Starship Methox RCS Thrusters

[Lar]

Is oxygen gas now considered lox?

No, it's gox.

[Llian Rhydderch]

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.

[TheRadicalModerate]

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?

[Semmel]

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.

[Semmel]

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.

[livingjw]

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

[Norm38]

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.

[livingjw]

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

[Keldor]

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?

[livingjw]

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

[Pete]

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.

[ThomasGadd]

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.

[DistantTemple]

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.

[rsdavis9]

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?

[DistantTemple]

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!

[livingjw]

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

[DAZ]

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.

[Norm38]

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.

[livingjw]

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

[ThomasGadd]

I've seen diagrams both ways... the RCS tanks can store the mixed gas right?