puff of RCS and it's not a big deal if it takes 5 minutes for the vehicle to slowly turn around to the direction they want it in.  This is even true with docking since you can just move very slowly.

However, this does not hold for atmospheric control.  In this case, you have to overcome aerodynamic resistance to maneuvering, meaning the thrusters have to exert a lot of force for a more sustained period, and thus you burn a lot of fuel.  Remember, this phase of flight is very short, with the bulk of the maneuvering needing RCS happening in the last seconds transitioning from belly flop to vertical landing.  In order for pumps to work, they would need to keep up with the thrusters, so you would have to vaporize on the order of 10s to 100s of kgs of fuel per second, which takes a huge amount of power.

To put things in perspective, 1 kW will vaporize 1 gram of methane every 5 seconds.  But if the assumption about the bulk of RCS fuel being used in less than 5 seconds to perform the horizontal-vertical transition we need many thousands of times higher rate if we want an under provisioned pressurized tank!  Probably less that a gigawatt, but certainly quite a few megawatts...

It's very hard to imagine the components needed to produce and transfer this sort of wattage being lighter than just using a bigger pressure vessel.

- The size of the RCS / OMS tanks is determined by the worst case usage scenario. This was stated in an earlier post. Lets assume that it is the flip maneuver, then the RCS / OMS tanks would be sized to perform that maneuver without the pumps turning on.

- The pumps are not sized to maintain pressure continuously. They are there to recharge the tanks after completion of a usage sequence. Pumps are sized to refill the RCS / OMS tanks in a reasonable time, probably on the order of many minutes, and they are pumping liquid, so the power is relatively low.

- I modified the diagram to clarify operation. I also added a coolant loop which cools thrusters and heats RCS tank gasses.

- I get .53 seconds to vaporize 1 gm of methane with a kW of power.

- About 1 second if you want to heat the methane up to about 333K (60C).

John
Title: Re: Starship Methox RCS Thrusters
Post by: Pete on 10/12/2019 08:05 pm
To put things in perspective, 1 kW will vaporize 1 gram of methane every 5 seconds. 
Trivial correction: 1 kW will vaporize TEN grams in 5 seconds, you slipped a digit.
Does not invalidate your point though.


Edit.
Ah, sorry. this was already pointed out.
Title: Re: Starship Methox RCS Thrusters
Post by: ThomasGadd on 10/12/2019 08:55 pm
The RCS system will have to be robust against single point of failure. 
The pumps can be the same ones used for inter ship transfer.
Title: Re: Starship Methox RCS Thrusters
Post by: DistantTemple on 10/12/2019 09:22 pm
SpaceX is (portrays itself as) laser focussed on as few vehicles as possible; F9, FH, Dragon1, Dragon2(Cargo), Crew Dragon, SH and SS(Initial? chomper?, tanker, HSF, oh and starlink ). However the various toolbox of engines, thrusters, technologies, control systems and tanks open many doors for cobbling together other craft, despite SX saying "Spaceship will do it all". These Methox thrusters which you postulate at 15 tonnes thrust, could be part of a family of engines, for all kinds of landers for the Moon, moons of Mars, and orbital manoeuvring. And of course the apparently wacky "horizontal landing" of SS - at least in low gravity like the Moon!
This might seem off topic, but many things that Elon does have additional future uses, like Tesla batteries on SS and tunnelling on Mars! This engine will be vastly simpler, and easier to maintain than Raptor, and likely cheap. As space operations expand it will find very many uses.
Title: Re: Starship Methox RCS Thrusters
Post by: rsdavis9 on 10/12/2019 09:22 pm
So given the high wattage necessary for vaporization. A preburner with no turbine but just enough O2 or CH4 to vaporize the fuel/oxidizer. It seems that they would be very simple?
Title: Re: Starship Methox RCS Thrusters
Post by: DistantTemple on 10/12/2019 09:34 pm
So given the high wattage necessary for vaporization. A preburner with no turbine but just enough O2 or CH4 to vaporize the fuel/oxidizer. It seems that they would be very simple?
could you run the LOX or LCH4 in separate cooling loops on the nozzle to cool it, vaporize them and maintain tank pressure during longer burns? Just a guess....  Perhaps only have a small local tank, almost part of each thruster where the liquids are flashed to gas as they are needed! thus LOX and LCH4 are piped to the engine, and only for the first seconds is electric heating needed, then the flashing is done by the nozzle! Electric pumps would still need to pump the liquids at high pressure. (a la Electron rocket)
However livingjw and others have worked in the business.... and I have not .... so I am ready to be disabused if this is fanciful!
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 10/12/2019 09:41 pm
So given the high wattage necessary for vaporization. A preburner with no turbine but just enough O2 or CH4 to vaporize the fuel/oxidizer. It seems that they would be very simple?

I think there should be enough heat from thruster coolant loop to maintain RCS tanks temperature.

John
Title: Re: Starship Methox RCS Thrusters
Post by: DAZ on 10/12/2019 10:27 pm
As the SS/SH RCS thrusters are pressure fed can they be both multimode gaseous and liquid?  The low-power vernier thrusters on the SS would still probably need to be gas mode only, but the high power RCS thrusters it seems could benefit using pressure-fed liquid propellants.  Even if the high power RCS thrusters could only be fed liquid and used as a coolant with the boiled off coolant then fed into the engine this could solve multiple problems.
Title: Re: Starship Methox RCS Thrusters
Post by: Norm38 on 10/12/2019 11:20 pm
Here is a simplified schematic of a rechargeable RCS / OMS system:

- LCH4 and LOX comes from header tanks.

- electric pumps pump in liquids and heaters gasify to maintain pressures above set lower limit.

- electric heaters maintain desired temperature range.

- A coolant loop could be added to reduce electrical needs by transferring heat from thrusters back to the RCS tanks.

John

Does this system also produce the 3 Bar for the tanks?  If not, what does?  We don’t want to need helium or nitrogen.
Something on Mars will have to press the tanks before launch. That can’t be the raptors. Though they can keep the tanks pressed during their burn.
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 10/13/2019 12:24 am
Here is a simplified schematic of a rechargeable RCS / OMS system:

- LCH4 and LOX comes from header tanks.

- electric pumps pump in liquids and heaters gasify to maintain pressures above set lower limit.

- electric heaters maintain desired temperature range.

- A coolant loop could be added to reduce electrical needs by transferring heat from thrusters back to the RCS tanks.

John

Does this system also produce the 3 Bar for the tanks?  If not, what does?  We don’t want to need helium or nitrogen.
Something on Mars will have to press the tanks before launch. That can’t be the raptors. Though they can keep the tanks pressed during their burn.

Header tanks are actively controlled to maintain 3 bar pressure. If pressure drops, add heat. If pressure is too high, cool or vent.

John
Title: Re: Starship Methox RCS Thrusters
Post by: ThomasGadd on 10/13/2019 10:06 am
I've seen diagrams both ways... the RCS tanks can store the mixed gas right? 
Title: Re: Starship Methox RCS Thrusters
Post by: Semmel on 10/13/2019 11:58 am
I've seen diagrams both ways... the RCS tanks can store the mixed gas right?

You mean mixed gas and liquid? As a mixture of oxygen and methane at high pressure is not going to stay at that pressure for long.
Title: Re: Starship Methox RCS Thrusters
Post by: rakaydos on 10/13/2019 01:48 pm
How severe is the mass penalty for multiple small tanks over monolithic large tanks?

I'm wondering about reduncancy, and whether it would be worth giving individual RCS blocks their own pressure vessels. Replenished from a central source, but with fuel on hand if the central source fails.
Title: Re: Starship Methox RCS Thrusters
Post by: Pete on 10/13/2019 02:13 pm
How severe is the mass penalty for multiple small tanks over monolithic large tanks?

I'm wondering about reduncancy, and whether it would be worth giving individual RCS blocks their own pressure vessels. Replenished from a central source, but with fuel on hand if the central source fails.
The added failure modes will likely make this sort of system *less* reliable than a simpler system.
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 10/13/2019 02:23 pm
How severe is the mass penalty for multiple small tanks over monolithic large tanks?

I'm wondering about redundancy, and whether it would be worth giving individual RCS blocks their own pressure vessels. Replenished from a central source, but with fuel on hand if the central source fails.

- Little penalty for the pressure tanks. Tank weight is proportional to volume everything else being equal.

- Some penalty for additional mounting fixtures and plumbing.

- System Designs should be "fail safe" where every possible; that is, one component failure should not bring down the system. A for and aft RCS system with cross feed capability would probably be adequate.

John
Title: Re: Starship Methox RCS Thrusters
Post by: ThomasGadd on 10/13/2019 03:48 pm
I've seen diagrams both ways... the RCS tanks can store the mixed gas right?

You mean mixed gas and liquid? As a mixture of oxygen and methane at high pressure is not going to stay at that pressure for long.

Sorry I was clear... they will be extracting from the LOX and LCH4 header tanks into a new GOX and GCH4 RCS tanks. 
Can that new (GOX and GCH4) be a single tank? 
Title: Re: Starship Methox RCS Thrusters
Post by: ulm_atms on 10/13/2019 04:15 pm
I've seen diagrams both ways... the RCS tanks can store the mixed gas right?

You mean mixed gas and liquid? As a mixture of oxygen and methane at high pressure is not going to stay at that pressure for long.

Sorry I was clear... they will be extracting from the LOX and LCH4 header tanks into a new GOX and GCH4 RCS tanks. 
Can that new (GOX and GCH4) be a single tank?

If I saw this config I would never ride on the thing even if paid.  When you mix GOX and GCH4...or any oxidizer and fuel.....you get a bomb, plain and simple.

I state this with certainty...you will never see a single tank with both in it.
Title: Re: Starship Methox RCS Thrusters
Post by: ThomasGadd on 10/13/2019 04:35 pm
I've seen diagrams both ways... the RCS tanks can store the mixed gas right?

You mean mixed gas and liquid? As a mixture of oxygen and methane at high pressure is not going to stay at that pressure for long.

Sorry I was clear... they will be extracting from the LOX and LCH4 header tanks into a new GOX and GCH4 RCS tanks. 
Can that new (GOX and GCH4) be a single tank?

If I saw this config I would never ride on the thing even if paid.  When you mix GOX and GCH4...or any oxidizer and fuel.....you get a bomb, plain and simple.

I state this with certainty...you will never see a single tank with both in it.

I understand that argument but there is no ignition source. 

Automobile gasoline tanks have air and gaseous gasoline in them. 
Title: Re: Starship Methox RCS Thrusters
Post by: ulm_atms on 10/13/2019 04:52 pm
I've seen diagrams both ways... the RCS tanks can store the mixed gas right?

You mean mixed gas and liquid? As a mixture of oxygen and methane at high pressure is not going to stay at that pressure for long.

Sorry I was clear... they will be extracting from the LOX and LCH4 header tanks into a new GOX and GCH4 RCS tanks. 
Can that new (GOX and GCH4) be a single tank?

If I saw this config I would never ride on the thing even if paid.  When you mix GOX and GCH4...or any oxidizer and fuel.....you get a bomb, plain and simple.

I state this with certainty...you will never see a single tank with both in it.

I understand that argument but there is no ignition source. 

Automobile gasoline tanks have air and gaseous gasoline in them.

In very minute quantities of ox to gas.  It may flame up if you smoke will filling up...but it won't go boom..and only flame on the outside where there is more air.  When you start the car, it also pulls a vacuum on the tank to keep that exact thing from happening.  That's why leaving the gas cap off sets off the check engine light..it's a vacuum error.  It's also about the correct fuel to air mixture.  The gas tank is really too fuel rich to burn.

But....

If you have GO2 and GCH4 under pressure in the same RCS tank...it's going to go boom if ignited...no question, as since the tank would have to be at the correct mixture ratio to work at all...it will ignite with very very little energy.  As far as ignition sources...static shock can easily happen.  In a compressed state...it takes so little energy for ignition.  Just the act of pressurizing the system may be enough to set it off.
Title: Re: Starship Methox RCS Thrusters
Post by: Pete on 10/13/2019 05:12 pm
The correct technical term for a high-pressure vessel containing a near-stoichiometric mixture of Oxygen and Methane gas is...

a Bomb.

All it needs is an ignition source.
Which could be almost anything, really.
A bit of heat, the tiniest spark, a shock to the containment vessel, a trace of contaminant in the contents or vessel...
Really about anything.

And 1 kg of Methane/Oxygen mixture contains about 3 times as much energy wanting to be released as 1kg of TNT.
Title: Re: Starship Methox RCS Thrusters
Post by: CorvusCorax on 10/17/2019 10:56 pm
I was thinking, could a MethOX thruster be "boosted"

We have quite a number of Tesla packs on board, in the car they are rated for 500kW each, but let's assume they can provide 2MW short term boost when using space rated next gen (plaid) packs, and put a few in parallel, let's say with a 10 MW Thermoelectric system

(for example using microwaves or a magnetic field to induce a heating current in the plasma within the thurster's throat.

Thus, turning the purely chemical thruster into a electrochemical hybrid engine.


To double the specific impulse - let's say from 350s  to 700s, one would have to quadruple the temperature.

At an assumed Isp of 350 s, at MethOX thruster.
350s = 3433 m/s / 9.81 m/s˛
3433 m/s = 3433 N * s/kg
That means the specific fuel consumption is around 0.3 kg per second and kiloNewton

A 50 Kilonewton thruster would therefore have a fuel flow of 15 kg/s

Assuming a temperature of aprox 3500 K of the methox exhaust, the system would have to superheat the plasma in the throat center (but hopefully not anywhere close to the wall)  to 14000K.

Both steam and Carbon Dioxide are in the ballpark range of a specific heat capacity of 2kJ per kg and Kelvin.

The energy required to heat 15kg of gas from 3500K to 14000k is therefore roughly 10^4 K * 15 kg * 2kJ = 3*10^5 kJ
we have that per second, which makes it 3*10^5 kW of power

that's 30 Megawatt.

So I guess a doubling of ISP at full thrust isn't possible, but at 1/3 of the thrust it might start to be feasible.
Especially if the bats can be recharged using solar panels.

All we need is a 10 MW induction coil around the thruster's throat...  Hmmm... we usually have cryogenic liquids on board anyway, high temperature superconductors anyone?

Edit: Had a slight misconception there. After all, when doubling the ISP at the same mass flow, the thrust would double, too. That means our 50 kN thruster operating at 1/3 throttle, boosted with 10MW  would still put out 2/3 of its regular thrust or around 33 kN.  *slaps head*

Title: Re: Starship Methox RCS Thrusters
Post by: DistantTemple on 10/17/2019 11:12 pm
Really:
Quote from: CorvusCorax
which makes it 3*10^5 kW of power

that's 30 Megawatt.

3*105 = 300,000 in my book!

so thats 300,000KW = 300MW

So has such a coil ever been made? Or any other way of transfering that power.

Items on this scale that come to mind are include
"A new gas-fired power plant with a total capacity of 300MW has entered commercial operation in Lincolnshire.

The Ł100 million Spalding peaking plant is highly flexible and will provide backup for renewable generation sources like solar and wind power."
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 10/17/2019 11:15 pm
OT  Might want to take this discussion to advanced concepts.

John
Title: Re: Starship Methox RCS Thrusters
Post by: alang on 10/18/2019 06:35 am
I've seen diagrams both ways... the RCS tanks can store the mixed gas right?
,
You mean mixed gas and liquid? As a mixture of oxygen and methane at high pressure is not going to stay at that pressure for long.

Sorry I was clear... they will be extracting from the LOX and LCH4 header tanks into a new GOX and GCH4 RCS tanks. 
Can that new (GOX and GCH4) be a single tank?

If I saw this config I would never ride on the thing even if paid.  When you mix GOX and GCH4...or any oxidizer and fuel.....you get a bomb, plain and simple.

I state this with certainty...you will never see a single tank with both in it.

I understand that argument but there is no ignition source. 

Automobile gasoline tanks have air and gaseous gasoline in them.

Were you around for the discussion of obscure ignition sources for the composite overwrapped pressure vessels (COPV's) submerged in liquid oxygen! This was after the SpaceX on pad explosion.
Apologies for not having a link, but a lot of knowledgable  people agreed that there will always be an ignition source.
Title: Re: Starship Methox RCS Thrusters
Post by: ThomasGadd on 10/18/2019 02:57 pm
I've seen diagrams both ways... the RCS tanks can store the mixed gas right?
,
You mean mixed gas and liquid? As a mixture of oxygen and methane at high pressure is not going to stay at that pressure for long.

Sorry I was clear... they will be extracting from the LOX and LCH4 header tanks into a new GOX and GCH4 RCS tanks. 
Can that new (GOX and GCH4) be a single tank?

If I saw this config I would never ride on the thing even if paid.  When you mix GOX and GCH4...or any oxidizer and fuel.....you get a bomb, plain and simple.

I state this with certainty...you will never see a single tank with both in it.

I understand that argument but there is no ignition source. 

Automobile gasoline tanks have air and gaseous gasoline in them.

Were you around for the discussion of obscure ignition sources for the composite overwrapped pressure vessels (COPV's) submerged in liquid oxygen! This was after the SpaceX on pad explosion.
Apologies for not having a link, but a lot of knowledgable  people agreed that there will always be an ignition source.

I was, but there is a big difference between LOX and LCH4 and there gaseous versions. 
On further thought though they will keep the gases stored separately to pressurize the main tanks. 
If there is no engine heat available they can use these tanks pressurize the main tanks
Title: Re: Starship Methox RCS Thrusters
Post by: edzieba on 03/27/2020 03:06 pm
All quiet on the hot-gas RCS front? The plan back at the Mk.1 unveil was for Mk.1 & 2 to use cold-gas RCS only, with hot-gas RCS added to Mk.3. Obviously with the switch to the SN series that timeline is no longer valid, but if hot-gas RCS is as imminent as it was previously going to be we should be seeing some evidence of component testing.
Title: Re: Starship Methox RCS Thrusters
Post by: cdebuhr on 03/27/2020 03:31 pm
All quiet on the hot-gas RCS front? The plan back at the Mk.1 unveil was for Mk.1 & 2 to use cold-gas RCS only, with hot-gas RCS added to Mk.3. Obviously with the switch to the SN series that timeline is no longer valid, but if hot-gas RCS is as imminent as it was previously going to be we should be seeing some evidence of component testing.
The could be heavily into testing hot gas thrusters ... how would we know?  I'd expect most of the test campaign to happen in McGregor, and these things will be so tiny compared to things like Raptor, or even Merlin, that I doubt anyone would even notice.  I expect the first sign of hot gas RCS at BC will be when they install RCS packs that look different from what came before, and barring any tweets from Elon, we won't really know until they light them off for the first time.
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 03/28/2020 04:59 pm
Did this a while back. Notional schematic of gaseous RCS system.
Title: Re: Starship Methox RCS Thrusters
Post by: Jcc on 03/28/2020 06:09 pm
Did this a while back. Notional schematic of gaseous RCS system.

I assume the pumps, etc, will be able to operate in microgravity. How do you avoid the possibility of feeding them gas instead of liquid, or is that not an issue?
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 03/28/2020 06:30 pm
Did this a while back. Notional schematic of gaseous RCS system.

I assume the pumps, etc, will be able to operate in microgravity. How do you avoid the possibility of feeding them gas instead of liquid, or is that not an issue?

- Pumps are small and are only used to recharge the RCS system when needed. Capillary flow devices in the tanks can be used to insure propellants are maintained at the pumps entrances.

- Also, you are taking a small amount of liquid from almost nearly full header tanks. As an alternative, you could get liquid from residual propellants in the main tanks, but that would be harder. You could also pump gases from the main tank, but the pumps would be large and take much more power.

https://www.youtube.com/watch?v=3lwy8xxJxKo

John
Title: Re: Starship Methox RCS Thrusters
Post by: rakaydos on 04/30/2020 09:50 pm
Bumping this for it's relevance to the Lunar Starship's landing system.
Title: Re: Starship Methox RCS Thrusters
Post by: John Alan on 04/30/2020 11:05 pm
For those who have L2 access...
(those that don't, this should block you (link will not work)... click and pic will download to your pc (for me anyway))

https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=47345.0;attach=1628795;sess=45275

Notice... the following in this pic...
The huge horizontal gas bottle 'farm' (in two parts) behind the small motor test cell behind the two Raptor cells...
The small 'plumage' as it's called between the two gas dump pipes on the small motor cell... (compare with the Raptor 'plumage')
(Note - 'plumage' is ScaryDare's name for for what happens when you fire a rocket engine horizontally over the Texas bare ground)

Now, look at livingjw's post up above...
https://forum.nasaspaceflight.com/index.php?topic=46574.msg2062890#msg2062890
I see a ground-based version of THAT in this pic... IMHO

My guess... the gas/gas Methox thruster like noted in the recent Moon lander version has been in the works for some time now.

JA  ;)
Title: Starship lunar landing engines
Post by: envy887 on 05/01/2020 12:55 pm
This is a thread specifically to discuss the landing engines SpaceX appears to have added for the lunar version of Starship. I'd like to consider their size, power, cycle, type, relation to other SpaceX engines, or to other companies' engines.

I'l start with my observations from the renders:
- There are 9 engines, mounted in 3 triple clusters
- The nozzle exit diameter is about 80-100 cm
- The engines are canted out about 25-30 degrees from vertical
- The need to hover a 200-400 t Starship in lunar gravity suggests about 80 to 100 kN thrust each - between SuperDraco and RL-10 in thrust.
- The plume color suggests they are not NTO/MMH and are methalox (maybe hydrolox, but with SpaceX that's unlikely).

Are they likely to be gas-gas methalox at this large size? How would the plumbing be set up and where would the prop storage be? How would they be pressurized? If they are liquid, pumped or pressure-fed? And how long would it take SpaceX to develop a new small liquid engine?

If pressure-fed liquid-liquid, could they be SuperDracos modified for cryo methane regen cooling and spark ignition?
Title: Re: Starship lunar landing engines
Post by: Thunderscreech on 05/01/2020 01:13 pm
Musk has talked about hot methalox RCS before for landing Starship and suggested that they needed to be pretty hefty, seems like it's plausible this could be those or an evolution of those, especially if they're something already in the pipe. 

I think he said the methalox RCS would share injector design heritage with the SuperDraco but be fed with methane and oxygen.  I wonder if it would be practical to use gas-fed for these the way the RCS was described, though, seems like it could be a real challenge to generate that much gaseous methane and oxygen for a burn of more than a couple seconds at this scale, so maybe these can re-use Raptor injectors (with gas-pressurized delivery, that would have a lesser requirement on pressurization than burning the gaseous LH2 & O2 directly, right?)
Title: Re: Starship lunar landing engines
Post by: rakaydos on 05/01/2020 01:21 pm
Please use previous thread: https://forum.nasaspaceflight.com/index.php?topic=46574.0
Title: Re: Starship lunar landing engines
Post by: envy887 on 05/01/2020 01:22 pm
Musk has talked about hot methalox RCS before for landing Starship and suggested that they needed to be pretty hefty, seems like it's plausible this could be those or an evolution of those, especially if they're something already in the pipe. 

I think he said the methalox RCS would share injector design heritage with the SuperDraco but be fed with methane and oxygen.  I wonder if it would be practical to use gas-fed for these the way the RCS was described, though, seems like it could be a real challenge to generate that much gaseous methane and oxygen for a burn of more than a couple seconds at this scale, so maybe these can re-use Raptor injectors (with gas-pressurized delivery, that would have a lesser requirement on pressurization than burning the gaseous LH2 & O2 directly, right?)

We're pretty sure the hot RCS are gas-gas engines, which makes the SuperDraco liquid-liquid design almost useless as a starting point. At minimum the injector and cooling would need to be completely redesigned for gas-gas, and with adding spark ignition that basically makes it a whole new engine. Which is why Musk said the RCS thrusters would be more similar to Raptor than to SuperDraco, because Raptor injects both propellants to the MCC as gases.

The landing engines could be gas-gas, but these are much more powerful than RCS in terms of thrust and total impulse so the gas storage requirements get intense. They would need a volume equal to 30 or more Falcon COPVs to hold the propellants, while with liquid propellants they would need 1/10th that volume. Since tank mass scales with volume, this could be a significant mass optimization.
Title: Re: Starship lunar landing engines
Post by: envy887 on 05/01/2020 01:28 pm
Please use previous thread: https://forum.nasaspaceflight.com/index.php?topic=46574.0

I'm not discussing the RCS system. These engines are, to me, clearly a separate system even if there may be some overlap in function and design.

The difference between these and the RCS is like the difference between Draco system and the SuperDracos on Dragon.
Title: Re: Starship lunar landing engines
Post by: butters on 05/01/2020 01:33 pm
Please use previous thread: https://forum.nasaspaceflight.com/index.php?topic=46574.0

I'm not discussing the RCS system. These engines are, to me, clearly a separate system even if there may be some overlap in function and design.

The difference between these and the RCS is like the difference between Draco system and the SuperDracos on Dragon.

This seems like a faulty premise. If we think the RCS thrusters are approximately 10-ton thrust, why would that be insufficiently powerful for lunar landing using about 9 of them? Gaseous propellant storage and delivery might be an additional challenge given the duty cycle for the landing thrusters, but they seem to be same thruster design as used for RCS.
Title: Re: Starship lunar landing engines
Post by: rakaydos on 05/01/2020 02:07 pm
Please use previous thread: https://forum.nasaspaceflight.com/index.php?topic=46574.0

I'm not discussing the RCS system. These engines are, to me, clearly a separate system even if there may be some overlap in function and design.

The difference between these and the RCS is like the difference between Draco system and the SuperDracos on Dragon.
From the thread you are not discussing:
For those who have L2 access...
(those that don't, this should block you (link will not work)... click and pic will download to your pc (for me anyway))

https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=47345.0;attach=1628795;sess=45275

Notice... the following in this pic...
The huge horizontal gas bottle 'farm' (in two parts) behind the small motor test cell behind the two Raptor cells...
The small 'plumage' as it's called between the two gas dump pipes on the small motor cell... (compare with the Raptor 'plumage')
(Note - 'plumage' is ScaryDare's name for for what happens when you fire a rocket engine horizontally over the Texas bare ground)

Now, look at livingjw's post up above...
https://forum.nasaspaceflight.com/index.php?topic=46574.msg2062890#msg2062890
I see a ground-based version of THAT in this pic... IMHO

My guess... the gas/gas Methox thruster like noted in the recent Moon lander version has been in the works for some time now.

JA  ;)
...there's a REASON we use old threads when possible.
Title: Re: Starship lunar landing engines
Post by: envy887 on 05/01/2020 02:52 pm
Please use previous thread: https://forum.nasaspaceflight.com/index.php?topic=46574.0

I'm not discussing the RCS system. These engines are, to me, clearly a separate system even if there may be some overlap in function and design.

The difference between these and the RCS is like the difference between Draco system and the SuperDracos on Dragon.

This seems like a faulty premise. If we think the RCS thrusters are approximately 10-ton thrust, why would that be insufficiently powerful for lunar landing using about 9 of them? Gaseous propellant storage and delivery might be an additional challenge given the duty cycle for the landing thrusters, but they seem to be same thruster design as used for RCS.

I don't think the current RCS is 10 tonne class, as that info is 4 years old, from the 2016 ITS version which was much larger. There was some talk about using 10 tonne RCS for landing the booster on the launch mount, but that plan has also been shelved.

100 kN is quite high for even for a "rough" RCS on the current size Starship. And landing engines need a much larger total impulse than makes sense for an RCS system which typically operates in small bursts.

The RCS thrusters rendered into the 2017 BFS are shown below, with a person to scale. There are a large and a fine set, with nozzles that are about 8-10 cm and 25-30 cm at exit, not 80-100 cm. The exit angles suggest these are nearly fully expanded, which means a new combustion chamber and throat are required to make a version with a 1 m class nozzle exit. I would estimate those are around 3 kN and 30 kN, not 100 kN.

Image from: https://www.spacex.com/sites/spacex/files/making_life_multiplanetary-2017.pdf

The 2020 Starship is nearly the same size as the 2017 BFS, so the RCS requirements will be the same. It's possible that they ditched the large RCS and only have the small set plus the landing engines. But 9 engines as mounted won't give 3-axis control, so I'm dubious of that. And still, the landing engines would be 20x or 30x larger in thrust and much more than that in total impulse, so it's doubtful they are simply a scaled-up version of the same system.
Title: Re: Starship lunar landing engines
Post by: John Alan on 05/01/2020 03:10 pm
My guess...  is SpaceX has developed FOUR sizes of Methane and Oxygen rocket engines...  ???

Small and tuned for very fast pulsing like action... 3kN as per above post...
Medium and tuned to be throttled or pulsed... 30kN as per envy887 post...

And this Moon landing class, Large as being discussed here in this thread... est is 100kN...
These are Throttle only my guess... and MAY be liquid fed once started, to avoid a large gas storage issue...

SO... could you make a rocket engine that starts on gas/gas and throttles into a liquid/liquid source?
Sure... It's a mini Raptor...  :o
OR not...  :-\

On edit... SO
400 N (Draco RCS)
3 kN (SS/SL RCS thruster) speculated
30 kN (crosswind landing SS/SL thruster) speculated
70 kN (Super Draco D2 launch abort)
100 kN (Moon landing engine (name TBD)) ----- > And subject of THIS thread...
900 kN (Merlin) old school KeroLox workhorse
2000 kN (Raptor) FFSC and state of the art IMHO
How's that for a family of rocket engines and thrusters under the SpaceX brand...  8)

My guess...  ;)

Later edit... added all SpaceX engines to above list... also added and then separate posted the below same of
Could you make a rocket engine that is gas/gas at its core but MAYBE also fed liquid/liquid using electric high-pressure pumps?
Starts on gas/gas and is then fed liquid through chamber and nozzle wall heat exchangers to, in effect, supercharge the engine to 100 kN?
Title: Re: Starship lunar landing engines
Post by: rakaydos on 05/01/2020 03:38 pm
My guess...  is SpaceX has developed FOUR sizes of Methane and Oxygen rocket engines...  ???

Small and tuned for very fast pulsing like action... 3kN as per above post...
Medium and tuned to be throttled or pulsed... 30kN as per envy887 post...

And this Moon landing class, Large as being discussed here in this thread... est is 100kN...
These are Throttle only my guess... and MAY be liquid fed once started, to avoid a large gas storage issue...

SO... could you make a rocket engine that starts on gas/gas and throttles into a liquid/liquid source?
Sure... It's a mini Raptor...  :o
OR not...  :-\

On edit... SO
3 kN (SS/SL RCS thruster) speculated
30 kN (crosswind landing SS/SL thruster) speculated
100 kN (Moon landing engine (name TBD)) ----- > And subject of THIS thread...
900 kN (Merlin) old school KeroLox workhorse
2000 kN (Raptor) FFSC and state of the art IMHO
How's that for a family of engines under the SpaceX brand...  8)

My guess...  ;)
"It takes 10 years to design and build a new rocket engine."
as late as 6 years ago, the plan was for a 12m booster to land on the launch mount in potentially heavy winds. I would believe that the RCS designed for that role would be approaching maturity around now, and while technically overpowered for Starship, it's ready.
Title: Re: Starship lunar landing engines
Post by: envy887 on 05/01/2020 03:51 pm
My guess...  is SpaceX has developed FOUR sizes of Methane and Oxygen rocket engines...  ???

Small and tuned for very fast pulsing like action... 3kN as per above post...
Medium and tuned to be throttled or pulsed... 30kN as per envy887 post...

And this Moon landing class, Large as being discussed here in this thread... est is 100kN...
These are Throttle only my guess... and MAY be liquid fed once started, to avoid a large gas storage issue...

SO... could you make a rocket engine that starts on gas/gas and throttles into a liquid/liquid source?
Sure... It's a mini Raptor...  :o
OR not...  :-\

On edit... SO
3 kN (SS/SL RCS thruster) speculated
30 kN (crosswind landing SS/SL thruster) speculated
100 kN (Moon landing engine (name TBD)) ----- > And subject of THIS thread...
900 kN (Merlin) old school KeroLox workhorse
2000 kN (Raptor) FFSC and state of the art IMHO
How's that for a family of engines under the SpaceX brand...  8)

My guess...  ;)
"It takes 10 years to design and build a new rocket engine."
as late as 6 years ago, the plan was for a 12m booster to land on the launch mount in potentially heavy winds. I would believe that the RCS designed for that role would be approaching maturity around now, and while technically overpowered for Starship, it's ready.

The landing engines may be derived from the ITS 100 kN RCS, but this application is no longer RCS. And Starship certainly has a separate RCS system that is probably mostly unrelated, at a systems level, to this landing system.

Or these could be completely unrelated. ITS had larger tanks generating more boiloff, and a larger margin for mass, so a high pressure gas-gas system made sense. Shoehorning it into this smaller vehicle may not.
Title: Re: Starship lunar landing engines
Post by: Lampyridae on 05/01/2020 03:54 pm
The nose cone and a barrel section seem unused. There's also a central pipe seen through the windows... my guess is a greatly enlarged header tank. Probably not for descent but ascent. I was thinking that the mini-Raptors/methDracos would run off that liquid fuel line somehow.

As to why the "ascent fuel" is overhead, my guess is there are 3 good reasons:
1. Jettison lower tank sections for an emergency ascent
2. Tanks are less likely to be punctured by damage.
3. Overhead radiation shield.
Title: Re: Starship lunar landing engines
Post by: John Alan on 05/01/2020 04:01 pm
Could you make a rocket engine that is gas/gas at its core but MAYBE also fed liquid/liquid using electric high-pressure pumps?
Starts on gas/gas and is then fed liquid through chamber and nozzle wall heat exchangers to, in effect, supercharge the 30 KN engine to 100 kN?

On edit...
And what cycle would you call THAT...  ;D
Title: Re: Starship lunar landing engines
Post by: xvel on 05/01/2020 04:12 pm
Could you make a rocket engine that is gas/gas at its core but MAYBE also fed liquid/liquid using electric high-pressure pumps?
Starts on gas/gas and is then fed liquid through chamber and nozzle wall heat exchangers to, in effect, supercharge the 30 KN engine to 100 kN?

raptor main combustion chamber is gasgas
Title: Re: Starship lunar landing engines
Post by: John Alan on 05/01/2020 04:17 pm
Could you make a rocket engine that is gas/gas at its core but MAYBE also fed liquid/liquid using electric high-pressure pumps?
Starts on gas/gas and is then fed liquid through chamber and nozzle wall heat exchangers to, in effect, supercharge the 30 KN engine to 100 kN?

raptor main combustion chamber is gasgas

Ok... main long term feed source is what I meant... Raptor is liquid fed... 'Core" was an incorrect term

On edit...
Let me clarify... a Rocket engine with an unusual start-up and duty cycle and operating cycle... for landing purposes...

Lights off using gas/gas from a high-pressure source directly into the chamber to warm it up... time needed maybe 10 seconds... thrust is not great and the chamber is oversized along with the nozzle for the tiny feed flow... BUT it conserves high-pressure gas reserves and landing does not need instant high thrust...

Once the chamber and nozzle warm-up, the electric pumps start slowly to start getting the liquid side primed... again the idea is slow starting fuel/oxidizer efficient usage... two sets of injectors... small set of gas/gas bottle-fed and larger set fed from the electric pump liquid fed, chamber and nozzle wall gasified set... another 10 seconds to get it up to full thrust...

THEN... gas bottle source is cut off... and the electric pump speed literally is the throttle on these landing engines...

Again... what would you call that rocket engine cycle?
Title: Re: Starship lunar landing engines
Post by: LMT on 05/01/2020 04:20 pm
Could you make a rocket engine that is gas/gas at its core but MAYBE also fed liquid/liquid using electric high-pressure pumps?

That would qualify as "complex", as the word is used in the HLS contract award (https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=50806.0;attach=1755515;sess=44864):

Quote from: Stephen Jurczyk
...SpaceX's proposed propulsion system is notably complex and comprised of likewise complex inividual subsystems that have yet to be developed, tested, and certified with very little schedule margin to accommodate delays.  One notable example of this issue concerns SpaceX’s proposed reaction control system (RCS), which is very complex when compared to flight-proven systems and will require considerable development time. Additionally, there is significant risk associated with successful development of the integrated propulsion system given the proposed approach for integrating and testing the individual elements of the system. While I note that SpaceX has proposed a robust and aggressive plan for early systems demonstrations, which lends credibility to its proposed execution, this plan does not adequately address the risk of potential delay in development, as well as concomitant delay to SpaceX’s demonstration mission.

--

Are the soft-landing rockets pressure-fed gaseous methox, or pump-fed liquid methalox, or both?

Just guessing from a peek through the open door (https://forum.nasaspaceflight.com/index.php?topic=49144.msg2075463#msg2075463), soft-landing rocketry might take up half the cargo deck.  The actual rockets may extend beneath the floor, into the curved space above the LCH4 tank, where nozzles are mounted.  That all suggests, to my mind, oversized RCS gaseous tanks within the cargo deck proper.

If that's the case, soft-landing rockets are methox, only.  Otherwise, why devote so much space?
Title: Re: Starship lunar landing engines
Post by: envy887 on 05/01/2020 04:32 pm
Could you make a rocket engine that is gas/gas at its core but MAYBE also fed liquid/liquid using electric high-pressure pumps?
Starts on gas/gas and is then fed liquid through chamber and nozzle wall heat exchangers to, in effect, supercharge the 30 KN engine to 100 kN?

raptor main combustion chamber is gasgas

It's possible to inject liquid into an engine at various places to increase thrust at expense of ISP, but its a lot of complexity with no real advantages for this use case IMO. Pressure-fed liquid engines are pretty simple.
Title: Re: Starship lunar landing engines
Post by: livingjw on 05/01/2020 04:38 pm
Musk has talked about hot methalox RCS before for landing Starship and suggested that they needed to be pretty hefty, seems like it's plausible this could be those or an evolution of those, especially if they're something already in the pipe. 

I think he said the methalox RCS would share injector design heritage with the SuperDraco but be fed with methane and oxygen.  I wonder if it would be practical to use gas-fed for these the way the RCS was described, though, seems like it could be a real challenge to generate that much gaseous methane and oxygen for a burn of more than a couple seconds at this scale, so maybe these can re-use Raptor injectors (with gas-pressurized delivery, that would have a lesser requirement on pressurization than burning the gaseous LH2 & O2 directly, right?)

We're pretty sure the hot RCS are gas-gas engines, which makes the SuperDraco liquid-liquid design almost useless as a starting point. At minimum the injector and cooling would need to be completely redesigned for gas-gas, and with adding spark ignition that basically makes it a whole new engine. Which is why Musk said the RCS thrusters would be more similar to Raptor than to SuperDraco, because Raptor injects both propellants to the MCC as gases.

The landing engines could be gas-gas, but these are much more powerful than RCS in terms of thrust and total impulse so the gas storage requirements get intense. They would need a volume equal to 30 or more Falcon COPVs to hold the propellants, while with liquid propellants they would need 1/10th that volume. Since tank mass scales with volume, this could be a significant mass optimization.

- I agree. I think the soft landing engines will be liquid fueled. I am working on a RCS and Landing System based on my earlier RCS system. I am just adding two pressurized tanks to store "soft landing" propellants. These tanks are pressurized by the GOx and GCH4 pressure tanks.

- These soft landing tanks can be refilled from header tanks (or main if that is desired) by depressurizing them, pumping in the propellants using small electric pumps, then re-pressurizing.

- I think the tanks can be fit around the top of the LOx main CH4 tank.

John
Title: Re: Starship lunar landing engines
Post by: LMT on 05/01/2020 05:08 pm
- I think the tanks can be fit around the top of the LOx tank.

Isn't LOX tank integral with LCH4 tank?

Or is there reason to expect some useful space around LCH4 tank, between nozzle clusters?  SpaceX indicated that space was already heavily utilized, in their 2017 cutout.
Title: Re: Starship lunar landing engines
Post by: livingjw on 05/01/2020 05:13 pm
- I think the tanks can be fit around the top of the LOx tank.

Isn't LOX tank integral with LCH4 tank?

Or is there reason to expect some useful space around LCH4 tank, between nozzles?

ARRRG! I did it myself! I meant the main CH4 tank. Thanks.

John
Title: Re: Starship lunar landing engines
Post by: rsdavis9 on 05/01/2020 06:10 pm
- I think the tanks can be fit around the top of the LOx tank.

Isn't LOX tank integral with LCH4 tank?

Or is there reason to expect some useful space around LCH4 tank, between nozzles?

I think we should make Elon move the LOX tank back on top so we stop making this mistake.! :)

ARRRG! I did it myself! I meant the main CH4 tank. Thanks.

John
Title: Re: Starship lunar landing engines
Post by: John Alan on 05/01/2020 06:26 pm
Going back to the original post of yours this morning...

This is a thread specifically to discuss the landing engines SpaceX appears to have added for the lunar version of Starship. I'd like to consider their size, power, cycle, type, relation to other SpaceX engines, or to other companies' engines.

I'l start with my observations from the renders:
- There are 9 engines, mounted in 3 triple clusters
- The nozzle exit diameter is about 80-100 cm
- The engines are canted out about 25-30 degrees from vertical
- The need to hover a 200-400 t Starship in lunar gravity suggests about 80 to 100 kN thrust each - between SuperDraco and RL-10 in thrust.
- The plume color suggests they are not NTO/MMH and are methalox (maybe hydrolox, but with SpaceX that's unlikely).

Are they likely to be gas-gas methalox at this large size? How would the plumbing be set up and where would the prop storage be? How would they be pressurized? If they are liquid, pumped or pressure-fed? And how long would it take SpaceX to develop a new small liquid engine?

If pressure-fed liquid-liquid, could they be SuperDracos modified for cryo methane regen cooling and spark ignition?

Yep... works on the Moon...
What about using the same mounting location and all that gains plume and debris wise and landing leg weight wise ... and landing on Mars...

Needs to be about 250 kN class?... and fit in the same space... meaning much higher chamber pressures...
Need a 1/8 scale Raptor engine in the long run... you think Raptor scales size wise... I bet it does with a bit of luck...

That said... that's overkill for the moon...
BUT I could see them replacing 3 of 9 on a Moon bound one and trying it out someday...

See... this goes back to that 'plumage' comment I made elsewhere...
Does that mark look like 1/20th of a Raptor... or 1/8th...
I think one look at that gas tank farm made them realize it's gotta be liquid fed...
And by the size of the pipes plumbed around to that corner in that earlier pic... I wonder...

Oh well... time will tell... back to the 100 kN version being discussed today...  ;)
Title: Re: Starship Methox RCS Thrusters
Post by: su27k on 05/02/2020 03:46 am
So a liquid methalox landing engine? Not a huge development, but not small either, just for the last tens of meters of lunar landing, isn't it a bit wasteful?

Do we expect this engine will have other uses? Will it be used on Mars landing too?

Could they speed up the development by reusing some stuff from Raptor? Could Raptor be down-scaled easily to create a mini-Raptor for this?

Could this engine share commonality with smaller RCS engines?
Title: Re: Starship Methox RCS Thrusters
Post by: Khadgars on 05/02/2020 04:01 am
are we sure they not using the super dracos?
Title: Re: Starship Methox RCS Thrusters
Post by: aero on 05/02/2020 04:09 am
Would it be less complicated plumbing to feed the landing engines from the header tanks? Maybe not as the LOX header is much nearer the nose than are the landing engines.
Title: Re: Starship Methox RCS Thrusters
Post by: MATTBLAK on 05/02/2020 04:12 am
What about the previous, abandoned sub-scale version of the Raptors?
Title: Re: Starship Methox RCS Thrusters
Post by: Coastal Ron on 05/02/2020 04:21 am
It would sure be convenient to use the same propellant for the landing thrusters as they use for the main engines. Simplifies refueling and reuse.

Looking forward to getting official clarity on this...
Title: Re: Starship Methox RCS Thrusters
Post by: MATTBLAK on 05/02/2020 04:23 am
I wonder how difficult it would be to make a new version of a Methalox Super Draco, with nozzle extensions?
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 05/02/2020 04:55 am
Would it be less complicated plumbing to feed the landing engines from the header tanks? Maybe not as the LOX header is much nearer the nose than are the landing engines.

Can't use the header tanks because they are low pressure tanks. You need tanks that can handle pressures in the 30 - 60 bar range, assuming they are using pressure fed "soft landing" rockets, which is a safe bet.

John
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 05/02/2020 04:56 am
What about the previous, abandoned sub-scale version of the Raptors?

Too big.
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 05/02/2020 04:59 am
I wonder how difficult it would be to make a new version of a Methalox Super Draco, with nozzle extensions?

Pretty straight forward. You already have a pressurization system in the form of the high pressure GOx and GCH4 tanks. High pressure GOx and GCH4 are used to feed the RCS thrusters and to start the Raptors.

John
Title: Re: Starship Methox RCS Thrusters
Post by: tbellman on 05/02/2020 09:30 am
are we sure they not using the super dracos?

I think it is quite unlikely.  SuperDraco uses hypergolic propellants, so you would need some way of filling that up as well, not just methane and oxygen.

The current incarnation of SuperDraco is also single-use, ever since they replaced a valve with a burst disc after the Dragon 2 explosion.
Title: Re: Starship Methox RCS Thrusters
Post by: RotoSequence on 05/02/2020 09:45 am
If they need to regasify the liquid oxygen and liquid methane (stored as liquids for volume and storage tank mass concerns) before burning it in their RCS system, how do they do so? Will they need some sort of heat engine and gas storage bottle buffer to run the smaller thrusters for small maneuvers and corrections without firing up their, presumably fuel burning, heat engine?
Title: Re: Starship Methox RCS Thrusters
Post by: rakaydos on 05/02/2020 11:26 am
So a liquid methalox landing engine? Not a huge development, but not small either, just for the last tens of meters of lunar landing, isn't it a bit wasteful?

Do we expect this engine will have other uses? Will it be used on Mars landing too?

Could they speed up the development by reusing some stuff from Raptor? Could Raptor be down-scaled easily to create a mini-Raptor for this?

Could this engine share commonality with smaller RCS engines?
these would be the rocket engines designed as the ITS boosters landing RCS, back when was planned to land on the launch mount in high winds. It takes 10 years to build a rocket engine after all. A bit oversized for starship's own RCS, but if it's scales, the RCS could be smaller than planned while the landing thrusters could be bigger than planned.
Title: Re: Starship Methox RCS Thrusters
Post by: oiorionsbelt on 05/02/2020 01:47 pm
With respect to the assumed size of these landing engines. As they are mission critical I would expect engine out capability.
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 05/02/2020 04:13 pm
So a liquid methalox landing engine? Not a huge development, but not small either, just for the last tens of meters of lunar landing, isn't it a bit wasteful?

Do we expect this engine will have other uses? Will it be used on Mars landing too?

Could they speed up the development by reusing some stuff from Raptor? Could Raptor be down-scaled easily to create a mini-Raptor for this?

Could this engine share commonality with smaller RCS engines?
these would be the rocket engines designed as the ITS boosters landing RCS, back when was planned to land on the launch mount in high winds. It takes 10 years to build a rocket engine after all. A bit oversized for starship's own RCS, but if it's scales, the RCS could be smaller than planned while the landing thrusters could be bigger than planned.

Where 10 years for a pump fed liquid rocket engine is reasonable, a pressure fed rocket is much simpler. You need to develop efficient injectors, ignition and combustion chamber. No pumps or pre-burners are necessary. SpaceX having experience in these areas, from Merlin and SuperDraco, should be able to develop "soft landing" engines in much less time.

John
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 05/02/2020 04:20 pm
If they need to regasify the liquid oxygen and liquid methane (stored as liquids for volume and storage tank mass concerns) before burning it in their RCS system, how do they do so? Will they need some sort of heat engine and gas storage bottle buffer to run the smaller thrusters for small maneuvers and corrections without firing up their, presumably fuel burning, heat engine?

- The high pressure LOx and LCH4 tanks are only for the "soft landing" engines, not the RCS thrusters. "soft landing" rockets would use liquid liquid injectors (possibly pintle). RCS thrusters continue to be be gas gas.

John
Title: Re: Starship Methox RCS Thrusters
Post by: woods170 on 05/02/2020 05:01 pm
What about the previous, abandoned sub-scale version of the Raptors?

That was only 20% smaller than production Raptor. The engines shown in the artwork are much smaller than sub-scale Raptor.
Title: Re: Starship Methox RCS Thrusters
Post by: Nomadd on 05/02/2020 05:12 pm
these would be the rocket engines designed as the ITS boosters landing RCS, back when was planned to land on the launch mount in high winds. It takes 10 years to build a rocket engine after all. A bit oversized for starship's own RCS, but if it's scales, the RCS could be smaller than planned while the landing thrusters could be bigger than planned.
Merlin was flying less than 5 years after it was a vague idea. And I'm pretty sure SpaceX is a little better staffed and equipped than Tom Mueller's garage was 19 years ago.
Title: Re: Starship Methox RCS Thrusters
Post by: kkattula on 05/03/2020 02:31 am
If the Lunar Lander SS version still has header tanks*, which are designed to only be pressurized enough for Raptor pump input, (3 to 6 bar?), then making the headers capable of 20 to 30 bar, or adding new high pressure liquid tanks, for pressure fed engines seems like a lot of complication.

We know SpaceX are OK with starting turbo-pump engines in the last few seconds before hitting the ground. They do this with Merlins on Falcon, and plan to do it with Raptors on Starship and Super Heavy.  Why do we think they would desperately try to avoid this here?

I suggest they might use an expander or gas-generator cycle engine. Something designed to be simple and dependable, but not getting the highest possible Isp or T/W. (If GG they must be feeding the exhaust back into the main nozzle since we don't see secondary exhausts).

These engines don't need to be super efficient since they will only operate for a few seconds at relatively low total delta v. E.g. 30 seconds of hovering/descent on the Moon is less than 50 m/s.

 At a conservative Isp 320 and T/W of only 30, nine of these engines would only mass 3 tonnes, and use < 5 t of propellant for 50 m/s, at 280 t landed mass.

That's assuming 100 kN engines. They don't need 9 x 100 kN to land on the Moon, even with cosine losses. 5 would do.  However, 9 is just under the minimum to land a 250 t SS on Mars. If that's the plan, I expect they may actually be about 120 kN or slightly more. Or that they plan to increase the thrust later. If not, the engines may be just 70 kN to allow for 2-engine out.


*  I think it probably will have headers (somewhere) because, after returning to NRHO, it will need to wait around for months until the next surface mission, so propellant for station keeping in headers will be a lot easier to manage than in nearly empty main tanks, or as gas in high pressure tanks.
Title: Re: Starship Methox RCS Thrusters
Post by: steveleach on 05/03/2020 11:50 am
If they need to regasify the liquid oxygen and liquid methane (stored as liquids for volume and storage tank mass concerns) before burning it in their RCS system, how do they do so? Will they need some sort of heat engine and gas storage bottle buffer to run the smaller thrusters for small maneuvers and corrections without firing up their, presumably fuel burning, heat engine?

- The high pressure LOx and LCH4 tanks are only for the "soft landing" engines, not the RCS thrusters. "soft landing" rockets would use liquid liquid injectors (possibly pintle). RCS thrusters continue to be be gas gas.

John
I was trying to get my head around how all this would work, in terms of tanking, for all the different engines. You've got the Raptors (pump-fed liquid), the RCS (pressure-fed gas) and now the soft-landers (pressure-fed liquid), all using methane and oxygen, right? All three are presumably in use during moon landings. And you need to keep it all in the correct state and pressure for when it's needed.

Would this (oversimplified) diagram be in any way representative?
Title: Re: Starship Methox RCS Thrusters
Post by: envy887 on 05/03/2020 12:12 pm
If they need to regasify the liquid oxygen and liquid methane (stored as liquids for volume and storage tank mass concerns) before burning it in their RCS system, how do they do so? Will they need some sort of heat engine and gas storage bottle buffer to run the smaller thrusters for small maneuvers and corrections without firing up their, presumably fuel burning, heat engine?

- The high pressure LOx and LCH4 tanks are only for the "soft landing" engines, not the RCS thrusters. "soft landing" rockets would use liquid liquid injectors (possibly pintle). RCS thrusters continue to be be gas gas.

John
Does SuperDraco use a pintle injector?
Title: Re: Starship Methox RCS Thrusters
Post by: enbandi on 05/03/2020 12:25 pm
are we sure they not using the super dracos?

I think it is quite unlikely.  SuperDraco uses hypergolic propellants, so you would need some way of filling that up as well, not just methane and oxygen.

The current incarnation of SuperDraco is also single-use, ever since they replaced a valve with a burst disc after the Dragon 2 explosion.

Hypergol was my first concern also, when somebody suggested SuperDracos. But Gateway and the other lander concepts may rely already on that, in which case not as big a concern from logistics perspective. (Not sure, I di not kniw enough about GW, and the other dssigns, and hypergol can mean different propellants also).

But I agree on overall: Dracos and hypergol are much less probable as least practical and we newer heard a single word about hypergol on Starship.
Title: Re: Starship Methox RCS Thrusters
Post by: dondar on 05/03/2020 12:52 pm
If they need to regasify the liquid oxygen and liquid methane (stored as liquids for volume and storage tank mass concerns) before burning it in their RCS system, how do they do so? Will they need some sort of heat engine and gas storage bottle buffer to run the smaller thrusters for small maneuvers and corrections without firing up their, presumably fuel burning, heat engine?

- The high pressure LOx and LCH4 tanks are only for the "soft landing" engines, not the RCS thrusters. "soft landing" rockets would use liquid liquid injectors (possibly pintle). RCS thrusters continue to be be gas gas.

John
why not to use common  high pressure LOx and LCh4 tanks for RCS and landing trusters and to pump them up with some special pump when needed? Why RCS trusters have to be gas gas per se? Power flexibility can be resolved by the mixing ratios to some degree, and injectors are infinitely accurate (with earth G applied at least). I am completely out of my woods here :D.
Title: Re: Starship Methox RCS Thrusters
Post by: rsdavis9 on 05/03/2020 12:55 pm

That's assuming 100 kN engines. They don't need 9 x 100 kN to land on the Moon, even with cosine losses. 5 would do.  However, 9 is just under the minimum to land a 250 t SS on Mars. If that's the plan, I expect they may actually be about 120 kN or slightly more. Or that they plan to increase the thrust later. If not, the engines may be just 70 kN to allow for 2-engine out.

I was assuming about 320t landed mass
100t ship (minus 10t tiles, 10t fins)
150t payload
70t propellant back to LLO
(370s*force)*(ln(170t/100t)) =1925m/s (average 370s isp 70t propellant, going back empty)

320t*moongravity= .51MN
5*.1MN =.5MN (5 .1MN thrusters, almost enough)
9*.1MN = .9MN (more than enough)

And I think from what I have read that site selection on mars should provide landing spots without meters of loose soil. So no need for these landing thrusters.
Title: Re: Starship Methox RCS Thrusters
Post by: rsdavis9 on 05/03/2020 12:59 pm
If they need to regasify the liquid oxygen and liquid methane (stored as liquids for volume and storage tank mass concerns) before burning it in their RCS system, how do they do so? Will they need some sort of heat engine and gas storage bottle buffer to run the smaller thrusters for small maneuvers and corrections without firing up their, presumably fuel burning, heat engine?

- The high pressure LOx and LCH4 tanks are only for the "soft landing" engines, not the RCS thrusters. "soft landing" rockets would use liquid liquid injectors (possibly pintle). RCS thrusters continue to be be gas gas.

John
why not to use common  high pressure LOx and LCh4 tanks for RCS and landing trusters and to pump them up with some special pump when needed? Why RCS trusters have to be gas gas per se? Power flexibility can be resolved by the mixing ratios to some degree, and injectors are infinitely accurate (with earth G applied at least). I am completely out of my woods here :D.

gas gas makes it possible to fire up fast in any orientation and acceleration.
livingjw showed a electric pump to supply liquids to replenish the 30bar(50bar?) gas storage.
Also heaters to vaporize?
Title: Re: Starship Methox RCS Thrusters
Post by: rakaydos on 05/03/2020 03:38 pm
If they need to regasify the liquid oxygen and liquid methane (stored as liquids for volume and storage tank mass concerns) before burning it in their RCS system, how do they do so? Will they need some sort of heat engine and gas storage bottle buffer to run the smaller thrusters for small maneuvers and corrections without firing up their, presumably fuel burning, heat engine?

- The high pressure LOx and LCH4 tanks are only for the "soft landing" engines, not the RCS thrusters. "soft landing" rockets would use liquid liquid injectors (possibly pintle). RCS thrusters continue to be be gas gas.

John
why not to use common  high pressure LOx and LCh4 tanks for RCS and landing trusters and to pump them up with some special pump when needed? Why RCS trusters have to be gas gas per se? Power flexibility can be resolved by the mixing ratios to some degree, and injectors are infinitely accurate (with earth G applied at least). I am completely out of my woods here :D.

gas gas makes it possible to fire up fast in any orientation and acceleration.
livingjw showed a electric pump to supply liquids to replenish the 30bar(50bar?) gas storage.
Also heaters to vaporize?
So why have the soft landing engines use liquid/liquid, if there are perfectly functional gas/gas engines with gas/gas tankage already installed and ready to go?
Two engine types (Raptor and... I guess Harrier?), three sets of tanks (main/header/high pressure gas from raptor) instead of 3 engine types (raptor, harrier and RCS) and 4 sets of tanks.
Title: Re: Starship Methox RCS Thrusters
Post by: steveleach on 05/03/2020 05:03 pm
So why have the soft landing engines use liquid/liquid, if there are perfectly functional gas/gas engines with gas/gas tankage already installed and ready to go?
Two engine types (Raptor and... I guess Harrier?), three sets of tanks (main/header/high pressure gas from raptor) instead of 3 engine types (raptor, harrier and RCS) and 4 sets of tanks.
I'm so far from an expert that its laughable, but my limited understanding is that gas-fed engines are limited in the rate at which you can get the fuel into the engines, so are only suitable for low thrust uses like RCS. Pressure-fed liquid can shift more fuel to get more thrust, but not as much as pump-fed liquid.
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 05/03/2020 05:05 pm
If they need to regasify the liquid oxygen and liquid methane (stored as liquids for volume and storage tank mass concerns) before burning it in their RCS system, how do they do so? Will they need some sort of heat engine and gas storage bottle buffer to run the smaller thrusters for small maneuvers and corrections without firing up their, presumably fuel burning, heat engine?

- The high pressure LOx and LCH4 tanks are only for the "soft landing" engines, not the RCS thrusters. "soft landing" rockets would use liquid liquid injectors (possibly pintle). RCS thrusters continue to be be gas gas.

John
why not to use common  high pressure LOx and LCh4 tanks for RCS and landing trusters and to pump them up with some special pump when needed? Why RCS trusters have to be gas gas per se? Power flexibility can be resolved by the mixing ratios to some degree, and injectors are infinitely accurate (with earth G applied at least). I am completely out of my woods here :D.

gas gas makes it possible to fire up fast in any orientation and acceleration.
livingjw showed a electric pump to supply liquids to replenish the 30bar(50bar?) gas storage.
Also heaters to vaporize?
So why have the soft landing engines use liquid/liquid, if there are perfectly functional gas/gas engines with gas/gas tankage already installed and ready to go?
Two engine types (Raptor and... I guess Harrier?), three sets of tanks (main/header/high pressure gas from raptor) instead of 3 engine types (raptor, harrier and RCS) and 4 sets of tanks.

- Whether they will be gas or liquid fed will depend on how much delta V is required for soft landing maneuver versus worst case RCS maneuvering sequence.

- If they are close then it would make sense to just use gas fed rockets, but I suspect that the delta V requirements for the soft landing will be much greater. If they are, then liquid fed rockets in order to save tankage weight.Tank weight is proportional to volume, pressure and density. Liquids are denser than gases so lighter tanks.

- I am cutting numbers now for an integrated RCS / soft landing system. I will do it both ways and see which is lighter and by how much.

John
Title: Re: Starship Methox RCS Thrusters
Post by: ThomasGadd on 05/03/2020 05:18 pm
If they need to regasify the liquid oxygen and liquid methane (stored as liquids for volume and storage tank mass concerns) before burning it in their RCS system, how do they do so? Will they need some sort of heat engine and gas storage bottle buffer to run the smaller thrusters for small maneuvers and corrections without firing up their, presumably fuel burning, heat engine?

- The high pressure LOx and LCH4 tanks are only for the "soft landing" engines, not the RCS thrusters. "soft landing" rockets would use liquid liquid injectors (possibly pintle). RCS thrusters continue to be be gas gas.

John
why not to use common  high pressure LOx and LCh4 tanks for RCS and landing trusters and to pump them up with some special pump when needed? Why RCS trusters have to be gas gas per se? Power flexibility can be resolved by the mixing ratios to some degree, and injectors are infinitely accurate (with earth G applied at least). I am completely out of my woods here :D.

gas gas makes it possible to fire up fast in any orientation and acceleration.
livingjw showed a electric pump to supply liquids to replenish the 30bar(50bar?) gas storage.
Also heaters to vaporize?
So why have the soft landing engines use liquid/liquid, if there are perfectly functional gas/gas engines with gas/gas tankage already installed and ready to go?
Two engine types (Raptor and... I guess Harrier?), three sets of tanks (main/header/high pressure gas from raptor) instead of 3 engine types (raptor, harrier and RCS) and 4 sets of tanks.

- Whether they will be gas or liquid fed will depend on how much delta V is required for soft landing maneuver versus worst case RCS maneuvering sequence.

- If they are close then it would make sense to just use gas fed rockets, but I suspect that the delta V requirements for the soft landing will be much greater. If they are, then liquid fed rockets in order to save tankage weight.Tank weight is proportional to volume, pressure and density. Liquids are denser than gases so lighter tanks.

- I am cutting numbers now for an integrated RCS / soft landing system. I will do it both ways and see which is lighter and by how much.

John

One of the previous arguments was that the gas would be depleted.
Why can't the liquid to gas generation keep up with the RCS demand? 
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 05/03/2020 06:17 pm
If they need to regasify the liquid oxygen and liquid methane (stored as liquids for volume and storage tank mass concerns) before burning it in their RCS system, how do they do so? Will they need some sort of heat engine and gas storage bottle buffer to run the smaller thrusters for small maneuvers and corrections without firing up their, presumably fuel burning, heat engine?

- The high pressure LOx and LCH4 tanks are only for the "soft landing" engines, not the RCS thrusters. "soft landing" rockets would use liquid liquid injectors (possibly pintle). RCS thrusters continue to be be gas gas.

John
why not to use common  high pressure LOx and LCh4 tanks for RCS and landing trusters and to pump them up with some special pump when needed? Why RCS trusters have to be gas gas per se? Power flexibility can be resolved by the mixing ratios to some degree, and injectors are infinitely accurate (with earth G applied at least). I am completely out of my woods here :D.

gas gas makes it possible to fire up fast in any orientation and acceleration.
livingjw showed a electric pump to supply liquids to replenish the 30bar(50bar?) gas storage.
Also heaters to vaporize?
So why have the soft landing engines use liquid/liquid, if there are perfectly functional gas/gas engines with gas/gas tankage already installed and ready to go?
Two engine types (Raptor and... I guess Harrier?), three sets of tanks (main/header/high pressure gas from raptor) instead of 3 engine types (raptor, harrier and RCS) and 4 sets of tanks.

- Whether they will be gas or liquid fed will depend on how much delta V is required for soft landing maneuver versus worst case RCS maneuvering sequence.

- If they are close then it would make sense to just use gas fed rockets, but I suspect that the delta V requirements for the soft landing will be much greater. If they are, then liquid fed rockets in order to save tankage weight.Tank weight is proportional to volume, pressure and density. Liquids are denser than gases so lighter tanks.

- I am cutting numbers now for an integrated RCS / soft landing system. I will do it both ways and see which is lighter and by how much.

John

One of the previous arguments was that the gas would be depleted.
Why can't the liquid to gas generation keep up with the RCS demand?

If you do that, you basically have a Rutherford rocket engine. The Rutherford rocket pumps are driven by two motors said to put out ~100 HP to produce 5400 lbs of thrust. This is about 4000 hp/MN. If we need .5 - 1MN of thrust, we would need 2000 - 4000 HP. Also, there is no need to vaporize it, just send it to the rockets as a liquid. Not a good option.

John
Title: Re: Starship Methox RCS Thrusters
Post by: ThomasGadd on 05/03/2020 06:50 pm
Sorry John I wasn't clear...
As understand it
Liquid is pumped from the header tanks into a high pressure gas tank and heated if necessary to create high pressure gas source for the RCS. 
Repeat as necessary to keep it pressurized. 
Why can't this be a continuous process? 
Do we need two sets of tanks to alternate between?  Use one and fill the other then switch?
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 05/03/2020 06:53 pm
If they need to regasify the liquid oxygen and liquid methane (stored as liquids for volume and storage tank mass concerns) before burning it in their RCS system, how do they do so? Will they need some sort of heat engine and gas storage bottle buffer to run the smaller thrusters for small maneuvers and corrections without firing up their, presumably fuel burning, heat engine?

- The high pressure LOx and LCH4 tanks are only for the "soft landing" engines, not the RCS thrusters. "soft landing" rockets would use liquid liquid injectors (possibly pintle). RCS thrusters continue to be be gas gas.

John
I was trying to get my head around how all this would work, in terms of tanking, for all the different engines. You've got the Raptors (pump-fed liquid), the RCS (pressure-fed gas) and now the soft-landers (pressure-fed liquid), all using methane and oxygen, right? All three are presumably in use during moon landings. And you need to keep it all in the correct state and pressure for when it's needed.

Would this (oversimplified) diagram be in any way representative?

- I think so. You would also need lines and electric pumps from the header tanks to the HP gas tanks to recharge them after a sequence of maneuvers. When the Raptors are firing they may be used for HP gas replenishment but you would also need lines and electric pumps from the header tanks to the HP gas tanks to recharge them after a sequence of RCS maneuvers when the Raptors are not running. Also, after a long stay on Mars or the Moon, you will need to recharge your HP gas in order to start the raptors.

Here is a sequence of operation:

Assumptions:
   - HP propellant tanks are sized for "soft landing" delta V and are empty.
   - HP gas tanks are at pressure (~ 70-100 bar?) and are sized for worse case RCS maneuver sequence or "soft
      landing" HP liquid tanks pressurization, what ever is greater.

Preparation for Lunar landing assuming HP liquid tanks are empty:
   - liquid propellants flow from the header tanks into the HP "soft landing" tanks.
   - HP liquid tanks are sealed off and pressurized by HP gas tanks.
   - HP gases maintain HP liquid tank pressure during "soft landing" burn (~30-60 bar ?)

Preparation for launching Blue assumes you are going to use "soft takeoff" rockets:
   - Charge HP gas using header tank liquids pumped into HP gas tanks and heated to obtain to the desired pressure
      and temperature.
   - liquid propellants flow from the header tanks into the HP "soft takeoff" tanks.
   - HP liquid tanks are sealed off and pressurized by HP gas tanks.
   - HP gases maintain HP liquid tanks pressure during "soft takeoff" burn (~30-60 bar ?)

John

Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 05/03/2020 07:04 pm
Sorry John I wasn't clear...
As understand it
Liquid is pumped from the header tanks into a high pressure gas tank and heated if necessary to create high pressure gas source for the RCS. 
Repeat as necessary to keep it pressurized. 
Why can't this be a continuous process? 
Do we need two sets of tanks to alternate between?  Use one and fill the other then switch?

- It could be a continuous process. Too keep up with the needs of the soft landing rockets you need large electric pumps with thousands of hp. Also there is no need to gasify the propellants, that just takes more power.

- If you don't need to keep up, you can spread the pumping over a longer period of them and reduce the pump size and power by an order of magnitude. Its all about optimizing the mass of the system while maintaining needed functionality.


- Duh, still didn't answer your question. To keep up with a worse case RCS sequence (say a 180 degree in 20s turn) you would need ~400-800 hp, still a lot of power. If you spread the recharge over 200s you only need 40-80 hp pumps. Much less mass and power required.

John
Title: Re: Starship Methox RCS Thrusters
Post by: steveleach on 05/03/2020 07:15 pm
- I think so. You would also need lines and electric pumps from the header tanks to the HP gas tanks to recharge them after a sequence of maneuvers. When the Raptors are firing they may be used for HP gas replenishment but you would also need lines and electric pumps from the header tanks to the HP gas tanks to recharge them after a sequence of RCS maneuvers when the Raptors are not running. Also, after a long stay on Mars or the Moon, you will need to recharge your HP gas in order to start the raptors.

Here is a sequence of operation:

Assumptions:
   - HP propellant tanks are sized for "soft landing" delta V and are empty.
   - HP gas tanks are at pressure (~ 70-100 bar?) and are sized for worse case RCS maneuver sequence or "soft
      landing" tank pressurization, what ever is greater.

Preparation for Lunar landing assuming HP liquid tanks are empty:
   - liquid propellants flow from the header tanks into the HP "soft landing" tanks.
   - HP liquid tanks are sealed off and pressurized by HP gas tanks.
   - HP gases maintain HP liquid tank pressure during "soft landing" burn (~30-60 bar ?)

Preparation for launching Blue assumes you are going to use "soft takeoff" rockets:
   - Charge HP gas using header tank liquids pumped into HP gas tanks and heated to obtain to the desired pressure
      and temperature.
   - liquid propellants flow from the header tanks into the HP "soft takeoff" tanks.
   - HP liquid tanks are sealed off and pressurized by HP gas tanks.
   - HP gases maintain HP liquid tanks pressure during "soft takeoff" burn (~30-60 bar ?)

John
Thanks John.

I presume pumps that can handle those pressures (70-100 bar) are practical for this kind of application? Not too heavy, or requiring too much power?

Updated diagram attached.
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 05/03/2020 07:21 pm
- I think so. You would also need lines and electric pumps from the header tanks to the HP gas tanks to recharge them after a sequence of maneuvers. When the Raptors are firing they may be used for HP gas replenishment but you would also need lines and electric pumps from the header tanks to the HP gas tanks to recharge them after a sequence of RCS maneuvers when the Raptors are not running. Also, after a long stay on Mars or the Moon, you will need to recharge your HP gas in order to start the raptors.

Here is a sequence of operation:

Assumptions:
   - HP propellant tanks are sized for "soft landing" delta V and are empty.
   - HP gas tanks are at pressure (~ 70-100 bar?) and are sized for worse case RCS maneuver sequence or "soft
      landing" tank pressurization, what ever is greater.

Preparation for Lunar landing assuming HP liquid tanks are empty:
   - liquid propellants flow from the header tanks into the HP "soft landing" tanks.
   - HP liquid tanks are sealed off and pressurized by HP gas tanks.
   - HP gases maintain HP liquid tank pressure during "soft landing" burn (~30-60 bar ?)

Preparation for launching Blue assumes you are going to use "soft takeoff" rockets:
   - Charge HP gas using header tank liquids pumped into HP gas tanks and heated to obtain to the desired pressure
      and temperature.
   - liquid propellants flow from the header tanks into the HP "soft takeoff" tanks.
   - HP liquid tanks are sealed off and pressurized by HP gas tanks.
   - HP gases maintain HP liquid tanks pressure during "soft takeoff" burn (~30-60 bar ?)

John
Thanks John.

I presume pumps that can handle those pressures (70-100 bar) are practical for this kind of application? Not too heavy, or requiring too much power?

Updated diagram attached.

Nice, I think that clarifies the basic approach. Pumps similar to the Rutherford rocket pumps would work. One or two stages.

- These are only ROM numbers but less than 100 hp.

John
Title: Re: Starship Methox RCS Thrusters
Post by: ThomasGadd on 05/03/2020 07:47 pm
Sorry John I wasn't clear...
As understand it
Liquid is pumped from the header tanks into a high pressure gas tank and heated if necessary to create high pressure gas source for the RCS. 
Repeat as necessary to keep it pressurized. 
Why can't this be a continuous process? 
Do we need two sets of tanks to alternate between?  Use one and fill the other then switch?

- It could be a continuous process. Too keep up with the needs of the soft landing rockets you need large electric pumps with thousands of hp. Also there is no need to gasify the propellants, that just takes more power.

- If you don't need to keep up, you can spread the pumping over a longer period of them and reduce the pump size and power by an order of magnitude. Its all about optimizing the mass of the system while maintaining needed functionality.

John

Thanks that make sense
You end up with a trade off of two different systems total weight verses complexity.
These landing rockets will only be used on the moon, right? 

With the existing two (redundant) pumps rotating between through empty tanks how many tanks would be needed for the lunar landing?
What would be the weight difference of this verses a new liquid engine system?
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 05/03/2020 09:37 pm
Sorry John I wasn't clear...
As understand it
Liquid is pumped from the header tanks into a high pressure gas tank and heated if necessary to create high pressure gas source for the RCS. 
Repeat as necessary to keep it pressurized. 
Why can't this be a continuous process? 
Do we need two sets of tanks to alternate between?  Use one and fill the other then switch?

- It could be a continuous process. Too keep up with the needs of the soft landing rockets you need large electric pumps with thousands of hp. Also there is no need to gasify the propellants, that just takes more power.

- If you don't need to keep up, you can spread the pumping over a longer period of them and reduce the pump size and power by an order of magnitude. Its all about optimizing the mass of the system while maintaining needed functionality.

John

Thanks that make sense
You end up with a trade off of two different systems total weight verses complexity.
These landing rockets will only be used on the moon, right? 

With the existing two (redundant) pumps rotating between through empty tanks how many tanks would be needed for the lunar landing?
What would be the weight difference of this verses a new liquid engine system?

- That's the way I understand it. Might not need them if a landing pad is available or landing on rocky surfaces.

- I'm doing those calculations now. Stand by.

John
Title: Re: Starship Methox RCS Thrusters
Post by: warp99 on 05/03/2020 11:33 pm
- I think so. You would also need lines and electric pumps from the header tanks to the HP gas tanks to recharge them after a sequence of maneuvers. When the Raptors are firing they may be used for HP gas replenishment but you would also need lines and electric pumps from the header tanks to the HP gas tanks to recharge them after a sequence of RCS maneuvers when the Raptors are not running. Also, after a long stay on Mars or the Moon, you will need to recharge your HP gas in order to start the raptors.

Here is a sequence of operation:

Assumptions:
   - HP propellant tanks are sized for "soft landing" delta V and are empty.
   - HP gas tanks are at pressure (~ 70-100 bar?) and are sized for worse case RCS maneuver sequence or "soft
      landing" tank pressurization, what ever is greater.

Preparation for Lunar landing assuming HP liquid tanks are empty:
   - liquid propellants flow from the header tanks into the HP "soft landing" tanks.
   - HP liquid tanks are sealed off and pressurized by HP gas tanks.
   - HP gases maintain HP liquid tank pressure during "soft landing" burn (~30-60 bar ?)

Preparation for launching Blue assumes you are going to use "soft takeoff" rockets:
   - Charge HP gas using header tank liquids pumped into HP gas tanks and heated to obtain to the desired pressure
      and temperature.
   - liquid propellants flow from the header tanks into the HP "soft takeoff" tanks.
   - HP liquid tanks are sealed off and pressurized by HP gas tanks.
   - HP gases maintain HP liquid tanks pressure during "soft takeoff" burn (~30-60 bar ?)

John
Thanks John.

I presume pumps that can handle those pressures (70-100 bar) are practical for this kind of application? Not too heavy, or requiring too much power?

Updated diagram attached.
Great diagram.  However I think there will not be any header tanks. 

Header tanks sized for Earth landing contain 30 tonnes of propellant and are required to minimise slosh issues during the flip before landing and to balance the mass distribution during entry.

Header tanks sized for Mars landing contain 66 tonnes of propellant on my figures and are needed for slosh prevention, mass distribution and to provide insulated storage during the trip to Mars. 

Header tanks sized for a Moon landing would need to contain several hundred tonnes of landing propellant and a slightly smaller amount for takeoff.  They would have total capacity of around one third the total propellant mass and are not needed for mass distribution or slosh prevention with tail first braking and landing.  They are called the main tanks at this point and there is no need for additional tankage. 

Incidentally the main tanks probably will be externally insulated with MLI before being painted to reduce heat gain during transit to the Moon and loitering waiting for Orion.  This will also serve to reduce boiloff of propellant during the surface stay on the Moon.
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 05/04/2020 12:07 am
- I think so. You would also need lines and electric pumps from the header tanks to the HP gas tanks to recharge them after a sequence of maneuvers. When the Raptors are firing they may be used for HP gas replenishment but you would also need lines and electric pumps from the header tanks to the HP gas tanks to recharge them after a sequence of RCS maneuvers when the Raptors are not running. Also, after a long stay on Mars or the Moon, you will need to recharge your HP gas in order to start the raptors.

Here is a sequence of operation:

Assumptions:
   - HP propellant tanks are sized for "soft landing" delta V and are empty.
   - HP gas tanks are at pressure (~ 70-100 bar?) and are sized for worse case RCS maneuver sequence or "soft
      landing" tank pressurization, what ever is greater.

Preparation for Lunar landing assuming HP liquid tanks are empty:
   - liquid propellants flow from the header tanks into the HP "soft landing" tanks.
   - HP liquid tanks are sealed off and pressurized by HP gas tanks.
   - HP gases maintain HP liquid tank pressure during "soft landing" burn (~30-60 bar ?)

Preparation for launching Blue assumes you are going to use "soft takeoff" rockets:
   - Charge HP gas using header tank liquids pumped into HP gas tanks and heated to obtain to the desired pressure
      and temperature.
   - liquid propellants flow from the header tanks into the HP "soft takeoff" tanks.
   - HP liquid tanks are sealed off and pressurized by HP gas tanks.
   - HP gases maintain HP liquid tanks pressure during "soft takeoff" burn (~30-60 bar ?)

John
Thanks John.

I presume pumps that can handle those pressures (70-100 bar) are practical for this kind of application? Not too heavy, or requiring too much power?

Updated diagram attached.
Great diagram.  However I think there will not be any header tanks. 

Header tanks sized for Earth landing contain 30 tonnes of propellant and are required to minimise slosh issues during the flip before landing and to balance the mass distribution during entry.

Header tanks sized for Mars landing contain 66 tonnes of propellant on my figures and are needed for slosh prevention, mass distribution and to provide insulated storage during the trip to Mars. 

Header tanks sized for a Moon landing would need to contain several hundred tonnes of landing propellant and a slightly smaller amount for takeoff.  They would have total capacity of around one third the total propellant mass and are not needed for mass distribution or slosh prevention with tail first braking and landing.  They are called the main tanks at this point and there is no need for additional tankage. 

Incidentally the main tanks probably will be externally insulated with MLI before being painted to reduce heat gain during transit to the Moon and loitering waiting for Orion.  This will also serve to reduce boiloff of propellant during the surface stay on the Moon.

Doesn't absolutely have to have header tanks, as long as Moonship has some form of capillary liquid collection system in the main tanks in order to feed electric pumps.

John
Title: Re: Starship Methox RCS Thrusters
Post by: CorvusCorax on 05/10/2020 12:03 am
So Long story short we assume Moonship will have 4 engine types:

1. 3x Sea level raptor (SLR) (3)
2. 3x Vacuum Raptor (VR)
3. 9x Yet unnamed Liquid Methalox pressure fed engine for landing (Kestrel on Meth ?) (KM)
4. ?x Gasseous Methane-Oxygen RCS thrusters (RCS)

The propellant system would be symmetric, with each system duplicated for both (liquid/gasseous) Oxygen and (liquid/gasseous) Methane

As such there'd be (at least) 3 tank pairs:
1. High pressure gaseous propellant tank  (HPG(x)T) (where x is O or M)
2. High pressure liquid propellant tank (HPL(x)T)
3. Low pressure (main) liquid propellant tank (M(x)T)

Although for redundancy and weight distribution it would probably make sense to have the high pressure tanks in triplets, each feeding and getting fed their own subset of RCS and main engines

HPG(x)Ts have 3 inlets:

1. Gas feeds from Raptor preburners (both SL and Vac) (high volume, high pressure feed needed during raptor operation)
2. Pump feeds from main tank through evaporation heater (low volume, high pressure feed to re-pressure after long coast/sitting on the moon)
3. Valve controlled crossfeed between the 3 systems (redundancy)

HPG(x)Ts have 4 outlets:

1. Pressurization for main tank
2. Pressurization for HPL(x)Ts
3. RCS thrusters
4. Crossfeed between systems

HPL(x)Ts have 2 inlets:

1. Liquid propellant feed from main tank (possibly through propellant tank)
2. Pressurization from HPG(x)Ts

HPL(x)Ts have 2 outlets:

1. Kestrel on Meth (KM) landing engines
2. Drain to main tank

M(x)Ts have 3 inlets:

1. Drain from HPL(x)Ts
2. Propellant transfer system feed (might double as GST for pad fueling)
3. Pressurization from HPG(x)Ts

M(x)Ts have 4 outlets:

1. Propellant transfer system drain (might double for circularisation for pad GST)
2. Pumped feed to HPL(x)Ts
3. Supply for Sea level and Vacuum Raptors
4. outboard vent (to get rid of surplus ulage pressure due to boiloff)

Ideally you'd have a temperature control system of some sorts, for long missions, using a separate coolant loop, radiators and heat pumps.

Purpose:
1. Cool down main M(x)T to prevent boiloff
2. Heat HPG(x)T (prevent condensation)
3. Evaporate propellant to replenish HPG(x)T
4. Temperature control of mechanical and electronic components to prevent heat/cold soak (valves, computers, batteries, engines, etc...)


Would that work?
Does anyone want to make a diagram?


Edit: Shouldn't forget, both Rapor and Kestrel on Meth would need high pressure gas feeds for ignitors
Raptors might also need high pressure gas for startup/spoolup (unless they use helium)
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 05/10/2020 03:51 am
Good start. Did you remember HPG lines for turbine spin up? I'm working on some diagrams and calculations. Be a couple of days yet.

John
Title: Re: Starship Methox RCS Thrusters
Post by: DusanC on 05/10/2020 05:52 am
So Long story short we assume Moonship will have 4 engine types:

1. 3x Sea level raptor (SLR) (3)
2. 3x Vacuum Raptor (VR)
3. 9x Yet unnamed Liquid Methalox pressure fed engine for landing (Kestrel on Meth ?) (KM)
4. ?x Gasseous Methane-Oxygen RCS thrusters (RCS)

The propellant system would be symmetric, with each system duplicated for both (liquid/gasseous) Oxygen and (liquid/gasseous) Methane

As such there'd be (at least) 3 tank pairs:
1. High pressure gaseous propellant tank  (HPG(x)T) (where x is O or M)
2. High pressure liquid propellant tank (HPL(x)T)
3. Low pressure (main) liquid propellant tank (M(x)T)

Although for redundancy and weight distribution it would probably make sense to have the high pressure tanks in triplets, each feeding and getting fed their own subset of RCS and main engines

HPG(x)Ts have 3 inlets:

1. Gas feeds from Raptor preburners (both SL and Vac) (high volume, high pressure feed needed during raptor operation)
2. Pump feeds from main tank through evaporation heater (low volume, high pressure feed to re-pressure after long coast/sitting on the moon)
3. Valve controlled crossfeed between the 3 systems (redundancy)

HPG(x)Ts have 4 outlets:

1. Pressurization for main tank
2. Pressurization for HPL(x)Ts
3. RCS thrusters
4. Crossfeed between systems

HPL(x)Ts have 2 inlets:

1. Liquid propellant feed from main tank (possibly through propellant tank)
2. Pressurization from HPG(x)Ts

HPL(x)Ts have 2 outlets:

1. Kestrel on Meth (KM) landing engines
2. Drain to main tank

M(x)Ts have 3 inlets:

1. Drain from HPL(x)Ts
2. Propellant transfer system feed (might double as GST for pad fueling)
3. Pressurization from HPG(x)Ts

M(x)Ts have 4 outlets:

1. Propellant transfer system drain (might double for circularisation for pad GST)
2. Pumped feed to HPL(x)Ts
3. Supply for Sea level and Vacuum Raptors
4. outboard vent (to get rid of surplus ulage pressure due to boiloff)

Ideally you'd have a temperature control system of some sorts, for long missions, using a separate coolant loop, radiators and heat pumps.

Purpose:
1. Cool down main M(x)T to prevent boiloff
2. Heat HPG(x)T (prevent condensation)
3. Evaporate propellant to replenish HPG(x)T
4. Temperature control of mechanical and electronic components to prevent heat/cold soak (valves, computers, batteries, engines, etc...)


Would that work?
Does anyone want to make a diagram?


Edit: Shouldn't forget, both Rapor and Kestrel on Meth would need high pressure gas feeds for ignitors
Raptors might also need high pressure gas for startup/spoolup (unless they use helium)
Before I make a schematic, why wouldn't HPLT with heater/evaporator on the line after it subtitute multiple HPGT?

Edit: Made a new topic just for tanks and pipelines discussion https://forum.nasaspaceflight.com/index.php?topic=50897.0
Title: Re: Starship Methox RCS Thrusters
Post by: kkattula on 05/10/2020 08:06 am
At a rough estimate, the landing engines might need to hover for 20 seconds in one lunar gravity, Maybe less if they know exactly where they're going to touchdown and just turn on briefly to cushion the fall.  Maybe more if they need time to find the best spot, or correct for main engine descent positional errors. (They won't have GPS on the Moon, at least initially.) Also more if they want to take off using the landing engines, but the tanks can be topped up again from the main tanks for that.

So assume 30 m/s delta v:

For a (maximal) 120 t dry SS, with 150 t cargo and 180 t return propellant, that's 450 t landing mass.

At 320 Isp, that's about 4.4 t of liquid landing propellant.  At an average density of 0.8 that's about 5.5 m3 tank volume.

For pressure fed engines, that's a lot of high pressure gas needed to keep the pressure up around 30 to 40 bar, until the tanks are almost empty.


Whereas, if say expander cycle engines were used, the landing tanks might only need to be pressurized to 3 or 4 bar.  Besides using a little less propellant due to higher Isp.

The engines could even be fed directly from the main tanks, which would be still pressurized from the Raptor autogenous feed.

Or just tap the main tank pressurant gasses. The pressure in the huge main tanks would barely drop for that amount, either way.
Title: Re: Starship Methox RCS Thrusters
Post by: rakaydos on 05/10/2020 09:34 am
At a rough estimate, the landing engines might need to hover for 20 seconds in one lunar gravity, Maybe less if they know exactly where they're going to touchdown and just turn on briefly to cushion the fall.  Maybe more if they need time to find the best spot, or correct for main engine descent positional errors. (They won't have GPS on the Moon, at least initially.) Also more if they want to take off using the landing engines, but the tanks can be topped up again from the main tanks for that.

So assume 30 m/s delta v:

For a (maximal) 120 t dry SS, with 150 t cargo and 180 t return propellant, that's 450 t landing mass.

At 320 Isp, that's about 4.4 t of liquid landing propellant.  At an average density of 0.8 that's about 5.5 m3 tank volume.

For pressure fed engines, that's a lot of high pressure gas needed to keep the pressure up around 30 to 40 bar, until the tanks are almost empty.


Whereas, if say expander cycle engines were used, the landing tanks might only need to be pressurized to 3 or 4 bar.  Besides using a little less propellant due to higher Isp.

The engines could even be fed directly from the main tanks, which would be still pressurized from the Raptor autogenous feed.

Or just tap the main tank pressurant gasses. The pressure in the huge main tanks would barely drop for that amount, either way.
Don't be over complicating the landing thruster design - it just needs to soften the impact to a survivable litho break, and advanced rocket engines take longer to develop. The schedules long, the part is wrong.
Title: Re: Starship Methox RCS Thrusters
Post by: kkattula on 05/10/2020 01:30 pm
Don't be over complicating the landing thruster design - it just needs to soften the impact to a survivable litho break, and advanced rocket engines take longer to develop. The schedules long, the part is wrong.

The schedule would only be long IF they were starting from scratch recently.  We have no real idea what they might have been playing with on that scale over the last few years. Plus there are a few existing, working, LOX\LCH4 engines, at that size, out there that they could buy or licence. 

100 kN is near the sweet spot for expanders. The turbines run at around room temperature, on cryo fuel vaporized by regen cooling of the chamber and nozzle. Not a harsh regime.

It could be argued that the extra high pressure tanks, lines, and recharging requirements of pressure fed engines are the real unnecessary complications. Compared to low-temp turbo-pumps on engines, that probably already have regen cooling, and low pressure propellant lines from existing tanks.
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 05/10/2020 02:35 pm
At a rough estimate, the landing engines might need to hover for 20 seconds in one lunar gravity, Maybe less if they know exactly where they're going to touchdown and just turn on briefly to cushion the fall.  Maybe more if they need time to find the best spot, or correct for main engine descent positional errors. (They won't have GPS on the Moon, at least initially.) Also more if they want to take off using the landing engines, but the tanks can be topped up again from the main tanks for that.

So assume 30 m/s delta v:

For a (maximal) 120 t dry SS, with 150 t cargo and 180 t return propellant, that's 450 t landing mass.

At 320 Isp, that's about 4.4 t of liquid landing propellant.  At an average density of 0.8 that's about 5.5 m3 tank volume.

For pressure fed engines, that's a lot of high pressure gas needed to keep the pressure up around 30 to 40 bar, until the tanks are almost empty.


Whereas, if say expander cycle engines were used, the landing tanks might only need to be pressurized to 3 or 4 bar.  Besides using a little less propellant due to higher Isp.

The engines could even be fed directly from the main tanks, which would be still pressurized from the Raptor autogenous feed.

Or just tap the main tank pressurant gasses. The pressure in the huge main tanks would barely drop for that amount, either way.

Definitely worth looking at.

John
Title: Re: Starship Methox RCS Thrusters
Post by: ThomasGadd on 05/10/2020 02:49 pm
Don't be over complicating the landing thruster design - it just needs to soften the impact to a survivable litho break, and advanced rocket engines take longer to develop. The schedules long, the part is wrong.

The schedule would only be long IF they were starting from scratch recently.  We have no real idea what they might have been playing with on that scale over the last few years. Plus there are a few existing, working, LOX\LCH4 engines, at that size, out there that they could buy or licence. 

100 kN is near the sweet spot for expanders. The turbines run at around room temperature, on cryo fuel vaporized by regen cooling of the chamber and nozzle. Not a harsh regime.

It could be argued that the extra high pressure tanks, lines, and recharging requirements of pressure fed engines are the real unnecessary complications. Compared to low-temp turbo-pumps on engines, that probably already have regen cooling, and low pressure propellant lines from existing tanks.

RCS systems are in all SS. 

Is it worth having a few verses many high pressure tanks that you are rotating through topping off? 

The existing RCS will have existing redundant, small pumps to keep the high pressure tanks topped up. 
Does it make sense to use larger pumps during landing?  Or just larger tanks? 

A trade off is the the total system weight verses complexity. 

Are the landing engines used elsewhere?
Would these new landing engines be capable of wind sheer countering during EDL? 
Title: Re: Starship Methox RCS Thrusters
Post by: LMT on 05/10/2020 09:34 pm
At a rough estimate, the landing engines might need to hover for 20 seconds in one lunar gravity...

...if say expander cycle engines were used, the landing tanks might only need to be pressurized to 3 or 4 bar. 

100 kN is near the sweet spot for expanders. The turbines run at around room temperature, on cryo fuel vaporized by regen cooling of the chamber and nozzle.

That's a fuel-only, not a dual, expander cycle?

What's the envisioned heat exchanger function at the start of soft-landing descent?
Title: Re: Starship Methox RCS Thrusters
Post by: kkattula on 05/11/2020 12:26 am
At a rough estimate, the landing engines might need to hover for 20 seconds in one lunar gravity...

...if say expander cycle engines were used, the landing tanks might only need to be pressurized to 3 or 4 bar. 

100 kN is near the sweet spot for expanders. The turbines run at around room temperature, on cryo fuel vaporized by regen cooling of the chamber and nozzle.

That's a fuel-only, not a dual, expander cycle?

What's the envisioned heat exchanger function at the start of soft-landing descent?

Fuel only probably. As pointed out, complexity should be kept to a minimum and ultra-efficiency is not needed for these engines.

Typically expanders use ambient temperature of the metal chamber and nozzle for the initial heating of the cryo fluid, then gradually spool up, over a second or two, to full working temperature. 

This link (https://space.stackexchange.com/questions/39536/how-do-expander-cycle-engines-inject-liquid-fuel-during-startup) explains how the RL-10 handles it. Note: an RL-10 has been experimentally run on CH4.

If between uses the landing engines are in a spot that gets too cold, there are simple mitigation options. e.g. Running electrical heaters or torch igniters to pre-warm the engines, before use.

There's no doubt these engines would be more complex than pressure fed, but eliminating high pressure liquid cryo tanks, nine sets of high pressure feed lines, and all the rest, may well be worth it. If something breaks, you've probably just lost one of nine landing engines, not the whole system.
Title: Re: Starship Methox RCS Thrusters
Post by: Norm38 on 05/11/2020 02:12 am
So a system is still needed to press the main tanks, but now that’s a 10 bar pump, not a 100 bar pump?  And no accumulator tanks?
Or vastly smaller tanks?  What do the maneuvering RCS need?

Edit:  From the tanks schematics thread, there are 70 bar tanks for the maneuvering RCS thrusters.  But those would have to get much larger to power the landing engines.  So that's the tradeoff to expander complexity.
Title: Re: Starship Methox RCS Thrusters
Post by: CorvusCorax on 05/11/2020 03:03 am
At a rough estimate, the landing engines might need to hover for 20 seconds in one lunar gravity...

...if say expander cycle engines were used, the landing tanks might only need to be pressurized to 3 or 4 bar. 

100 kN is near the sweet spot for expanders. The turbines run at around room temperature, on cryo fuel vaporized by regen cooling of the chamber and nozzle.

That's a fuel-only, not a dual, expander cycle?

What's the envisioned heat exchanger function at the start of soft-landing descent?

Fuel only probably. As pointed out, complexity should be kept to a minimum and ultra-efficiency is not needed for these engines.

Typically expanders use ambient temperature of the metal chamber and nozzle for the initial heating of the cryo fluid, then gradually spool up, over a second or two, to full working temperature. 

This link (https://space.stackexchange.com/questions/39536/how-do-expander-cycle-engines-inject-liquid-fuel-during-startup) explains how the RL-10 handles it. Note: an RL-10 has been experimentally run on CH4.

If between uses the landing engines are in a spot that gets too cold, there are simple mitigation options. e.g. Running electrical heaters or torch igniters to pre-warm the engines, before use.

There's no doubt these engines would be more complex than pressure fed, but eliminating high pressure liquid cryo tanks, nine sets of high pressure feed lines, and all the rest, may well be worth it. If something breaks, you've probably just lost one of nine landing engines, not the whole system.

You just voiced a very important reason why an expander cycle engine would probably be a bad idea for the landing. It's spool-up is way too slow. For final approach and touchdown you need an engine that can be deep throttled, is high thrust and react to throttle changes ultra fast - both up and down.

An expander cycle engine can be deep throttled, but its neither high thrust nor does it react fast, especially when its cold. (It can also have quite horrible transients when thrust is reduced due to the expanding residual propellant in the cooling channels.)
Title: Re: Starship Methox RCS Thrusters
Post by: kkattula on 05/11/2020 06:04 am
At a rough estimate, the landing engines might need to hover for 20 seconds in one lunar gravity...

...if say expander cycle engines were used, the landing tanks might only need to be pressurized to 3 or 4 bar. 

100 kN is near the sweet spot for expanders. The turbines run at around room temperature, on cryo fuel vaporized by regen cooling of the chamber and nozzle.

That's a fuel-only, not a dual, expander cycle?

What's the envisioned heat exchanger function at the start of soft-landing descent?

Fuel only probably. As pointed out, complexity should be kept to a minimum and ultra-efficiency is not needed for these engines.

Typically expanders use ambient temperature of the metal chamber and nozzle for the initial heating of the cryo fluid, then gradually spool up, over a second or two, to full working temperature. 

This link (https://space.stackexchange.com/questions/39536/how-do-expander-cycle-engines-inject-liquid-fuel-during-startup) explains how the RL-10 handles it. Note: an RL-10 has been experimentally run on CH4.

If between uses the landing engines are in a spot that gets too cold, there are simple mitigation options. e.g. Running electrical heaters or torch igniters to pre-warm the engines, before use.

There's no doubt these engines would be more complex than pressure fed, but eliminating high pressure liquid cryo tanks, nine sets of high pressure feed lines, and all the rest, may well be worth it. If something breaks, you've probably just lost one of nine landing engines, not the whole system.

You just voiced a very important reason why an expander cycle engine would probably be a bad idea for the landing. It's spool-up is way too slow. For final approach and touchdown you need an engine that can be deep throttled, is high thrust and react to throttle changes ultra fast - both up and down.

An expander cycle engine can be deep throttled, but its neither high thrust nor does it react fast, especially when its cold. (It can also have quite horrible transients when thrust is reduced due to the expanding residual propellant in the cooling channels.)

DC-X ?

For a lunar landing, there is plenty of time, even after MECO, for the landing engines to spool up.
Title: Re: Starship Methox RCS Thrusters
Post by: DusanC on 05/11/2020 06:12 am
There's some very interesting engine talks here but I'm getting pretty confused with parallel discussion about RCS and Moon landing engines in the RCS thrusters thread.

Could the Methoxs Moon landing engines have their dedicated thread?
Title: Re: Starship Methox RCS Thrusters
Post by: Norm38 on 05/11/2020 12:54 pm
^^^  But it's all one system though.  70 bar HP tanks to power the maneuvering RCS and also press the main tanks.  Those 70 bar tanks can be much larger to power the landing engines.  Or be much smaller if the landing engines run off the main tanks at 6 bar.  If we're discussing tradeoffs, one thread is good for that.
Title: Re: Starship Methox RCS Thrusters
Post by: spacexfanatic on 05/11/2020 12:59 pm
What about an escape tower that could be used for landing/orbiting moon, Starship could ignite her engine after reaching a security distance as is the case for cold launch missiles.
Title: Re: Starship Methox RCS Thrusters
Post by: DusanC on 05/11/2020 01:34 pm
^^^  But it's all one system though.  70 bar HP tanks to power the maneuvering RCS and also press the main tanks.  Those 70 bar tanks can be much larger to power the landing engines.  Or be much smaller if the landing engines run off the main tanks at 6 bar.  If we're discussing tradeoffs, one thread is good for that.
70 bar tanks for pressurizing the main tanks would be large and heavy. Simple CH4 burner would pressure the main tanks much easier if needed.

Dunno, mixing engine talk with tankage talk in RCS engine talk is counterintuitive to me.
Title: Re: Starship Methox RCS Thrusters
Post by: drzerg on 05/12/2020 01:33 pm
From where do we know that there will be pressure fed moon landing engines? Why not small open cycle or electric pump fed? Best tank is no tank.
Title: Re: Starship Methox RCS Thrusters
Post by: kkattula on 05/13/2020 01:37 am
From where do we know that there will be pressure fed moon landing engines? Why not small open cycle or electric pump fed? Best tank is no tank.

We don't. Hence all the speculation, and how it affects the presumably pressure fed RCS system.

Recent speculation on (I think) the prototypes thread, was that the pump powered by a Tesla motor might be for providing high pressure gas for RCS, without needing large storage tanks, just small accumulators.

This wouldn't be nearly large enough for landing engines, and if they were doing electric pumping, liquid would be more logical than gas, given the landing propellant volumes.


IMHO, if the pump and motor are for pressurizing gas, it's to refill large COPV tanks for RCS and other uses. I expect the landing engines to use a fairly basic, expander or electric pump cycle.
Title: Re: Starship Methox RCS Thrusters
Post by: Johnnyhinbos on 05/13/2020 04:05 am
From where do we know that there will be pressure fed moon landing engines? Why not small open cycle or electric pump fed? Best tank is no tank.

We don't. Hence all the speculation, and how it affects the presumably pressure fed RCS system.

Recent speculation on (I think) the prototypes thread, was that the pump powered by a Tesla motor might be for providing high pressure gas for RCS, without needing large storage tanks, just small accumulators.

This wouldn't be nearly large enough for landing engines, and if they were doing electric pumping, liquid would be more logical than gas, given the landing propellant volumes.


IMHO, if the pump and motor are for pressurizing gas, it's to refill large COPV tanks for RCS and other uses. I expect the landing engines to use a fairly basic, expander or electric pump cycle.
The tesla motor operates a hydraulic pump. Hydraulic fluid. So if you want to put it in a RCS system you’ll have to explain how...
Title: Re: Starship Methox RCS Thrusters
Post by: ThomasGadd on 05/13/2020 12:41 pm
From where do we know that there will be pressure fed moon landing engines? Why not small open cycle or electric pump fed? Best tank is no tank.

We don't. Hence all the speculation, and how it affects the presumably pressure fed RCS system.

Recent speculation on (I think) the prototypes thread, was that the pump powered by a Tesla motor might be for providing high pressure gas for RCS, without needing large storage tanks, just small accumulators.

This wouldn't be nearly large enough for landing engines, and if they were doing electric pumping, liquid would be more logical than gas, given the landing propellant volumes.


IMHO, if the pump and motor are for pressurizing gas, it's to refill large COPV tanks for RCS and other uses. I expect the landing engines to use a fairly basic, expander or electric pump cycle.

The existing RCS has two edge cases:
 in space operations small, precise thrust
 cross wind landing large, precise thrust
 the tanks are sized for landing operations using small efficient liquid pumps. 
 
With the new lunar landing (and take off):
 why not use the existing EDL RCS system
 the tanks are sized for space  operations using small efficient liquid pumps
 landing operations use larger efficient liquid pumps feeding the common tank system. 
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 05/13/2020 02:01 pm
From where do we know that there will be pressure fed moon landing engines? Why not small open cycle or electric pump fed? Best tank is no tank.

We don't. Hence all the speculation, and how it affects the presumably pressure fed RCS system.

Recent speculation on (I think) the prototypes thread, was that the pump powered by a Tesla motor might be for providing high pressure gas for RCS, without needing large storage tanks, just small accumulators.

This wouldn't be nearly large enough for landing engines, and if they were doing electric pumping, liquid would be more logical than gas, given the landing propellant volumes.


IMHO, if the pump and motor are for pressurizing gas, it's to refill large COPV tanks for RCS and other uses. I expect the landing engines to use a fairly basic, expander or electric pump cycle.

The existing RCS has two edge cases:
 in space operations small, precise thrust
 cross wind landing large, precise thrust
 the tanks are sized for landing operations using small efficient liquid pumps. 
 
With the new lunar landing (and take off):
 why not use the existing EDL RCS system
 the tanks are sized for space  operations using small efficient liquid pumps
 landing operations use larger efficient liquid pumps feeding the common tank system.

- There is no indication that the RCS uses liquid rockets. Indications are the are gaseous fed.

- RCS rockets could be sized for EDL, we don't know, but there is no indication that they are liquid or pump fed. Indications are that they are gaseous, pressure fed.

John
Title: Re: Starship Methox RCS Thrusters
Post by: ThomasGadd on 05/13/2020 02:11 pm
From where do we know that there will be pressure fed moon landing engines? Why not small open cycle or electric pump fed? Best tank is no tank.

We don't. Hence all the speculation, and how it affects the presumably pressure fed RCS system.

Recent speculation on (I think) the prototypes thread, was that the pump powered by a Tesla motor might be for providing high pressure gas for RCS, without needing large storage tanks, just small accumulators.

This wouldn't be nearly large enough for landing engines, and if they were doing electric pumping, liquid would be more logical than gas, given the landing propellant volumes.


IMHO, if the pump and motor are for pressurizing gas, it's to refill large COPV tanks for RCS and other uses. I expect the landing engines to use a fairly basic, expander or electric pump cycle.

The existing RCS has two edge cases:
 in space operations small, precise thrust
 cross wind landing large, precise thrust
 the tanks are sized for landing operations using small efficient liquid pumps. 
 
With the new lunar landing (and take off):
 why not use the existing EDL RCS system
 the tanks are sized for space  operations using small efficient liquid pumps
 landing operations use larger efficient liquid pumps feeding the common tank system.

- There is no indication that the RCS uses liquid rockets. Indications are the are gaseous fed.

- RCS rockets could be sized for EDL, we don't know, but there is no indication that they are liquid or pump fed. Indications are that they are gaseous, pressure fed.

John

Sorry I wasn't clear, liquid pumps refill the gaseous high pressure tanks.
Title: Re: Starship Methox RCS Thrusters
Post by: Johnnyhinbos on 05/13/2020 02:20 pm
I will note that one RCS pod on Hopper has been removed (each RCS location around Hopper was comprised of two RCS pods from F9 and only the left / right thrusters were plumbed for each pod).
Title: Re: Starship Methox RCS Thrusters
Post by: ThomasGadd on 05/16/2020 06:31 pm
The planned RCS thruster is optimized for sea level operation. 
What would a vacuum optimized version look like? 
Title: Re: Starship Methox RCS Thrusters
Post by: steveleach on 05/16/2020 07:51 pm
The planned RCS thruster is optimized for sea level operation. 
What would a vacuum optimized version look like?
Huh? Why would you optimise RCS for sea level?

Seriously confusd  ???
Title: Re: Starship Methox RCS Thrusters
Post by: _MECO on 05/16/2020 08:22 pm
I will note that one RCS pod on Hopper has been removed (each RCS location around Hopper was comprised of two RCS pods from F9 and only the left / right thrusters were plumbed for each pod).

If you're making that observation from seeing that Hopper itself is missing one, you should know that Musk presented Yusaku Maezawa one of the RCS pods as a gift.

https://mobile.twitter.com/thesheetztweetz/status/1196774568200941568
Title: Re: Starship Methox RCS Thrusters
Post by: tbellman on 05/16/2020 08:56 pm
Huh? Why would you optimise RCS for sea level?

Seriously confusd  ???

Because the high thrust is needed when Starship is landing (especially in high winds).  On-orbit maneuvering will probably only need a few kilonewtons of thrust.

Also, one of the big things optimizing for sea-level, is having a smaller nozzle.  A vacuum-optimized nozzle can lead to flow separation when fired in a dense atmosphere, which can damage the nozzle, and I suspect it can also lead to unpredictable thrust vectors.  So if you want to fire the RCS thrusters at sea-level, they must not be vacuum-optimized.  Firing an engine optimized for sea-level in vacuum, on the other hand, only wastes some performance.

It might be, though, that thrusters in the lateral and longitudinal directions can be optimized differently.  Longitudinal thrusters (i.e. forward and backward motion) are probably only needed on-orbit, and so can be optimized for vacuum.  They are also the ones that need to fire when doing propellant transfer, which may be when most thrust is needed, and for a fairly long time, at that.  Lateral (sideways) thrusters on the other hand, need to be used during landing, and needs to be optimized for sea-level.  On-orbit, I suspect you will only need small amounts of lateral impulse, so it would be more acceptable for them to be inefficient in vacuum.
Title: Re: Starship Methox RCS Thrusters
Post by: steveleach on 05/16/2020 09:18 pm
Huh? Why would you optimise RCS for sea level?

Seriously confusd  ???
A vacuum-optimized nozzle can lead to flow separation when fired in a dense atmosphere, which can damage the nozzle, and I suspect it can also lead to unpredictable thrust vectors.  So if you want to fire the RCS thrusters at sea-level, they must not be vacuum-optimized.  Firing an engine optimized for sea-level in vacuum, on the other hand, only wastes some performance.
Ah, yes. That makes sense.

Thanks.
Title: Re: Starship Methox RCS Thrusters
Post by: Johnnyhinbos on 05/16/2020 10:11 pm
I will note that one RCS pod on Hopper has been removed (each RCS location around Hopper was comprised of two RCS pods from F9 and only the left / right thrusters were plumbed for each pod).

If you're making that observation from seeing that Hopper itself is missing one, you should know that Musk presented Yusaku Maezawa one of the RCS pods as a gift.

https://mobile.twitter.com/thesheetztweetz/status/1196774568200941568
Huh - yup!
Title: Re: Starship Methox RCS Thrusters
Post by: rsdavis9 on 05/17/2020 12:34 pm
Huh? Why would you optimise RCS for sea level?

Seriously confusd  ???
A vacuum-optimized nozzle can lead to flow separation when fired in a dense atmosphere, which can damage the nozzle, and I suspect it can also lead to unpredictable thrust vectors.  So if you want to fire the RCS thrusters at sea-level, they must not be vacuum-optimized.  Firing an engine optimized for sea-level in vacuum, on the other hand, only wastes some performance.
Ah, yes. That makes sense.

Thanks.

When they do the flip they might want to use a little thrust to settle propellant aftwards before the main engines start up? Or/and they could use the fins to have an angle of attack to the rear. So maybe some longitudinal larger thrust RCS?
Title: Re: Starship Methox RCS Thrusters
Post by: niwax on 05/18/2020 12:59 pm
Question regarding the RCS thrusters: Raptor already uses ignition torches in the preburners, which are like small combustion chambers themselves. Could a methalox thruster be based on a repurposed Raptor preburner?
Title: Re: Starship Methox RCS Thrusters
Post by: Keldor on 05/18/2020 01:58 pm
Question regarding the RCS thrusters: Raptor already uses ignition torches in the preburners, which are like small combustion chambers themselves. Could a methalox thruster be based on a repurposed Raptor preburner?

This sounds akin to taking the starter motor out of a truck, attaching it to your bike and calling it an electric scooter.  It might be physically possible, but it's hard to imagine that it would be the best option by any metric.
Title: Re: Starship Methox RCS Thrusters
Post by: DistantTemple on 05/18/2020 07:44 pm
Question regarding the RCS thrusters: Raptor already uses ignition torches in the preburners, which are like small combustion chambers themselves. Could a methalox thruster be based on a repurposed Raptor preburner?

This sounds akin to taking the starter motor out of a truck, attaching it to your bike and calling it an electric scooter.  It might be physically possible, but it's hard to imagine that it would be the best option by any metric.
I'm NO expert, but the preburners are physically extremely integrated into the raptor engine. Not something that could be taken out. ALso AIUI each depends on the environment the other creates. Also each burns a massively ASYMMETRIC mixture, so the overwhelming majority of the gaseous output of each is either unused oxygen, or unburnt methane. So they are totally unsuited to work individually as rocket engines. Then their mass flow is  jointly  approaching one tonne (1/2?) per second.... which is totally inappropriate for a thruster!
So my  amateur opinion is absolutely and categorically, "hell no that's totally impossible!" and if you then think about redesigning from that point .... well they would be completely different and unrecognisable, and it would be harder than starting from scratch!
Title: Re: Starship Methox RCS Thrusters
Post by: dulises on 06/12/2020 08:43 am
I guess you already watched it, but just in case. Methalox RCS, heritage from Raptors, tests in the coming year.

https://www.youtube.com/watch?v=KEnz8V97Qck&feature=youtu.be&t=2134 (https://www.youtube.com/watch?v=KEnz8V97Qck&feature=youtu.be&t=2134)
Title: Re: Starship Methox RCS Thrusters
Post by: yg1968 on 08/09/2020 07:15 pm
For the exact quote, see below:

[...] here’s a lightly edited version of what was said [at 35 minutes of the video by Nicholas Cummings of SpaceX]:

Quote
For the terminal descent of Starship, a few tens of meters before we touchdown on the lunar surface, We actually use a high thrust RCS system so we don’t impinge on the surface of the moon with a high thrust Raptor engine. The thrusters planned have a lot of heritage in the Raptor design itself. It uses the same methane and oxygen propellants as Raptor, so there’s a lot of commonality there. Going to be ramping up some very rapid testing activities in the coming year.

https://forum.nasaspaceflight.com/index.php?topic=46645.msg2095198#msg2095198
Title: Re: Starship Methox RCS Thrusters
Post by: DigitalMan on 08/09/2020 11:00 pm
Elon previously mentioned Raptor is scalable. Perhaps it will be based on a scaled down Raptor. If so it will be interesting to see what changes are made to it.
Title: Re: Starship Methox RCS Thrusters
Post by: yg1968 on 08/19/2020 03:09 am
Quote from: Elon Musk
Question by Tim Todd: Raptors will only do majority of the work flipping on the first couple flights right? Won't the hot gas thrusters do more of the work eventually?

Answer by Elon Musk: It’s counter-intuitive, but Raptor has so much thrust at high Isp with liquid (high density) propellant & pump-fed (light tanks), that it beats hot gas for the flip. That said, hot gas beats the heck out of N2 for orbital manuevers & stabilizing ship if landing in high winds!

https://twitter.com/elonmusk/status/1295907719317204992
Title: Re: Starship Methox RCS Thrusters
Post by: eriblo on 03/16/2021 08:21 pm
Some recent discussion on the topic of "use the same 100 kN thruster for RCS and Moon landing" had me doing a bit of a comparison with the Shuttle so I decided to dust of this rather more on-topic thread hoping against all reason that some relevant discussion might find its way here ;)

The Shuttle crew operations manual (PDF) (http://390651main_shuttle_crew_operations_manual.pdf) has some nice tables of average orbiter RCS acceleration on page 9.2-3 (p. 1009 in the PDF). For a heavy payload (260 klbs) in normal orbital mode these are:

AxisLinear acceleration [ft/s2]
+X0.21
-X0.22
±Y0.16
+Z0.32*
-Z0.42
*There is also a HIGH mode that can fire all thrusters to back away from a payload at 0.94 ft/s2.

AxisAngular acceleration [deg/s2]
±Roll0.92
+Pitch1.23
-Pitch0.85
±Yaw0.70

From these numbers it looks like 0.2 ft/s2 (~6cm/s2) and 1 deg/s2 are reasonable for general maneuvering. Using a quick and dirty model for a fully fueled Starship at 1450 t with payload consisting of four stacked solid solid cylinders (Skirt: 5 m, 27 t. Oxygen: 12 m, 939t. Methane: 9 m, 261 t. Nose: 23 m, 223 t) i get a center of mass at 16.8 m (right at the methane header tank) and a moment of inertia for pitch/yaw of ~1.3e8 kgm2 with roll about 1/10 of that. Placing RCS thrusters at the "corners" (front end of forward flaps and aft end of the skirt) gives a average moment arm of ~22 m for pitch/yaw and 4.5 m for roll, so pitch/yaw is the limiting case.

1450 t * 6 cm/s2 = 90 kN
1.3e8 kgm2 * 1 deg/s2 / 22 m = 2 x 53 kN

So you can make an argument for 100 kN RCS thrusters if you want a fully loaded Starship to be able to make higher acceleration maneuvers like the back-off mentioned above (which would need four of them). For general Shuttle like maneuverability something like 25 kN thrusters in a decently redundant configuration should be enough and that is only if you insist on moving the full Starship massing 3 x the ISS. For docking for example you would rather be docking things to it - in most Starship dockings at least one of them will be <300 t for which something like 5-10 kN is more reasonable (which should still be plenty for a 1450 t Starship to hold position and control attitude).
Title: Re: Starship Methox RCS Thrusters
Post by: Daan on 04/18/2021 04:33 pm
By the look of it, Lunar Starship is going to use an array of 24 or 30 hot gas RCS thrusters to land. It's probably possible to get to a decent thrust estimate if we look at Lunar Starship mass, thruster angle, and landing TWR
Title: Re: Starship Methox RCS Thrusters
Post by: TrueBlueWitt on 04/18/2021 05:26 pm
Some recent discussion on the topic of "use the same 100 kN thruster for RCS and Moon landing" had me doing a bit of a comparison with the Shuttle so I decided to dust of this rather more on-topic thread hoping against all reason that some relevant discussion might find its way here ;)

The Shuttle crew operations manual (PDF) (http://390651main_shuttle_crew_operations_manual.pdf) has some nice tables of average orbiter RCS acceleration on page 9.2-3 (p. 1009 in the PDF). For a heavy payload (260 klbs) in normal orbital mode these are:

AxisLinear acceleration [ft/s2]
+X0.21
-X0.22
±Y0.16
+Z0.32*
-Z0.42
*There is also a HIGH mode that can fire all thrusters to back away from a payload at 0.94 ft/s2.

AxisAngular acceleration [deg/s2]
±Roll0.92
+Pitch1.23
-Pitch0.85
±Yaw0.70

From these numbers it looks like 0.2 ft/s2 (~6cm/s2) and 1 deg/s2 are reasonable for general maneuvering. Using a quick and dirty model for a fully fueled Starship at 1450 t with payload consisting of four stacked solid solid cylinders (Skirt: 5 m, 27 t. Oxygen: 12 m, 939t. Methane: 9 m, 261 t. Nose: 23 m, 223 t) i get a center of mass at 16.8 m (right at the methane header tank) and a moment of inertia for pitch/yaw of ~1.3e8 kgm2 with roll about 1/10 of that. Placing RCS thrusters at the "corners" (front end of forward flaps and aft end of the skirt) gives a average moment arm of ~22 m for pitch/yaw and 4.5 m for roll, so pitch/yaw is the limiting case.

1450 t * 6 cm/s2 = 90 kN
1.3e8 kgm2 * 1 deg/s2 / 22 m = 2 x 53 kN

So you can make an argument for 100 kN RCS thrusters if you want a fully loaded Starship to be able to make higher acceleration maneuvers like the back-off mentioned above (which would need four of them). For general Shuttle like maneuverability something like 25 kN thrusters in a decently redundant configuration should be enough and that is only if you insist on moving the full Starship massing 3 x the ISS. For docking for example you would rather be docking things to it - in most Starship dockings at least one of them will be <300 t for which something like 5-10 kN is more reasonable (which should still be plenty for a 1450 t Starship to hold position and control attitude).

I still think they should maximize on their strengths, and ISP, by going the Electron - Rutherford way.
They will already have most the battery capacity onboard, and can regenerate using solar.

Anyone else's thoughts on going with Electric Pumped Fed Hot Gas Design for the Starship RCS/Landing Thrusters?
Title: Re: Starship Methox RCS Thrusters
Post by: Michael S on 04/18/2021 08:56 pm

I still think they should maximize on their strengths, and ISP, by going the Electron - Rutherford way.
They will already have most the battery capacity onboard, and can regenerate using solar.

Anyone else's thoughts on going with Electric Pumped Fed Hot Gas Design for the Starship RCS/Landing Thrusters?


Can you clarify what you mean by a "Electric Pumped Fed Hot Gas"?
The way I understand it, an electric pump system, like in the Rutherford engine, moves liquids. A hot gas system is inherently pressure fed because the fuel and oxidizer are already under high pressure. 

edit: spelling oxidizer correctly
Title: Re: Starship Methox RCS Thrusters
Post by: _MECO on 04/18/2021 09:29 pm

I still think they should maximize on their strengths, and ISP, by going the Electron - Rutherford way.
They will already have most the battery capacity onboard, and can regenerate using solar.

Anyone else's thoughts on going with Electric Pumped Fed Hot Gas Design for the Starship RCS/Landing Thrusters?


Can you clarify what you mean by a "Electric Pumped Fed Hot Gas"?
The way I understand it, an electric pump system, like in the Rutherford engine, moves liquids. A hot gas system is inherently pressure fed because the fuel and oxidized are already under high pressure.
This. Electric pumps can be for Methalox, or for charging Methox tanks. I would imagine electric pump-charged Methox is better because you don't need giant banks of batteries.
Title: Re: Starship Methox RCS Thrusters
Post by: xvel on 04/18/2021 09:29 pm
even if such a thing would exist, it would not have pumps, but compressors.
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 04/19/2021 01:32 am
Pump power is inversely proportional to fluid density. You don't want to pump low density gases.

John
Title: Re: Starship Methox RCS Thrusters
Post by: _MECO on 04/19/2021 02:16 am
Pump power is inversely proportional to fluid density. You don't want to pump low density gases.

John
Unless you have a large gaseous RCS reservoir that you can continuously top up slowly when not in use. Or else just pump some methalox and then vaporize it with resistive heaters. Either or. Pumping boiloff into the RCS tanks would help with cooling though.
Title: Re: Starship Methox RCS Thrusters
Post by: su27k on 04/19/2021 04:26 pm
Over at reddit, people are saying this video (actually mainly the audio part) may be test firing of the hot gas thruster at McGregor:

https://www.youtube.com/watch?v=vrGS2UtSP5k
Title: Re: Starship Methox RCS Thrusters
Post by: DigitalMan on 04/19/2021 04:35 pm
Interesting. The patterns seem unlikely to be possible from a Raptor. But how do you rule out testing of a Super Draco?
Title: Re: Starship Methox RCS Thrusters
Post by: xvel on 04/19/2021 05:14 pm
it's not unlikely, it's impossible, it was some sort of pressure feed engine
Title: Re: Starship Methox RCS Thrusters
Post by: _MECO on 04/19/2021 05:30 pm
Interesting. The patterns seem unlikely to be possible from a Raptor. But how do you rule out testing of a Super Draco?
Aren't Super Dracos only meant to fire for constant duration to pull Crew Dragon off a booster? And they seem like they would be so loud. I mean look at this.

https://www.youtube.com/watch?v=lIGVi_rMFGw

Either they've got a solitary Raptor preburner that's a complete jalopy running with cold spark plugs and has a backfiring problem, or that's Starship methox RCS.
Title: Re: Starship Methox RCS Thrusters
Post by: DigitalMan on 04/19/2021 05:37 pm
Interesting. The patterns seem unlikely to be possible from a Raptor. But how do you rule out testing of a Super Draco?
Aren't Super Dracos only meant to fire for constant duration to pull Crew Dragon off a booster? And they seem like they would be so loud. I mean look at this.

https://www.youtube.com/watch?v=lIGVi_rMFGw

Either they've got a solitary Raptor preburner that's a complete jalopy running with cold spark plugs and has a backfiring problem, or that's Starship methox RCS.

Comparing a video/audio taken feet away from the engine compared to video/audio at a substantial distance isn't great for comparing loudness, I suspect.

Yea, I don't recall footage of a SuperDraco pulsing like the mystery audio contains, but it doesn't mean someone else hasn't seen it. I am just trying to approach from the point of view that if we can't prove it is a new hot-gas thruster for Starship, lets try to exclude existing components like Draco, SuperDraco.

Title: Re: Starship Methox RCS Thrusters
Post by: StarshipTrooper on 04/19/2021 05:39 pm
I hope they do a hop test with a belt of 24 of those thrusters. That would be a sight!
Title: Re: Starship Methox RCS Thrusters
Post by: _MECO on 04/19/2021 05:44 pm
Interesting. The patterns seem unlikely to be possible from a Raptor. But how do you rule out testing of a Super Draco?
Aren't Super Dracos only meant to fire for constant duration to pull Crew Dragon off a booster? And they seem like they would be so loud. I mean look at this.

https://www.youtube.com/watch?v=lIGVi_rMFGw

Either they've got a solitary Raptor preburner that's a complete jalopy running with cold spark plugs and has a backfiring problem, or that's Starship methox RCS.

Comparing a video/audio taken feet away from the engine compared to video/audio at a substantial distance isn't great for comparing loudness, I suspect.

Yea, I don't recall footage of a SuperDraco pulsing like the mystery audio contains, but it doesn't mean someone else hasn't seen it. I am just trying to approach from the point of view that if we can't prove it is a new hot-gas thruster for Starship, lets try to exclude existing components like Draco, SuperDraco.
That's why I say seem. Not very objective, but the pulsing is another story.

I hope they do a hop test with a belt of 24 of those thrusters. That would be a sight!

The nice thing about that is you can rule of thumb it just by the ratio between Moon and Earth gravity. If a 120 ton Starship laden with 100 tons of cargo can land on the Moon then it should be able to do the same on Earth if the vehicle/cargo total mass was...

37 tons.

Dry mass of SS is 120 tons. Even if the TWR during a Moon landing is as high as 2 that means a hop can't be performed at all unless the test vehicle and fuel are significantly lighter than 70 tons. Sorry, if they're going to hop on those thrusters it's not going to be in a full scale vehicle. Unless maybe they fire a Raptor or two to simulate Lunar G a la the infamous Flying Beadstead? That would be a challenge because even though Raptors can gimbal a ton by rocket standards they don't have anything on that crazy swivel jet engine.
Title: Re: Starship Methox RCS Thrusters
Post by: StarshipTrooper on 04/19/2021 07:36 pm
There is a problem with your rule of thumb. Starship holds 1200 tons of fuel. Although you probably wouldn't have a circumstance where it would be full while you land on the Moon, even if you just wanted the ability to land with a 1/4 of a tank that would mean the thruster belt would have to put out ((80t(LSS) + 80t(payload) +300t fuel) * 1.5 (thrust to weight)) at 1/6 the Earth's gravity.

101 tons.
Title: Re: Starship Methox RCS Thrusters
Post by: _MECO on 04/19/2021 07:43 pm
There is a problem with your rule of thumb. Starship holds 1200 tons of fuel. Although you probably wouldn't have a circumstance where it would be full while you land on the Moon, even if you just wanted the ability to land with a 1/4 of a tank that would mean the thruster belt would have to put out ((80t(LSS) + 80t(payload) +300t fuel) * 1.5 (thrust to weight)) at 1/6 the Earth's gravity.

101 tons.
It was just a rule of thumb. Still works though because Starship's dry mass is more than 100 tons. Why do a hop though? Flying in a vacuum is relatively simple. And none of the engines have thrust vectoring to worry about. They'll also likely be pressure fed. I mean, the only real concerns are reliably getting the engines to fire in a vacuum and vibrational issues.
Title: Re: Starship Methox RCS Thrusters
Post by: Lars-J on 04/19/2021 07:45 pm
There is a problem with your rule of thumb. Starship holds 1200 tons of fuel. Although you probably wouldn't have a circumstance where it would be full while you land on the Moon, even if you just wanted the ability to land with a 1/4 of a tank that would mean the thruster belt would have to put out ((80t(LSS) + 80t(payload) +300t fuel) * 1.5 (thrust to weight)) at 1/6 the Earth's gravity.

101 tons.

The delta-v from NHRO to land on the moon is at least 2.5 km/s... So Lunar Starship will need to be able to land with enough propellant to make it back to NHRO and Gateway/Orion. How much propellant is that? I'll leave that to others.  :) But I suspect it is in the 1/3 to 1/4 full range.
Title: Re: Starship Methox RCS Thrusters
Post by: TheTraveller on 04/19/2021 08:01 pm
The delta-v from NHRO to land on the moon is at least 2.5 km/s... So Lunar Starship will need to be able to land with enough propellant to make it back to NHRO and Gateway/Orion. How much propellant is that? I'll leave that to others.  :) But I suspect it is in the 1/3 to 1/4 full range.

This shows 2.75 km/s for each trip = 5.5km/s down & back, PLUS 3.65 km/s for LSS to get there from LEO.
Total Dv budget of 9.15 km/s.

Didn't Elon quote 6.9 km/s as SS' number?

Which suggests LSS can't meet Orion in NRHO & take crew to/from the moon.

Where else can Orion, with a 1.2 km/s capability meet LSS & get back to Earth plus LSS can take crew to the moon & back?

HEO?
Title: Re: Starship Methox RCS Thrusters
Post by: _MECO on 04/19/2021 08:10 pm
The delta-v from NHRO to land on the moon is at least 2.5 km/s... So Lunar Starship will need to be able to land with enough propellant to make it back to NHRO and Gateway/Orion. How much propellant is that? I'll leave that to others.  :) But I suspect it is in the 1/3 to 1/4 full range.

This shows 2.75 km/s for each trip = 5.5km/s down & back, PLUS 3.65 km/s for LSS to get there from LEO.
Total Dv budget of 9.15 km/s.

Didn't Elon quote 6.9 km/s as SS' number?

Which suggests LSS can't meet Orion in NRHO & take crew to/from the moon.

Where else can Orion, with a 1.2 km/s capability meet LSS & get back to Earth plus LSS can take crew to the moon & back?

HEO?
6.9 km/s with 100 tons of cargo. You can go way further if you pack fewer granola bars. Also, I've been looking for a delta-v map that includes NRHO forever! Thank you so much for posting that. But WHAT on God's green Earth is that visual communication. Holy cow. It's like the attractiveness and succinctness of a dV map is inversely proportional to how interesting the information is that it conveys.
Title: Re: Starship Methox RCS Thrusters
Post by: StarshipTrooper on 04/19/2021 08:14 pm
Why do a hop though? Flying in a vacuum is relatively simple. And none of the engines have thrust vectoring to worry about. They'll also likely be pressure fed. I mean, the only real concerns are reliably getting the engines to fire in a vacuum and vibrational issues.

Why do a hop? Well, it's true it might not be needed.

On the other hand, you are dealing with an object in gravity, landing, and taking off. It is being supported by thrust from a rather unusual location rather high up but not above the craft with a center of gravity that would vary widely depending on payload and fuel. You have a system that contains... what?... 24 engines. They are potentially, individually controlled, to adapt to any failures. There could actually be vectoring of thrust by changing the power by different quadrants. They might want to verify their models...

They might want to test it.
Title: Re: Starship Methox RCS Thrusters
Post by: _MECO on 04/19/2021 08:59 pm
Why do a hop though? Flying in a vacuum is relatively simple. And none of the engines have thrust vectoring to worry about. They'll also likely be pressure fed. I mean, the only real concerns are reliably getting the engines to fire in a vacuum and vibrational issues.

Why do a hop? Well, it's true it might not be needed.

On the other hand, you are dealing with an object in gravity, landing, and taking off. It is being supported by thrust from a rather unusual location rather high up but not above the craft with a center of gravity that would vary widely depending on payload and fuel. You have a system that contains... what?... 24 engines. They are potentially, individually controlled, to adapt to any failures. There could actually be vectoring of thrust by changing the power by different quadrants. They might want to verify their models...

They might want to test it.
Gravity is a body force and so pulls on all parts of Starship the same. So long as it hasn't hit the ground yet or been subjected to black hole tidal forces or something, gravity is not a force that can dynamically affect the vehicle. Even including gravity, the free body diagram for SS on these waist thrusters is dead simple. There is a single mass and a single moment of Inertia that is centered at some point below the thruster belt on the centerline of the vehicle, and then you have 24 vectors of thrust force that can be pulsed on and off which point at some angle between straight out and straight down positioned radially about the vehicle. The only other force is gravity. It's constant and does not exert any moments so it plays very nice with the math.

For any given instantaneous moment in time, the linear motion of the vehicle is described by a vector sum of all the currently turned on thrusters acting on the mass of the vehicle (as well as the downward force of gravity) according to ΣF=Σma on each of the three axes. The rotational motion of the vehicle is described by a vector sum of all the cross products of the forces about three axes and the moment of inertia on those axes according to ΣT=ΣIα. With these six equations you can completely describe the behavior of the vehicle for any inputs. Convert that garbage into the S domain and you're on your way to designing a basic reaction control system.

Not having to deal with air is very nice.
Title: Re: Starship Methox RCS Thrusters
Post by: tbellman on 04/19/2021 09:02 pm
Also, I've been looking for a delta-v map that includes NRHO forever!

Not a map, but there is a table in the Wikipedia page for Delta-v budget (https://en.wikipedia.org/wiki/Delta-v_budget#Earth_Lunar_Gateway%E2%80%94high_thrust).
Title: Re: Starship Methox RCS Thrusters
Post by: _MECO on 04/19/2021 09:03 pm
Also, I've been looking for a delta-v map that includes NRHO forever!

Not a map, but there is a table in the Wikipedia page for Delta-v budget (https://en.wikipedia.org/wiki/Delta-v_budget#Earth_Lunar_Gateway%E2%80%94high_thrust).
Okay I'm desparate to share this with you or anyone who will listen. You know how I said the visual communication was atrocious? Look how much better this is. And I did it in MS Paint. You can immediately go from one place to another and it's much more obvious what is going on. I did exclude pure TLI delta-v but included it in figures for going to different Lunar orbits from Earth. And it's crusty as all get out. But still!
Title: Re: Starship Methox RCS Thrusters
Post by: baldusi on 04/19/2021 09:10 pm
Also, I've been looking for a delta-v map that includes NRHO forever!

Not a map, but there is a table in the Wikipedia page for Delta-v budget (https://en.wikipedia.org/wiki/Delta-v_budget#Earth_Lunar_Gateway%E2%80%94high_thrust).
Okay I'm desparate to share this with you or anyone who will listen. You know how I said the visual communication was atrocious? Look how much better this is. And I did it in MS Paint. You can immediately go from one place to another and it's much more obvious what is going on. I did exclude pure TLI delta-v but included it in figures for going to different Lunar orbits from Earth. And it's crusty as all get out. But still!

Wikipedia is a collaboratory effort. Sign in, clean up a bit your graph, upload it and link it to the article.
Title: Re: Starship Methox RCS Thrusters
Post by: _MECO on 04/19/2021 09:20 pm
Also, I've been looking for a delta-v map that includes NRHO forever!

Not a map, but there is a table in the Wikipedia page for Delta-v budget (https://en.wikipedia.org/wiki/Delta-v_budget#Earth_Lunar_Gateway%E2%80%94high_thrust).
Okay I'm desparate to share this with you or anyone who will listen. You know how I said the visual communication was atrocious? Look how much better this is. And I did it in MS Paint. You can immediately go from one place to another and it's much more obvious what is going on. I did exclude pure TLI delta-v but included it in figures for going to different Lunar orbits from Earth. And it's crusty as all get out. But still!

Wikipedia is a collaboratory effort. Sign in, clean up a bit your graph, upload it and link it to the article.
I may not be afraid of complaining all over the place and armchair redesigning with MS Paint but contributing to the community is a whole other story. Then again, It would be really cool to see a unitary map combining all the big guns into one image. You could have the big Earth-Moon-Mars trio as the focus, with all the funky halo orbits and cycler orbits and lagrange points included. The rest of the solar system could be off to the bottom. That's a project!
Title: Re: Starship Methox RCS Thrusters
Post by: TheTraveller on 04/19/2021 09:41 pm
Add this one as well
Title: Re: Starship Methox RCS Thrusters
Post by: _MECO on 04/19/2021 09:55 pm
Add this one as well
Hey, you saved my doodle! I feel so honored, I'm sorry for how crusty it is. I regret that one. Wish it was NRHO instead of LLO but

(1) I didn't yet have the NRHO delta-v map that just got posted so I didn't know what the requirements were at the time and

(2) I didn't realize that Orion/EUS/ESM didn't have the delta-v to make LLO and come back. Which is sad, by the way.
Title: Re: Starship Methox RCS Thrusters
Post by: TheTraveller on 04/19/2021 10:21 pm
Add this one as well
Hey, you saved my doodle! I feel so honored, I'm sorry for how crusty it is. I regret that one. Wish it was NRHO instead of LLO but

(1) I didn't yet have the NRHO delta-v map that just got posted so I didn't know what the requirements were at the time and

(2) I didn't realize that Orion/EUS/ESM didn't have the delta-v to make LLO and come back. Which is sad, by the way.

Nice doodle.

Yes Orion with 1.2 km/s is a bit limited.
Simple to fix.
Orion docks with LSS in HEO.
Transfers crew, LSS does direct entry, lands moon & returns direct to HEO, no LLO.
Crew transfer to Orion, which lands Earth.
LSS is refuelled for next transfer.

Of course SpaceX could use their Crew Dragon to do this as well.
Which gives SpaceX the ability to send private crew to the moon.
Title: Re: Starship Methox RCS Thrusters
Post by: Kiwi53 on 04/20/2021 02:56 am

Orion docks with LSS in HEO.
Transfers crew, LSS does direct entry, lands moon & returns direct to HEO, no LLO.
Crew transfer to Orion, which lands Earth.
LSS is refuelled for next transfer.

Of course SpaceX could use their Crew Dragon to do this as well.
Which gives SpaceX the ability to send private crew to the moon.

And if SpaceX was to do that before Artemis III, and publish the price per seat and per tonne, well that would undoubtedly be the coup de grace for SLS.
I reckon Elon Musk would be extremely tempted to do that, particularly if Artemis III slips say to 2025.
Title: Re: Starship Methox RCS Thrusters
Post by: sdsds on 04/20/2021 05:17 am
[...] I did it in MS Paint. [...]  But still!

It's good!

One place it might be improved is somehow showing that the NRHO and LLO trajectories don't (usually) intersect.  In particular the low lunar orbits used for Apollo missions were essentially equatorial rather than polar. The astrogation to get from an NHRO to one of those isn't totally straight-forward, to say the least.
Title: Re: Starship Methox RCS Thrusters
Post by: su27k on 04/20/2021 12:37 pm
I hope they do a hop test with a belt of 24 of those thrusters. That would be a sight!

The nice thing about that is you can rule of thumb it just by the ratio between Moon and Earth gravity. If a 120 ton Starship laden with 100 tons of cargo can land on the Moon then it should be able to do the same on Earth if the vehicle/cargo total mass was...

37 tons.

Dry mass of SS is 120 tons. Even if the TWR during a Moon landing is as high as 2 that means a hop can't be performed at all unless the test vehicle and fuel are significantly lighter than 70 tons. Sorry, if they're going to hop on those thrusters it's not going to be in a full scale vehicle. Unless maybe they fire a Raptor or two to simulate Lunar G a la the infamous Flying Beadstead? That would be a challenge because even though Raptors can gimbal a ton by rocket standards they don't have anything on that crazy swivel jet engine.

John did a calculation of the thrust needed for lunar landing here (https://forum.nasaspaceflight.com/index.php?topic=49622.msg2169758#msg2169758), it needs ~163 ton force, so should be able to hop on Earth.
Title: Re: Starship Methox RCS Thrusters
Post by: _MECO on 04/20/2021 03:03 pm
[...] I did it in MS Paint. [...]  But still!

It's good!

One place it might be improved is somehow showing that the NRHO and LLO trajectories don't (usually) intersect.  In particular the low lunar orbits used for Apollo missions were essentially equatorial rather than polar. The astrogation to get from an NHRO to one of those isn't totally straight-forward, to say the least.
Good delta-v maps will also include maximum plane change delta-v alongside the primary indicator for transfer dV required. In the diagram the only orbit change that absolutely needs such a plane change is a complete 90 degree shift from a polar LLO to an equatorial LLO. Granted the details of how to efficiently insert from NRHO to an efficient trans-Earth return trajectory currently escape me, but still.
Title: Re: Starship Methox RCS Thrusters
Post by: _MECO on 04/20/2021 03:06 pm
I hope they do a hop test with a belt of 24 of those thrusters. That would be a sight!

The nice thing about that is you can rule of thumb it just by the ratio between Moon and Earth gravity. If a 120 ton Starship laden with 100 tons of cargo can land on the Moon then it should be able to do the same on Earth if the vehicle/cargo total mass was...

37 tons.

Dry mass of SS is 120 tons. Even if the TWR during a Moon landing is as high as 2 that means a hop can't be performed at all unless the test vehicle and fuel are significantly lighter than 70 tons. Sorry, if they're going to hop on those thrusters it's not going to be in a full scale vehicle. Unless maybe they fire a Raptor or two to simulate Lunar G a la the infamous Flying Beadstead? That would be a challenge because even though Raptors can gimbal a ton by rocket standards they don't have anything on that crazy swivel jet engine.

John did a calculation of the thrust needed for lunar landing here (https://forum.nasaspaceflight.com/index.php?topic=49622.msg2169758#msg2169758), it needs ~163 ton force, so should be able to hop on Earth.
That would give a 120 ton dry mass Starship with 100 tons of cargo a Lunar TWR with over 4 assuming no return fuel and a TWR of more than 3 with 100 tons of placeholder methalox also aboard. Not bad! I retract what I said earlier if that's true.
Title: Re: Starship Methox RCS Thrusters
Post by: kkattula on 04/20/2021 04:16 pm
LEO direct to LLO is only about 4000 m/s, if you don't go via NRHO.
Title: Re: Starship Methox RCS Thrusters
Post by: _MECO on 04/20/2021 06:50 pm
LEO direct to LLO is only about 4000 m/s, if you don't go via NRHO.
Okay, this is why I have reservations about actually attempting to construct my own delta-v map. Using premade values can lead you astray because there may be information that wasn't included but is otherwise important, and calculating figures yourself can also give wrong values because you didn't come up with the most efficient route. Take that figure I gave for going from NRHO to an equatorial LLO. I said the plane change delta-V was twice LLO orbital velocity (an impulsive plane change would kill all polar velocity and add it back in at 90 degrees to the original direction) but I know that's not most efficient because performing the plane change before you circularize the NRHO would allow you to do it at lower total velocity and thus it would be cheaper. My grasp on orbital mechanics is really weak so I think I'd like to leave it alone altogether.
Title: Re: Starship Methox RCS Thrusters
Post by: kdhilliard on 04/26/2021 11:34 am
LEO direct to LLO is only about 4000 m/s, if you don't go via NRHO.
Okay, this is why I have reservations about actually attempting to construct my own delta-v map. ...

Your graphic is very clear, and with our return to the Moon we should have such a map which isn't as cluttered with dozens of other destinations.  You should collaborate with someone on the numbers, make a quality product, and if you are too shy to upload it to Wikipedia yourself, you can put it up on an image sharing site with an appropriately free license notice and then drop a line on the Wikipedia article's talk page.  If they like it, they'll upload it.


...
Didn't Elon quote 6.9 km/s as SS' number?

Which suggests LSS can't meet Orion in NRHO & take crew to/from the moon.
...

From the artwork, it appears that SpaceX's HLS proposal has raised their upper tank domes, thus increasing propellant capacity and available delta-v.  In a couple of the other threads TheRadicalModerate has done calculations with this increased capacity showing it closes the LEO -> NRHO -> LunarSurface -> NRHO budget with a respectable payload.  Refueling in NRHO gives you more.
Title: Re: Starship Methox RCS Thrusters
Post by: StarshipTrooper on 05/09/2021 12:26 am
New thruster tests sound recorded from McGregor.
  https://www.youtube.com/watch?v=XwT76nm_gNo (https://www.youtube.com/watch?v=XwT76nm_gNo)
Title: Re: Starship Methox RCS Thrusters
Post by: Joseph Peterson on 05/09/2021 11:51 am
Good delta-v maps will also include maximum plane change delta-v alongside the primary indicator for transfer dV required. In the diagram the only orbit change that absolutely needs such a plane change is a complete 90 degree shift from a polar LLO to an equatorial LLO. Granted the details of how to efficiently insert from NRHO to an efficient trans-Earth return trajectory currently escape me, but still.

Hopefully this helps you understand the math as much as it helped me.

Quote
Rendezvous Strategies in the Vicinity of Earth-Moon Lagrangian Points
Stephanie Lizy-Destrez1*, Laurent Beauregard1, Emmanuel Blazquez1, Antonino Campolo1,2, Sara Manglativi1,2 and Victor Quet1
1ISAE-SUPAERO, Toulouse, France
2Dipartimento di Scienze e Tecnologie Aerospaziali (DAER), Politecnico di Milano, Milan, Italy

In the context of Human Spaceflight exploration mission scenario, with the Lunar Orbital Platform- Gateway (LOP-G) orbiting about Earth-Moon Lagrangian Point (EML), Rendezvous and Docking (RVD) operational activities are mandatory and critical for the deployment and utilization of the LOP-G (station assembly, crew rotations, cargo delivery, lunar sample return). There is extensive experience with RVD in the two-body problem: in Low Earth Orbit (LEO) to various space stations, or around quasi-circular Low Lunar Orbits (LLO), the latter by Apollo by means of manual RVD. However, the RVD problem in non-Keplerian environments has rarely been addressed and no RVD has been performed to this date in the vicinity of Lagrangian points (LP) where Keplerian dynamics are no longer applicable. Dynamics in such regions are more complex, but multi-body dynamics also come with strong advantages that need to be further researched by the work proposed here. The aim of this paper is to present methods and results of investigations conducted to first set up strategies for far and close rendezvous between a target (the LOP-G, for example) and a chaser (cargo, crew vehicle, ascent and descent vehicle, station modules, etc.) depending on target and chaser orbit. Semi-analytical tools have been developed to compute and model families of orbits about the Lagrangian points in the Circular Restricted Three Body Problem (CR3BP) like NRHO, DRO, Lyapunov, Halo and Lissajous orbits. As far as close rendezvous is concerned, implementation of different linear and non-linear models used to describe cis-lunar relative motion will be discussed and compared, in particular for NRHO and DRO.

https://www.frontiersin.org/articles/10.3389/fspas.2018.00045/full
Title: Re: Starship Methox RCS Thrusters
Post by: StarshipTrooper on 05/14/2021 07:45 pm
Intriguing audio of the (meth/lox?) thrusters being tested at McGregor, recorded by Nicotine Jenkins on youTube.

Although it's mostly just audio, you can get a sense that some serious progress is being made.

SpaceX McGregor Engine/Thruster Test 04/13/2021 ANGLE #1
https://www.youtube.com/watch?v=vrGS2UtSP5k (https://www.youtube.com/watch?v=vrGS2UtSP5k)

SpaceX McGregor Engine/Thruster Test 04/13/2021 ANGLE #2
https://www.youtube.com/watch?v=DmSsfWK5UIg (https://www.youtube.com/watch?v=DmSsfWK5UIg)

SpaceX McGregor Engine/Thruster Test 04/15/2021
https://www.youtube.com/watch?v=Wa7TPCjgRHk (https://www.youtube.com/watch?v=Wa7TPCjgRHk)

SpaceX McGregor Thruster Test #1 05/04/2021
https://www.youtube.com/watch?v=XwT76nm_gNo (https://www.youtube.com/watch?v=XwT76nm_gNo)

SpaceX McGregor Thruster Test #2 05/04/2021
https://www.youtube.com/watch?v=cK579HjnlhY (https://www.youtube.com/watch?v=cK579HjnlhY)

SpaceX McGregor Thruster Test #1 05/10/2021(audio only no visuals)
https://www.youtube.com/watch?v=D9OGjr24mO8 (https://www.youtube.com/watch?v=D9OGjr24mO8)

SpaceX McGregor Thruster Test #2 05/10/2021(audio only no visuals)
https://www.youtube.com/watch?v=8P8qXwxPQH8&t=138s (https://www.youtube.com/watch?v=8P8qXwxPQH8&t=138s)

SpaceX McGregor Thruster Test 05/12/2021(audio only no visuals)
https://www.youtube.com/watch?v=ThvCrYfw0ow (https://www.youtube.com/watch?v=ThvCrYfw0ow)
Title: Re: Starship Methox RCS Thrusters
Post by: BT52 on 05/14/2021 10:40 pm
Due to shear size of SS and its mass. Hot gas thrusters could made sense. Especially for tight manoeuvring during landing. Maybe is even enabler for possible precise tover landings.

So yeah its application could provide precise grunt for SS and even more importantly for SH. So  i m wondering ways how could SpaceX raise efficacy so far crude devices.

Would electric turbopump made sense? I think it could.
1. Made central redundant parallel HIGH pressure electric turbo pump assembly
2. Raise chamber and nozzle pressure on demand pretty much anywhere on ship
3. Avoid preburners and use batteries instead. At slight mass cost but for much easier fire and refire in split second
4. Reduce COPVS complications and its heat losses at minimum. And due its high pressures avoid higher RUD probability because of it.

I would argue that way you could made design of HRCS much more simpler. Especially when you are transferring problem to already known variables - battery, electric engines and electric valves, engine mappings. Plus hey you could pull some Tesla guys for advice in spare time or two. All that areas could be quite easily redundant and controllable in case of any anomaly's. Plus response would be second to none. 
Title: Re: Starship Methox RCS Thrusters
Post by: steveleach on 05/14/2021 10:49 pm
Due to shear size of SS and its mass. Hot gas thrusters could made sense. Especially for tight manoeuvring during landing. Maybe is even enabler for possible precise tover landings.

So yeah its application could provide precise grunt for SS and even more importantly for SH. So  i m wondering ways how could SpaceX raise efficacy so far crude devices.

Would electric turbopump made sense? I think it could.
1. Made central redundant parallel HIGH pressure electric turbo pump assembly
2. Raise chamber and nozzle pressure on demand pretty much anywhere on ship
3. Avoid preburners and use batteries instead. At slight mass cost but for much easier fire and refire in split second
4. Reduce COPVS complications and its heat losses at minimum. And due its high pressures avoid higher RUD probability because of it.

I would argue that way you could made design of HRCS much more simpler. Especially when you are transferring problem to already known variables - battery, electric engines and electric valves, engine mappings. Plus hey you could pull some Tesla guys for advice in spare time or two. All that areas could be quite easily redundant and controllable in case of any anomaly's. Plus response would be second to none.
You mean pumping cryogenic liquids then burning gas, all without a preburner?
Title: Re: Starship Methox RCS Thrusters
Post by: BT52 on 05/14/2021 11:02 pm
Yeah sorta.

Quote
An electric cycle engine uses electric pumps to pressurize the propellants from a low-pressure fuel tank to high-pressure combustion chamber levels, generally from 0.2 to 0.3 MPa (29 to 44 psi) to 10 to 20 MPa (1,500 to 2,900 psi). The pumps are powered by an electric motor, with electricity from a battery bank. Source Wiki
Title: Re: Starship Methox RCS Thrusters
Post by: chopsticks on 05/14/2021 11:02 pm
Due to shear size of SS and its mass. Hot gas thrusters could made sense. Especially for tight manoeuvring during landing. Maybe is even enabler for possible precise tover landings.

So yeah its application could provide precise grunt for SS and even more importantly for SH. So  i m wondering ways how could SpaceX raise efficacy so far crude devices.

Would electric turbopump made sense? I think it could.
1. Made central redundant parallel HIGH pressure electric turbo pump assembly
2. Raise chamber and nozzle pressure on demand pretty much anywhere on ship
3. Avoid preburners and use batteries instead. At slight mass cost but for much easier fire and refire in split second
4. Reduce COPVS complications and its heat losses at minimum. And due its high pressures avoid higher RUD probability because of it.

I would argue that way you could made design of HRCS much more simpler. Especially when you are transferring problem to already known variables - battery, electric engines and electric valves, engine mappings. Plus hey you could pull some Tesla guys for advice in spare time or two. All that areas could be quite easily redundant and controllable in case of any anomaly's. Plus response would be second to none.
While electric turbopumped engines are cool and all, this is just adding unneeded complexity in my opinion. Just use pressure fed COPVs (gox and methane) from the autogenous pressurization system and feed the gas lines from the COPVs to small combustion chambers with a spark plug type igniter. Am I way off track to think that this is a simple and viable solution and probably the path they're taking?
Title: Re: Starship Methox RCS Thrusters
Post by: cuddihy on 05/14/2021 11:10 pm
Autogenous pressurization system requires running Raptors to maintain pressure.
Title: Re: Starship Methox RCS Thrusters
Post by: BT52 on 05/14/2021 11:24 pm
Sure. They could do that.  Mine main concern is nozzle efficacy

(https://i.ibb.co/ncKMYqJ/image-2021-05-15-011258.png)

Maybe i complicate things too much. Still u need reliable heatexcangers, pressure management, bleed off valving.
Plus need to be independent from Raptors running.  System needs to standalone and quick and reliable.  Like  mr @chuddihy said

Autogenous pressurization system requires running Raptors to maintain pressure.
 
How much could u reach? 6-4 bars? Or SpaceX are running header tank higher? Hmm hard thermodynamic problem. Autogenous for header tanks is hard enough to deal in rocket.  I think electric HRCS could be option. I hope i would had time and resources to run some napkin sims to deal with that. Surely SpaceX are looking in any idea and solution. At end of day they are rocket scientist not me.
Title: Re: Starship Methox RCS Thrusters
Post by: chopsticks on 05/15/2021 12:32 am
Autogenous pressurization system requires running Raptors to maintain pressure.
Of course, I know that. The COPVs would be pressurized when Raptors are running and store the pressurized gases for later use.
Title: Re: Starship Methox RCS Thrusters
Post by: tyrred on 05/15/2021 01:34 am
Intriguing audio of the (meth/lox?) thrusters being tested at McGregor, recorded by Nicotine Jenkins on youTube.

Although it's mostly just audio, you can get a sense that some serious progress is being made.

SpaceX McGregor Engine/Thruster Test 04/13/2021 ANGLE #1
https://www.youtube.com/watch?v=vrGS2UtSP5k (https://www.youtube.com/watch?v=vrGS2UtSP5k)

SpaceX McGregor Engine/Thruster Test 04/13/2021 ANGLE #2
https://www.youtube.com/watch?v=DmSsfWK5UIg (https://www.youtube.com/watch?v=DmSsfWK5UIg)

SpaceX McGregor Engine/Thruster Test 04/15/2021
https://www.youtube.com/watch?v=Wa7TPCjgRHk (https://www.youtube.com/watch?v=Wa7TPCjgRHk)

SpaceX McGregor Thruster Test #1 05/04/2021
https://www.youtube.com/watch?v=XwT76nm_gNo (https://www.youtube.com/watch?v=XwT76nm_gNo)

SpaceX McGregor Thruster Test #2 05/04/2021
https://www.youtube.com/watch?v=cK579HjnlhY (https://www.youtube.com/watch?v=cK579HjnlhY)

SpaceX McGregor Thruster Test #1 05/10/2021(audio only no visuals)
https://www.youtube.com/watch?v=D9OGjr24mO8 (https://www.youtube.com/watch?v=D9OGjr24mO8)

SpaceX McGregor Thruster Test #2 05/10/2021(audio only no visuals)
https://www.youtube.com/watch?v=8P8qXwxPQH8&t=138s (https://www.youtube.com/watch?v=8P8qXwxPQH8&t=138s)

SpaceX McGregor Thruster Test 05/12/2021(audio only no visuals)
https://www.youtube.com/watch?v=ThvCrYfw0ow (https://www.youtube.com/watch?v=ThvCrYfw0ow)

Is this the closest thing we have on public side to an actual update?
Title: Re: Starship Methox RCS Thrusters
Post by: Kazioo on 05/15/2021 10:00 pm
Interesting comment from the person filming those tests:

Quote
These bursts are SO much louder than Raptor or Merlin tests. Although they've all been short, they're VERY impressive
Title: Re: Starship Methox RCS Thrusters
Post by: BT52 on 05/17/2021 07:09 pm
Interesting comment from the person filming those tests:

Quote
These bursts are SO much louder than Raptor or Merlin tests. Although they've all been short, they're VERY impressive

Guys aside mine stupid pump idea. Witch lambda will they run MRCS? Fuel rich or oxygen rich? I would imagine heat for those thrusters would not be such issue due its impulse nature and not "sustained" heat flux. Any thoughts
Title: Re: Starship Methox RCS Thrusters
Post by: livingjw on 05/17/2021 08:38 pm
Interesting comment from the person filming those tests:

Quote
These bursts are SO much louder than Raptor or Merlin tests. Although they've all been short, they're VERY impressive

Guys aside mine stupid pump idea. Witch lambda will they run MRCS? Fuel rich or oxygen rich? I would imagine heat for those thrusters would not be such issue due its impulse nature and not "sustained" heat flux. Any thoughts

fuel rich gives better Isp.

John
Title: Re: Starship Methox RCS Thrusters
Post by: su27k on 06/22/2021 04:17 am
https://www.nasaspaceflight.com/2021/06/orbital-demo-goal-potential-sn16-test/

Quote
Near the Raptor test stands is a facility that SpaceX is currently using to test Starship and Super Heavy RCS thrusters.

On his weekly flight, Gary Blair spotted the potential test stand for these hot gas thrusters used to steer the ship and booster to their destinations.

Although I couldn't make heads or tails of the photo in question.
Title: Re: Starship Methox RCS Thrusters
Post by: jthygesen on 06/22/2021 01:52 pm
Super Heavy Forward Dome on the Sleeving Stand and Skirt Moved Around

I assume that the photos in the quoted post show the hot gas thrusters.
What can we deduce from those photos?

Three “chambers” with two large pipes and two small pipes. LOx and liquid CH4? (or already gaseous?) and the smaller ones could perhaps be for a pilot light of sorts?

I could definitely also see these used in “cold gas” mode for minimum impulse bit, where the propellants are simply not ignited, but I have no knowledge or experience in this field so what do I know.
Title: Re: Starship Methox RCS Thrusters
Post by: Wolfram66 on 06/22/2021 07:27 pm
Super Heavy Forward Dome on the Sleeving Stand and Skirt Moved Around

I assume that the photos in the quoted post show the hot gas thrusters.
What can we deduce from those photos?

Three “chambers” with two large pipes and two small pipes. LOx and liquid CH4? (or already gaseous?) and the smaller ones could perhaps be for a pilot light of sorts?

I could definitely also see these used in “cold gas” mode for minimum impulse bit, where the propellants are simply not ignited, but I have no knowledge or experience in this field so what do I know.


Center port has a little something extra special on it... McGregor strikes again
Title: Re: Starship Methox RCS Thrusters
Post by: Gliderflyer on 06/22/2021 11:05 pm
It looks like they are using the same style igniter as the Raptor (not too surprising).

The prop lines also feed into the top of the chamber, so I guess they are either doing: 1) down-and-back regen, 2) pure heat-sink with no cooling, or 3) Armadillo-style film cooling. I doubt that room temp methane has enough heat capacity for regen, so I would guess film cooling (heat-sink would have poor duty cycle limits). Film cooling is rather distinctive visually, so we should be able to tell if they are using it whenever we get video of them firing.

On the igniter, it looks like they haven't hooked up the gox line yet, so that will be something to watch out for. The line that is attached looks like it taps off the fuel inlet and has no valve between it and the igniter, so they might be doing a fuel lead and lag with the fuel always running through the igniter as a purge.
Title: Re: Starship Methox RCS Thrusters
Post by: volker2020 on 06/23/2021 10:06 am
I am a little surprised that there is no casing around the RCS unit. Some of the elements look rather delicate, and I am wondering, whether they would survive a supersonic reentry of the booster.
Title: Re: Starship Methox RCS Thrusters
Post by: Welsh Dragon on 06/23/2021 10:23 am
I am a little surprised that there is no casing around the RCS unit. Some of the elements look rather delicate, and I am wondering, whether they would survive a supersonic reentry of the booster.
Why assume this is the finished article? It would seem obvious to me they'll add thruster and raceway covers etc later on.
Title: Re: Starship Methox RCS Thrusters
Post by: kevinof on 06/23/2021 10:31 am
I am a little surprised that there is no casing around the RCS unit. Some of the elements look rather delicate, and I am wondering, whether they would survive a supersonic reentry of the booster.
I always find these type of comments amusing. It’s as if the boys and girls at SpaceX are new to this spaceflight stuff and know nothing about hypersonic Re entry.

My bets are on them having all this already figured out.
Title: Re: Starship Methox RCS Thrusters
Post by: Norm38 on 06/24/2021 02:35 am
Interesting comment from the person filming those tests:

Quote
These bursts are SO much louder than Raptor or Merlin tests. Although they've all been short, they're VERY impressive

How is that even possible?  Some sort of pressure density function?  Higher pressure at small flow is louder than lower pressure at huge flow?
Title: Re: Starship Methox RCS Thrusters
Post by: Gliderflyer on 06/24/2021 02:45 am
Interesting comment from the person filming those tests:

Quote
These bursts are SO much louder than Raptor or Merlin tests. Although they've all been short, they're VERY impressive

How is that even possible?  Some sort of pressure density function?  Higher pressure at small flow is louder than lower pressure at huge flow?
Direction can play a large part. I was 1 mile from a 25k methane engine that was firing perpendicular to me, and you couldn't hear it over people talking; but a 1k alcohol engine at the same distance 100 feet in the air sounded like a jet taking off.

Propellant type, after-burning in the plume, atmospheric conditions, etc. can also play a large part.
Title: Re: Starship Methox RCS Thrusters
Post by: edzieba on 06/24/2021 08:29 am
https://twitter.com/elonmusk/status/1407969457411067905

So the hot-gas thrusters have been:
- Added for Starship flip
- Removed for Starship flip (Raptors only)
- Re-added for Starship in-orbit ops (and possibly Lunar Starship)
- Added to Super Heavy
- Removed from Super Heavy
Title: Re: Starship Methox RCS Thrusters
Post by: CorvusCorax on 06/24/2021 09:18 am
https://twitter.com/elonmusk/status/1407969457411067905

So the hot-gas thrusters have been:
- Added for Starship flip
- Removed for Starship flip (Raptors only)
- Re-added for Starship in-orbit ops (and possibly Lunar Starship)
- Added to Super Heavy
- Removed from Super Heavy

"for now" -- according to the tweet ;)
Title: Re: Starship Methox RCS Thrusters
Post by: kevinof on 06/24/2021 09:33 am
or the simpler way of looking at it is "they are in development but we're not going to let that development hold back the first flight".

https://twitter.com/elonmusk/status/1407969457411067905

So the hot-gas thrusters have been:
- Added for Starship flip
- Removed for Starship flip (Raptors only)
- Re-added for Starship in-orbit ops (and possibly Lunar Starship)
- Added to Super Heavy
- Removed from Super Heavy
Title: Re: Starship Methox RCS Thrusters
Post by: Cheapchips on 06/24/2021 09:45 am

They presumably have at least two more SN20 style tests in the works, given how they've run their testing to date.  They'd still have those opportunities to flight test the Methox RCS ahead of a catch attempt.
Title: Re: Starship Methox RCS Thrusters
Post by: _MECO on 06/24/2021 11:14 am

They presumably have at least two more SN20 style tests in the works, given how they've run their testing to date.  They'd still have those opportunities to flight test the Methox RCS ahead of a catch attempt.
Which is where you need them. Although I do wonder about the speed it takes for Super Heavy to swing around and do a boostback burn on methox vs nitrogen. I get that for the first orbital test the booster's going to be ending up in the drink but every second before reigniting those engines is hundreds more meters downrange.
Title: Re: Starship Methox RCS Thrusters
Post by: steveleach on 06/24/2021 12:37 pm

They presumably have at least two more SN20 style tests in the works, given how they've run their testing to date.  They'd still have those opportunities to flight test the Methox RCS ahead of a catch attempt.
Which is where you need them. Although I do wonder about the speed it takes for Super Heavy to swing around and do a boostback burn on methox vs nitrogen. I get that for the first orbital test the booster's going to be ending up in the drink but every second before reigniting those engines is hundreds more meters downrange.
I suspect Elon is ruthlessly constraining the scope of orbital launch #1 to minimise schedule delays, and any non-trivial attempts to improve landing accuracy have been rejected.

I bet there are dozens of conversations a day along the lines of...

Engineer: "we could test the microconflagulator on the orbital flight by just adding..."
Elon: "No"

Title: Re: Starship Methox RCS Thrusters
Post by: _MECO on 06/24/2021 01:54 pm

They presumably have at least two more SN20 style tests in the works, given how they've run their testing to date.  They'd still have those opportunities to flight test the Methox RCS ahead of a catch attempt.
Which is where you need them. Although I do wonder about the speed it takes for Super Heavy to swing around and do a boostback burn on methox vs nitrogen. I get that for the first orbital test the booster's going to be ending up in the drink but every second before reigniting those engines is hundreds more meters downrange.
I suspect Elon is ruthlessly constraining the scope of orbital launch #1 to minimise schedule delays, and any non-trivial attempts to improve landing accuracy have been rejected.

I bet there are dozens of conversations a day along the lines of...

Engineer: "we could test the microconflagulator on the orbital flight by just adding..."
Elon: "No"
Of course. The best part is no part. The best launch vehicle is a steel bulkhead atop a shaft with a nuclear weapon at the bottom 8)
Title: Re: Starship Methox RCS Thrusters
Post by: steveleach on 06/24/2021 02:01 pm

They presumably have at least two more SN20 style tests in the works, given how they've run their testing to date.  They'd still have those opportunities to flight test the Methox RCS ahead of a catch attempt.
Which is where you need them. Although I do wonder about the speed it takes for Super Heavy to swing around and do a boostback burn on methox vs nitrogen. I get that for the first orbital test the booster's going to be ending up in the drink but every second before reigniting those engines is hundreds more meters downrange.
I suspect Elon is ruthlessly constraining the scope of orbital launch #1 to minimise schedule delays, and any non-trivial attempts to improve landing accuracy have been rejected.

I bet there are dozens of conversations a day along the lines of...

Engineer: "we could test the microconflagulator on the orbital flight by just adding..."
Elon: "No"
Of course. The best part is no part. The best launch vehicle is a steel bulkhead atop a shaft with a nuclear weapon at the bottom 8)
Sorry, I didn't mean to imply that there would never be RCS on it. Just that Elon could well be saying "not for launch #1" a lot.
Title: Re: Starship Methox RCS Thrusters
Post by: Kazioo on 06/24/2021 02:45 pm

So the hot-gas thrusters have been:
- Added for Starship flip
- Removed for Starship flip (Raptors only)
- Re-added for Starship in-orbit ops (and possibly Lunar Starship)
- Added to Super Heavy
- Removed from Super Heavy

A change in operational design vs a change in one prototype to reduce the amount of variables. Completely different, unrelated things.
Title: Re: Starship Methox RCS Thrusters
Post by: edzieba on 06/24/2021 03:48 pm

So the hot-gas thrusters have been:
- Added for Starship flip
- Removed for Starship flip (Raptors only)
- Re-added for Starship in-orbit ops (and possibly Lunar Starship)
- Added to Super Heavy
- Removed from Super Heavy

A change in operational design vs a change in one prototype to reduce the amount of variables. Completely different, unrelated things.
Adding and removing a paper engine vs. adding and removing a now actual physical engine (not the case until the last few months).
Title: Re: Starship Methox RCS Thrusters
Post by: envy887 on 06/24/2021 03:49 pm

So the hot-gas thrusters have been:
- Added for Starship flip
- Removed for Starship flip (Raptors only)
- Re-added for Starship in-orbit ops (and possibly Lunar Starship)
- Added to Super Heavy
- Removed from Super Heavy

A change in operational design vs a change in one prototype to reduce the amount of variables. Completely different, unrelated things.
Adding and removing a paper engine vs. adding and removing a now actual physical engine (not the case until the last few months).

Perhaps it's a fit check with a non-flight unit. They stacked and unstacked an entire booster a couple months ago.
Title: Re: Starship Methox RCS Thrusters
Post by: Hauerg on 06/24/2021 03:53 pm

So the hot-gas thrusters have been:
- Added for Starship flip
- Removed for Starship flip (Raptors only)
- Re-added for Starship in-orbit ops (and possibly Lunar Starship)
- Added to Super Heavy
- Removed from Super Heavy
They habe the time and the tools to do a test installation on flight hardware.
Somwhat.
Nothing to see here.

A change in operational design vs a change in one prototype to reduce the amount of variables. Completely different, unrelated things.
Adding and removing a paper engine vs. adding and removing a now actual physical engine (not the case until the last few months).
Title: Re: Starship Methox RCS Thrusters
Post by: edzieba on 06/24/2021 03:54 pm

So the hot-gas thrusters have been:
- Added for Starship flip
- Removed for Starship flip (Raptors only)
- Re-added for Starship in-orbit ops (and possibly Lunar Starship)
- Added to Super Heavy
- Removed from Super Heavy

A change in operational design vs a change in one prototype to reduce the amount of variables. Completely different, unrelated things.
Adding and removing a paper engine vs. adding and removing a now actual physical engine (not the case until the last few months).

Perhaps it's a fit check with a non-flight unit. They stacked and unstacked an entire booster a couple months ago.
"Any insufficiently flight-ready hardware is indistinguishable from a mass-simulator".
Title: Re: Starship Methox RCS Thrusters
Post by: envy887 on 06/24/2021 05:41 pm

So the hot-gas thrusters have been:
- Added for Starship flip
- Removed for Starship flip (Raptors only)
- Re-added for Starship in-orbit ops (and possibly Lunar Starship)
- Added to Super Heavy
- Removed from Super Heavy

A change in operational design vs a change in one prototype to reduce the amount of variables. Completely different, unrelated things.
Adding and removing a paper engine vs. adding and removing a now actual physical engine (not the case until the last few months).

Perhaps it's a fit check with a non-flight unit. They stacked and unstacked an entire booster a couple months ago.
"Any insufficiently flight-ready hardware is indistinguishable from a mass-simulator".

Nothing wrong with a mass simulator for parts they don't need yet. Flying ASAP is more important then landing on the first try.
Title: Re: Starship Methox RCS Thrusters
Post by: oldAtlas_Eguy on 06/24/2021 07:35 pm
They may have the thrusters. But what they may not have is the GCH4 and GO2 systems and storage worked out yet. So just use the Methox thrusters in cold gas mode with GN2 tanks.Would give enough control for the stage separation and attitude control prior to Raptor burns as well as for prop settling.
Title: Re: Starship Methox RCS Thrusters
Post by: sevenperforce on 06/24/2021 08:02 pm
On the igniter, it looks like they haven't hooked up the gox line yet, so that will be something to watch out for. The line that is attached looks like it taps off the fuel inlet and has no valve between it and the igniter, so they might be doing a fuel lead and lag with the fuel always running through the igniter as a purge.
Do they need a GOX line in the igniter?

The ignition sequence could be:

1. Press CH4 through igniter
2. Send GOX through chamber at low press
3. Spark to ignite
4. GOX and CH4 into chamber at full press