Author Topic: Starship performance improvements  (Read 14415 times)

Offline raketa

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Starship performance improvements
« on: 11/21/2023 04:07 pm »
Could the performance of the starship system be increased by shutting down sea-level Raptors and using Vacuum Raptors only, or would Earth's gravity negate complitely the delta-v advantage?

Offline Jim

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Re: Starship performance improvements
« Reply #1 on: 11/21/2023 04:33 pm »
Could the performance of the starship system be increased by shutting down sea-level Raptors and using Vacuum Raptors only, or would Earth's gravity negate complitely the delta-v advantage?

The vacuum ones can't gimbal. 

No need to look for increased performance now.
« Last Edit: 11/21/2023 04:33 pm by Jim »

Offline steveleach

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Re: Starship performance improvements
« Reply #2 on: 11/21/2023 07:00 pm »
Could the performance of the starship system be increased by shutting down sea-level Raptors and using Vacuum Raptors only, or would Earth's gravity negate complitely the delta-v advantage?

The vacuum ones can't gimbal. 

No need to look for increased performance now.
But presumably differential thrust would give some level of control, and potentially sufficient assuming the vehicle is on a roughly nominal trajectory to start with.

My understanding is that gravity losses decrease as the vehicle gains velocity, so would the ship at some point cross a threshold where the increase in ISP from just using the vac-raps outweighs the increase in gravity losses from the reduced thrust?

Obviously I'm not thinking about changes for IFT-3, but at some point down the road once the basics are all in place.


Online AmigaClone

Re: Starship performance improvements
« Reply #3 on: 11/21/2023 07:24 pm »
Could the performance of the starship system be increased by shutting down sea-level Raptors and using Vacuum Raptors only, or would Earth's gravity negate complitely the delta-v advantage?

The vacuum ones can't gimbal. 

No need to look for increased performance now.
But presumably differential thrust would give some level of control, and potentially sufficient assuming the vehicle is on a roughly nominal trajectory to start with.

My understanding is that gravity losses decrease as the vehicle gains velocity, so would the ship at some point cross a threshold where the increase in ISP from just using the vac-raps outweighs the increase in gravity losses from the reduced thrust?

Obviously I'm not thinking about changes for IFT-3, but at some point down the road once the basics are all in place.

One big issue with differential thrust is that one or two engines would need to reduce thrust in order to have that differential thrust - something that would not be desirable at many stages of a launch. Gimbaling the engines allows them to run at the same thrust - with the direction of that thrust being adjustable.

Offline John Santos

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Re: Starship performance improvements
« Reply #4 on: 11/21/2023 07:43 pm »
Differential thrust is a very dicey way of steering, especially with just a few very big engines.

This reminds me of Sioux City, where a DC10 lost all its hydraulics in a center engine explosion and the only way to steer was manual control of the throttles on the two wing engines.  About 1/3 of the passengers died when a last second roll at landing caused the plane to disintegrate. Two thirds surviving was a miracle, everyone on board could have very easily died.  On the other hand, more luck at landing and it might have come down on its wheels instead of a wing tip touching the ground and landed intact.  That is not something you want to plan on as SOP.

Even if in theory differential thrust could steer Starship, it would be something they would only want to do in a crisis.

Offline steveleach

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Re: Starship performance improvements
« Reply #5 on: 11/21/2023 07:59 pm »
I don't know about the OP, but I wasn't asking whether differential thrust is best in all situations, or claiming that it didn't have downsides.

I was asking about a very specific situation: the ship is on a relatively nominal trajectory and already at a high enough velocity that gravity losses are less of a factor.

Even if in theory differential thrust could steer Starship, it would be something they would only want to do in a crisis.
Very confidently stated, but do you know whether SpaceX agree with you?

Offline Jim

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Re: Starship performance improvements
« Reply #6 on: 11/21/2023 08:24 pm »

But presumably differential thrust would give some level of control, and potentially sufficient assuming the vehicle is on a roughly nominal trajectory to start with.


Doesn't react quickly enough and can't do roll control

Offline steveleach

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Re: Starship performance improvements
« Reply #7 on: 11/21/2023 10:59 pm »

But presumably differential thrust would give some level of control, and potentially sufficient assuming the vehicle is on a roughly nominal trajectory to start with.


Doesn't react quickly enough and can't do roll control
Quickly enough for what, and why is roll control important, at that stage in the flight?

Also, would the RCS mitigate any of this?

Offline Brigantine

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Re: Starship performance improvements
« Reply #8 on: 11/21/2023 11:03 pm »
Could the performance of the starship system be increased by shutting down sea-level Raptors and using Vacuum Raptors only, or would Earth's gravity negate complitely the delta-v advantage?
What has been discussed elsewhere (probably not by official SpaceX sources), in the early part of the 2nd stage burn you want all the thrust you can get to fight gravity. Later when you're closer to orbital velocity, and for departure burns, yes it is better to lean on the RVacs as much as possible, and you do that by deep throttling the RCs and maybe shutting down 1 or 2 of them depending on your risk tolerance. (and need for roll control)

Keep running 1 sea-level Raptor at 50% throttle for steering, but still run 86% of the fuel through the 3 RVacs.
(or 93% of the fuel through the 6 RVacs)

Once the thrusters for moon landings / orbital fuel transfer ullage control are developed, depending on how good they are they may be a better option, assuming they gimbal and they're not just for Depots.
« Last Edit: 11/21/2023 11:19 pm by Brigantine »

Offline InterestedEngineer

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Re: Starship performance improvements
« Reply #9 on: 11/21/2023 11:15 pm »
Could the performance of the starship system be increased by shutting down sea-level Raptors and using Vacuum Raptors only, or would Earth's gravity negate complitely the delta-v advantage?
What has been discussed elsewhere (probably not by official SpaceX sources), in the early part of the 2nd stage burn you want all the thrust you can get to fight gravity. Later when you're closer to orbital velocity, and for departure burns, yes it is better to lean on the RVacs as much as possible, and you do that by deep throttling the RCs and maybe shutting down 1 or 2 of them depending on your risk tolerance.

Keep running 1 sea-level Raptor at 50% throttle for steering, but still run 86% of the fuel through the 3 RVacs.
(or 93% of the fuel through the 6 RVacs)

Depends on the mission.   If you are doing an Oberth burn, it's possible that the longer thrust time from reduced thrust reduces the net deltaV more than running a gimbaling sea level engine or two.

Let's say we got 6 Vacuum Raptors and 3 sea levels.

Running two sea levels at 40% thrust and 6 Vacuum at full thrust, we get an Isp of 6*378 + .8 * 363 or an average of 376.2

However it's silly to plan a mission on what is likely to happen, you have to plan on single failures.

So if it's a VR that fails it's 5*378 + .8*363 => average of 375.9

The difference between that an a differentially throttled Vacuum Raptor system is .5%, or with a deltaV of 4km/sec 22 m/sec.

There's enough other stuff going on to not worry about 22m/sec.

Offline ugordan

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Re: Starship performance improvements
« Reply #10 on: 11/22/2023 06:36 am »
Quickly enough for what, and why is roll control important, at that stage in the flight?

Look up F9 flight 1 for importance of keeping any roll torque in check.

Also, would the RCS mitigate any of this?

Suppose one of the 3 RVacs shuts down? No reasonably sized RCS would be able to fight that thrust imbalance.

Online Oersted

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Re: Starship performance improvements
« Reply #11 on: 11/22/2023 07:27 am »
This discussion hughlights, I think, the amazing design of the Starship engine set-up. Great in-built flexibility catering to many different requirements during the various stages of flight.

Offline Barley

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Re: Starship performance improvements
« Reply #12 on: 11/22/2023 09:07 am »

Suppose one of the 3 RVacs shuts down? No reasonably sized RCS would be able to fight that thrust imbalance.

Suppose one of the one MVacs on a Falcon 9 shuts down?  I could ask a similar question about many launchers.
You're holding SS to a higher standard.  It may be justified in some cases (crewed) but not in general (starlink).

Offline ugordan

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Re: Starship performance improvements
« Reply #13 on: 11/22/2023 11:04 am »

Suppose one of the 3 RVacs shuts down? No reasonably sized RCS would be able to fight that thrust imbalance.

Suppose one of the one MVacs on a Falcon 9 shuts down?  I could ask a similar question about many launchers.
You're holding SS to a higher standard.

Not really. MVac on F9 is a single point of failure either way. One RVac on SS going down while you have strong steering authority (via sea level Raptors) is not necessarily a Loss Of Vehicle/Mission scenario, while with only RCS attitude control it most surely would be. Apples and oranges.

The argument for relying on RCS only and not using the gimbaling Raptors is IMHO not a sound argument from a vehicle engineering and design reliability standpoint.

Offline Twark_Main

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Re: Starship performance improvements
« Reply #14 on: 11/22/2023 11:59 am »
I don't know about the OP, but I wasn't asking whether differential thrust is best in all situations, or claiming that it didn't have downsides.

I was asking about a very specific situation: the ship is on a relatively nominal trajectory and already at a high enough velocity that gravity losses are less of a factor.

Even if in theory differential thrust could steer Starship, it would be something they would only want to do in a crisis.
Very confidently stated, but do you know whether SpaceX agree with you?

Elon Musk has been asked this before, and his answer was that differential thrust was out because it can't handle an engine out scenario.

Sorry, no source. Internet searchability has collapsed like the Roman Empire. It might be one of the Everyday Astronaut interviews.
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Offline steveleach

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Re: Starship performance improvements
« Reply #15 on: 11/22/2023 01:37 pm »
Elon Musk has been asked this before, and his answer was that differential thrust was out because it can't handle an engine out scenario.

Sorry, no source. Internet searchability has collapsed like the Roman Empire. It might be one of the Everyday Astronaut interviews.
Sounds reasonable, and I can well believe that SpaceX would choose to forgo any potential performance gains to avoid the risk of that unrecoverable engine-out scenario.

I'd still be interested to know whether there is a point where shutting down the (lower ISP) sea-level engines increases overall vehicle performance, or whether the higher thrust from 6 engines vs 3 always wins out. No-one has tried to answer that yet, as far as I can tell.

Offline RoboGoofers

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Re: Starship performance improvements
« Reply #16 on: 11/22/2023 02:29 pm »
Seeing the giant Mach diamonds on booster, I wonder about the plume interactions on starship. do the SL Raptors get a small ISP boost from the plumes from the vac engines (like an aerobell instead of an aerospike)?

this is new ground, right? I can't think of any stage in the past where SL and Vac engines were firing together.

Offline goretexguy

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Re: Starship performance improvements
« Reply #17 on: 11/22/2023 02:42 pm »
Seeing the giant Mach diamonds on booster, I wonder about the plume interactions on starship. do the SL Raptors get a small ISP boost from the plumes from the vac engines (like an aerobell instead of an aerospike)?

this is new ground, right? I can't think of any stage in the past where SL and Vac engines were firing together.

I've wondered about this, if the exhaust plumes from surrounding engines act like a nozzle extension and increase ISP. My heart says, "that would be cool if so" but my brain tells me this is unlikely. A rocket nozzle converts pressure/temperature into velocity. Unless the external plume can somehow increase the velocity of the gas leaving the nozzle, then the answer is no.

Of course, if somebody has more insight/math, I'd love to make my heart happy.

Offline eriblo

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Re: Starship performance improvements
« Reply #18 on: 11/22/2023 03:25 pm »
Seeing the giant Mach diamonds on booster, I wonder about the plume interactions on starship. do the SL Raptors get a small ISP boost from the plumes from the vac engines (like an aerobell instead of an aerospike)?

this is new ground, right? I can't think of any stage in the past where SL and Vac engines were firing together.

I've wondered about this, if the exhaust plumes from surrounding engines act like a nozzle extension and increase ISP. My heart says, "that would be cool if so" but my brain tells me this is unlikely. A rocket nozzle converts pressure/temperature into velocity. Unless the external plume can somehow increase the velocity of the gas leaving the nozzle, then the answer is no.

Of course, if somebody has more insight/math, I'd love to make my heart happy.
Indeed, the exhaust imparts thrust on the vehicle by the molecules/atoms bouncing off solid surfaces. What happens to a supersonic exhaust is by definition irrelevant since the molecules can not move upstream.

There are at least two caveats to this: Prandtl–Meyer expansion and recirculation.

A small fraction of the exhaust expands around the lip of the nozzle and travels sideways and even forwards, potentially bouncing of the aft end of the rocket. The fact that Starship and Super Heavy have their nozzle exits more or less in the same plane makes me think that this will not greatly affect the overall (already minuscule) effect.

Exhaust gas re-circulation is when the ambient atmosphere being dragged along with the rocket brings entrapped exhaust with it from below the rocket which can change the pressure at the bottom end (higher pressure = an increase in effective thrust forward). This effect should be negligible for the ship which operates at high velocities in a near vacuum with an engine skirt that would prevent most gas from entering anyway.

For the booster there might be some re-circulation but the presence of inner engines would if anything act to decrease the effect. The fact that we see the plumes combine and produce a large Mach diamond is a sign that the outer ring shields the inner ones from the ambient atmosphere and the pressure around the center engines is much lower.

Offline deltaV

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Re: Starship performance improvements
« Reply #19 on: 11/23/2023 05:19 pm »
My understanding is that gravity losses decrease as the vehicle gains velocity, so would the ship at some point cross a threshold where the increase in ISP from just using the vac-raps outweighs the increase in gravity losses from the reduced thrust?

In theory shutting the SL raptors down seconds early would improve performance slightly but that's probably not feasible in practice because the 3 fixed engines would not provide roll control or engine out capability.

Offline steveleach

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Re: Starship performance improvements
« Reply #20 on: 11/24/2023 04:55 pm »
My understanding is that gravity losses decrease as the vehicle gains velocity, so would the ship at some point cross a threshold where the increase in ISP from just using the vac-raps outweighs the increase in gravity losses from the reduced thrust?

In theory shutting the SL raptors down seconds early would improve performance slightly but that's probably not feasible in practice because the 3 fixed engines would not provide roll control or engine out capability.
Thanks deltaV. A few more questions, because I'm genuinely trying to learn; I'm not challenging your assertions.

Why just seconds (out of the ~6 minute burn) early?

How much roll control is required once out of the bulk of the atmosphere? Too much for RCS to handle?

How resiliant to late vac-rap engine out would a Starship mission be anyway? You'd presumably have to compensate with more use of the sea-level engines, and have more gravity losses, so would you have the margins to handle it? If you do have the margins, could you just relight one of the sea-level engines?

Offline Jim

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Re: Starship performance improvements
« Reply #21 on: 11/24/2023 05:15 pm »

Thanks deltaV. A few more questions, because I'm genuinely trying to learn; I'm not challenging your assertions.

Why just seconds (out of the ~6 minute burn) early?

How much roll control is required once out of the bulk of the atmosphere? Too much for RCS to handle?

Because engines are not aligned perfectly, they don't have the exact same thrust and engines shutting down induces torques is why gimbaled engines are needed for the whole burn duration.
« Last Edit: 11/24/2023 05:16 pm by Jim »

Offline Jim

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Re: Starship performance improvements
« Reply #22 on: 11/24/2023 05:17 pm »
you just relight one of the sea-level engines?

They are never shutdown while the vacuum ones are firing.

Offline steveleach

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Re: Starship performance improvements
« Reply #23 on: 11/24/2023 10:08 pm »
you just relight one of the sea-level engines?

They are never shutdown while the vacuum ones are firing.
They are if you shut them down, which was what I was asking about.

Offline steveleach

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Re: Starship performance improvements
« Reply #24 on: 11/24/2023 10:12 pm »

Thanks deltaV. A few more questions, because I'm genuinely trying to learn; I'm not challenging your assertions.

Why just seconds (out of the ~6 minute burn) early?

How much roll control is required once out of the bulk of the atmosphere? Too much for RCS to handle?

Because engines are not aligned perfectly, they don't have the exact same thrust and engines shutting down induces torques is why gimbaled engines are needed for the whole burn duration.
Jim, I appreciate that these may seem like stupid questions to you, but your answers simply aren't helping. If the question is worth your time, I'd appreciate it if you give more detail; if it's not, just ignore it.

Offline deltaV

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Re: Starship performance improvements
« Reply #25 on: 11/25/2023 02:03 am »
Thanks deltaV. A few more questions, because I'm genuinely trying to learn; I'm not challenging your assertions.

Why just seconds (out of the ~6 minute burn) early?

How much roll control is required once out of the bulk of the atmosphere? Too much for RCS to handle?

How resiliant to late vac-rap engine out would a Starship mission be anyway? You'd presumably have to compensate with more use of the sea-level engines, and have more gravity losses, so would you have the margins to handle it? If you do have the margins, could you just relight one of the sea-level engines?

I'm afraid I don't know the answers to your questions. Quantifying when it would make sense to shut down the SL engines requires trajectory optimization software and it's hard for an amateur like me to get access to that software due to ITAR laws. I've considered writing that software myself but it would be a fair amount of work for something that would be hard to share with anyone else due to ITAR.

Offline Jim

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Re: Starship performance improvements
« Reply #26 on: 11/25/2023 02:15 am »

Thanks deltaV. A few more questions, because I'm genuinely trying to learn; I'm not challenging your assertions.

Why just seconds (out of the ~6 minute burn) early?

How much roll control is required once out of the bulk of the atmosphere? Too much for RCS to handle?

Because engines are not aligned perfectly, they don't have the exact same thrust and engines shutting down induces torques is why gimbaled engines are needed for the whole burn duration.

Differn
Jim, I appreciate that these may seem like stupid questions to you, but your answers simply aren't helping. If the question is worth your time, I'd appreciate it if you give more detail; if it's not, just ignore it.

There will be some sort of roll induced by those variances that differential thrusting can't stop.  Also, differential thrusting is not quick reacting.

You lose control because

Offline Brigantine

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Re: Starship performance improvements
« Reply #27 on: 11/25/2023 02:32 am »
How much roll control is required once out of the bulk of the atmosphere? Too much for RCS to handle?

Because engines are not aligned perfectly, they don't have the exact same thrust and engines shutting down induces torques is why gimbaled engines are needed for the whole burn duration.
Jim, I appreciate that these may seem like stupid questions to you, but your answers simply aren't helping. If the question is worth your time, I'd appreciate it if you give more detail; if it's not, just ignore it.

To interpret Jim a bit... to compensate for engine misalignment and thrust differentials, to maintain the right attitude (pitch/yaw/roll) it takes more than an occasional puff of RCS, you need a trim - a constant baseline correction to offset a constant bias.

Suppose 3 RCs do this with a trim of just 0.6⁰ i.e. 0.01 radians at 50% thrust. That's correcting forces totaling 38 kN applied constantly for the rest of the 2nd stage burn (assuming similar leverage). If using cold gas, that's a ~1.8% hit to your ISP (7s).

Or if you assume you have very fine throttle control of RVacs (pitch & yaw sorted), and your roll RCS has 4x the leverage of the RCs, that's still 10 kN. Assume hot gas thrusters and that's a ~0.2% hit to your ISP (1s).

What force can the current Ship RCS manage?

Also compare with:
Running two sea levels at 40% thrust and 6 Vacuum at full thrust, we get an Isp of 6*378 + .8 * 363 or an average of 376.2

However it's silly to plan a mission on what is likely to happen, you have to plan on single failures.

So if it's a VR that fails it's 5*378 + .8*363 => average of 375.9

The difference between that an a differentially throttled Vacuum Raptor system is .5%, or with a deltaV of 4km/sec 22 m/sec.

There's enough other stuff going on to not worry about 22m/sec.
« Last Edit: 11/25/2023 02:41 am by Brigantine »

Offline Nomadd

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Re: Starship performance improvements
« Reply #28 on: 11/25/2023 02:47 am »
 I keep looking for updates on hot gas thrusters, but haven't seen anything since the early lunar lander award days. Those could relate to several things from lunar landing to killer rcs to putting excess tank gas to good use.
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Offline steveleach

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Re: Starship performance improvements
« Reply #29 on: 11/25/2023 09:01 am »
How much roll control is required once out of the bulk of the atmosphere? Too much for RCS to handle?

Because engines are not aligned perfectly, they don't have the exact same thrust and engines shutting down induces torques is why gimbaled engines are needed for the whole burn duration.
Jim, I appreciate that these may seem like stupid questions to you, but your answers simply aren't helping. If the question is worth your time, I'd appreciate it if you give more detail; if it's not, just ignore it.

To interpret Jim a bit... to compensate for engine misalignment and thrust differentials, to maintain the right attitude (pitch/yaw/roll) it takes more than an occasional puff of RCS, you need a trim - a constant baseline correction to offset a constant bias.

Suppose 3 RCs do this with a trim of just 0.6⁰ i.e. 0.01 radians at 50% thrust. That's correcting forces totaling 38 kN applied constantly for the rest of the 2nd stage burn (assuming similar leverage). If using cold gas, that's a ~1.8% hit to your ISP (7s).

Or if you assume you have very fine throttle control of RVacs (pitch & yaw sorted), and your roll RCS has 4x the leverage of the RCs, that's still 10 kN. Assume hot gas thrusters and that's a ~0.2% hit to your ISP (1s).

What force can the current Ship RCS manage?

Also compare with:
Running two sea levels at 40% thrust and 6 Vacuum at full thrust, we get an Isp of 6*378 + .8 * 363 or an average of 376.2

However it's silly to plan a mission on what is likely to happen, you have to plan on single failures.

So if it's a VR that fails it's 5*378 + .8*363 => average of 375.9

The difference between that an a differentially throttled Vacuum Raptor system is .5%, or with a deltaV of 4km/sec 22 m/sec.

There's enough other stuff going on to not worry about 22m/sec.
Yep, I get that RCS won't cut it for pitch and yaw, but I was under the impression that differential thrust does provide that. It's specifically roll-control that I'm not really understanding. How do single-engine upper stages handle roll control? As I understand it, you can't control roll with a single gimballing engine.

And yes, InterestedEngineer's numbers showed that its not worth it, though I'm not sure where his ISP numbers came from. Regardless, it looks like keeping 3 sea-level engines running at 40% (assuming 6 vac-raps at 100%) only drops overall ISP by 2% or so.

Offline catdlr

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Re: Starship performance improvements
« Reply #30 on: 11/25/2023 10:13 am »
Ok, I'll go out on a limb;

My prediction:
1) Starship will be lengthened by one ring segment (part of Version 2).
2) Three additional R-Vacs are added to Starship (part of Version 2).
3) Extra Starship engines means no need to gimble inner three sea-level raptor engines at Hot staging but they will be running (like they did for IFT-2) along with the Vac-Raptors.
4) Tower extension (allow the Chopstick's added length necessary to lift the lengthened Version-2 Starship up another 10 meters):
  a) The upper tower parts delivered this week from Robert's road will be utilized to increase the overall tower height   
       along with added tower rails.  All will be assembled at the Sanchez site.   
  b) When completed, the new tower top section will fitted on top of the existing tower but the existing pully will stay
       as is until Version-1 starship test is completed (i.e., no interruptions to testing)
  c) When Version-1 testing is completed (timing allows), the cabling will be restrung from the old tower top through
       the pullies of the new tower top
  d)  Old pullies and cantilevered arms supporting them from the original tower top will be set aside for a second tower.
5) SQD will not need to move any higher but will be beefed up to provide added protection from rocket blasts (can be completed before Version-2).

Erata: The chopstix chain control cables may need extra links and cableing extended (possible, I'm not sure).

Correction: Changed length of Version-2 starship to one additional ring segment (instead of the incorrect 10 meters - what was I thinking?)
« Last Edit: 11/25/2023 06:37 pm by catdlr »
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Offline Brigantine

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Re: Starship performance improvements
« Reply #31 on: 11/25/2023 10:16 am »
How do single-engine upper stages handle roll control? As I understand it, you can't control roll with a single gimballing engine.

Well, yes RCS. But AIUI being single engine with the thrust straight through CoM, they don't need to run continuously to trim the roll. Occasional puffs suffice. (no RVac misalignment effects to compensate for)

(I don't think anyone actually uses reaction wheels, KSP style?)

Offline Nomadd

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Re: Starship performance improvements
« Reply #32 on: 11/25/2023 01:42 pm »
Ok, I'll go out on a limb;

My prediction:
1) Starship will be lengthened by one ring segment (10 meters) (part of Version 2).
4) Tower ex8tension (allow the Chopstick's added length necessary to lift the lengthened Version-2 Starship up another 10 meters):
I don't think the word "meter" means what you think it means.
Those who danced were thought to be quite insane by those who couldn't hear the music.

Offline chopsticks

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Re: Starship performance improvements
« Reply #33 on: 11/25/2023 02:08 pm »
How much roll control is required once out of the bulk of the atmosphere? Too much for RCS to handle?

Because engines are not aligned perfectly, they don't have the exact same thrust and engines shutting down induces torques is why gimbaled engines are needed for the whole burn duration.
Jim, I appreciate that these may seem like stupid questions to you, but your answers simply aren't helping. If the question is worth your time, I'd appreciate it if you give more detail; if it's not, just ignore it.

To interpret Jim a bit... to compensate for engine misalignment and thrust differentials, to maintain the right attitude (pitch/yaw/roll) it takes more than an occasional puff of RCS, you need a trim - a constant baseline correction to offset a constant bias.

Suppose 3 RCs do this with a trim of just 0.6⁰ i.e. 0.01 radians at 50% thrust. That's correcting forces totaling 38 kN applied constantly for the rest of the 2nd stage burn (assuming similar leverage). If using cold gas, that's a ~1.8% hit to your ISP (7s).

Or if you assume you have very fine throttle control of RVacs (pitch & yaw sorted), and your roll RCS has 4x the leverage of the RCs, that's still 10 kN. Assume hot gas thrusters and that's a ~0.2% hit to your ISP (1s).

What force can the current Ship RCS manage?

Also compare with:
Running two sea levels at 40% thrust and 6 Vacuum at full thrust, we get an Isp of 6*378 + .8 * 363 or an average of 376.2

However it's silly to plan a mission on what is likely to happen, you have to plan on single failures.

So if it's a VR that fails it's 5*378 + .8*363 => average of 375.9

The difference between that an a differentially throttled Vacuum Raptor system is .5%, or with a deltaV of 4km/sec 22 m/sec.

There's enough other stuff going on to not worry about 22m/sec.
Yep, I get that RCS won't cut it for pitch and yaw, but I was under the impression that differential thrust does provide that. It's specifically roll-control that I'm not really understanding. How do single-engine upper stages handle roll control? As I understand it, you can't control roll with a single gimballing engine.

And yes, InterestedEngineer's numbers showed that its not worth it, though I'm not sure where his ISP numbers came from. Regardless, it looks like keeping 3 sea-level engines running at 40% (assuming 6 vac-raps at 100%) only drops overall ISP by 2% or so.
Yes, as Brigantine said, roll control on single engine upper stages is controlled by RCS. But it works fine in this case because there's only one source of thrust and its alignment is always straight through the center of mass. With multiple fixed engines not thrusting through the center of mass (Raptor vacuum engines) but rather around the periphery, there will always be slight misalignments and a roll motion will be induced. This doesn't happen with single engine upper stage because the thrust always pointing through the center of mass. And relying of differential thrust as your only yaw and pitch control isn't optimal either since turbopump engines don't react as fast as a gimballing engine does.

Offline alang

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Re: Starship performance improvements
« Reply #34 on: 11/25/2023 02:26 pm »
I don't know about the OP, but I wasn't asking whether differential thrust is best in all situations, or claiming that it didn't have downsides.

I was asking about a very specific situation: the ship is on a relatively nominal trajectory and already at a high enough velocity that gravity losses are less of a factor.

Even if in theory differential thrust could steer Starship, it would be something they would only want to do in a crisis.
Very confidently stated, but do you know whether SpaceX agree with you?

Elon Musk has been asked this before, and his answer was that differential thrust was out because it can't handle an engine out scenario.

Sorry, no source. Internet searchability has collapsed like the Roman Empire. It might be one of the Everyday Astronaut interviews.

The empire strikes back:
https://twitter.com/elonmusk/status/1184732755294162944?t=zaFX5s_symy084-aH0K2hg&s=01

Offline ugordan

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Re: Starship performance improvements
« Reply #35 on: 11/25/2023 02:33 pm »
(I don't think anyone actually uses reaction wheels, KSP style?)

On an upper stage of an LV? I'd be hella surprised if anyone ever did.

Offline steveleach

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Re: Starship performance improvements
« Reply #36 on: 11/25/2023 03:38 pm »
The empire strikes back:
Thanks for digging that out.

Sounds like they considered it and concluded it was a bad trade.

Thanks also to everyone who helped me understand all this.

Offline catdlr

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Re: Starship performance improvements
« Reply #37 on: 11/25/2023 06:35 pm »
Ok, I'll go out on a limb;

My prediction:
1) Starship will be lengthened by one ring segment (10 meters) (part of Version 2).
4) Tower ex8tension (allow the Chopstick's added length necessary to lift the lengthened Version-2 Starship up another 10 meters):
I don't think the word "meter" means what you think it means.

Yes, Sir, you are correct.  That's what happens when responding at 3:30 in the morning.  I corrected the post.  (to one ring segment).  Have a great day.  Thanks for the catch.
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Offline oldAtlas_Eguy

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Re: Starship performance improvements
« Reply #38 on: 11/25/2023 10:12 pm »
Ok, I'll go out on a limb;

My prediction:
1) Starship will be lengthened by one ring segment (10 meters) (part of Version 2).
4) Tower ex8tension (allow the Chopstick's added length necessary to lift the lengthened Version-2 Starship up another 10 meters):
I don't think the word "meter" means what you think it means.

Yes, Sir, you are correct.  That's what happens when responding at 3:30 in the morning.  I corrected the post.  (to one ring segment).  Have a great day.  Thanks for the catch.
Only one segment stretch to the prop tank section only solves the increased prop load to get to 1600t. To solve the needed extra volume to stack up to 140t of Starlinks (~100 total full size V2s [50 X 2 stacks])you would need to add also 2 more ring sections to the payload section as well. And now we are at the need for a 10m tower extension that has been alluded to.

A NOTE here is that just 2 Starships so equipped Starlink launches average per month is 2,400 full size V2 Starlinks a year. Also even if such Starship launches costs as much per launch as $75M. The cost of launch of each of these Starlinks would be only$750K each. A capability savings over even the V1.5 costs on F9 of $750K but having 8 times the throughput. Thus an equivalent cost for throughput V1.5s vs the V2 Full of $94K.

Offline catdlr

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Re: Starship performance improvements
« Reply #39 on: 11/26/2023 04:36 am »
Ok, I'll go out on a limb;

My prediction:
1) Starship will be lengthened by one ring segment (10 meters) (part of Version 2).
4) Tower ex8tension (allow the Chopstick's added length necessary to lift the lengthened Version-2 Starship up another 10 meters):
I don't think the word "meter" means what you think it means.

So I made a mistake in my previous statement. I didn't mean to say 1 ring segment, but rather a 10-meter increase in length (approximately 30 feet). I apologize for the confusion. It will require multiple rings to achieve this extension. However, increasing the length will affect the tower's capacity. My prediction is that the tower will need to be modified to accommodate this additional length. Please note that the 10-meter measurement is an approximation and not exact.  IMHO.  So, let's just wait and see, it's just my prediction, I will be wrong somewhere.

https://twitter.com/elonmusk/status/1644868577265172481
« Last Edit: 11/26/2023 10:51 am by catdlr »
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Offline Nevyn72

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Re: Starship performance improvements
« Reply #40 on: 11/26/2023 05:44 am »
Ok, I'll go out on a limb;

My prediction:
1) Starship will be lengthened by one ring segment (10 meters) (part of Version 2).
4) Tower ex8tension (allow the Chopstick's added length necessary to lift the lengthened Version-2 Starship up another 10 meters):
I don't think the word "meter" means what you think it means.

So I made a mistake in my previous statement. I didn't mean to say 1 ring segment, but rather a 10-meter increase in length (approximately 30 feet). I apologize for the confusion. It will require multiple rings to achieve this extension. However, increasing the length will affect the tower's capacity. My prediction is that the tower will need to be modified to accommodate this additional length. Please note that the 10-meter measurement is an approximation and not exact.  IMHO.  So, let's just wait and see, it's just my prediction, I will be wrong somewhere.

10 metres would be at least 5 standard rings...
How would you distribute them among the ships' sections?

Offline catdlr

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Re: Starship performance improvements
« Reply #41 on: 11/26/2023 07:19 am »
Ok, I'll go out on a limb;

My prediction:
1) Starship will be lengthened by one ring segment (10 meters) (part of Version 2).
4) Tower ex8tension (allow the Chopstick's added length necessary to lift the lengthened Version-2 Starship up another 10 meters):
I don't think the word "meter" means what you think it means.



So I made a mistake in my previous statement. I didn't mean to say 1 ring segment, but rather a 10-meter increase in length (approximately 30 feet). I apologize for the confusion. It will require multiple rings to achieve this extension. However, increasing the length will affect the tower's capacity. My prediction is that the tower will need to be modified to accommodate this additional length. Please note that the 10-meter measurement is an approximation and not exact.  IMHO.  So, let's just wait and see, it's just my prediction, I will be wrong somewhere.

10 meters would be at least 5 standard rings...
How would you distribute them among the ships' sections?

That's what I wanted members to start a discussion on. Elon mentioned a 5-10 meter increase. So that may be one ring section for each tank? As for the number of bands per ring section, 4-5??. Maybe he wants to also increase the payload on top of that.

Nevyn72, Good question, Let's hope some smart minds get together to start directing that in this thread.

Looks like the number of RVACs are being discussed here:
https://forum.nasaspaceflight.com/index.php?topic=59953.msg2544001#msg2544001

and here:
https://forum.nasaspaceflight.com/index.php?topic=58758.msg2544006#msg2544006
« Last Edit: 11/26/2023 07:48 am by catdlr »
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Offline Barley

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Re: Starship performance improvements
« Reply #42 on: 11/26/2023 04:53 pm »
Yes, as Brigantine said, roll control on single engine upper stages is controlled by RCS. But it works fine in this case because there's only one source of thrust and its alignment is always straight through the center of mass. With multiple fixed engines not thrusting through the center of mass (Raptor vacuum engines) but rather around the periphery, there will always be slight misalignments and a roll motion will be induced. This doesn't happen with single engine upper stage because the thrust always pointing through the center of mass. And relying of differential thrust as your only yaw and pitch control isn't optimal either since turbopump engines don't react as fast as a gimballing engine does.
Your assuming perfect alignment for a single engine, but imperfect alignment for multiple engines.  Something is wrong with that picture.



Offline eriblo

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Re: Starship performance improvements
« Reply #43 on: 11/26/2023 06:47 pm »
Yes, as Brigantine said, roll control on single engine upper stages is controlled by RCS. But it works fine in this case because there's only one source of thrust and its alignment is always straight through the center of mass. With multiple fixed engines not thrusting through the center of mass (Raptor vacuum engines) but rather around the periphery, there will always be slight misalignments and a roll motion will be induced. This doesn't happen with single engine upper stage because the thrust always pointing through the center of mass. And relying of differential thrust as your only yaw and pitch control isn't optimal either since turbopump engines don't react as fast as a gimballing engine does.
Your assuming perfect alignment for a single engine, but imperfect alignment for multiple engines.  Something is wrong with that picture.
Not really. The center of mass of the stage will follow a curve in 3D space as the propellant decreases. If the tanks are symmetric this curve will be close to a straight line. How many engines can you align with a line?
« Last Edit: 11/26/2023 06:49 pm by eriblo »

Offline Brigantine

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Re: Starship performance improvements
« Reply #44 on: 11/26/2023 09:31 pm »
Your assuming perfect alignment for a single engine, but imperfect alignment for multiple engines.  Something is wrong with that picture.
To clarify, I was considering roll only, assuming a single engine with TVC (e.g. F9 S2 MVac), vs n fixed RVacs.
TVC will put the thrust vector running straight through CoM, even if it is mounted with some misalignment

At present, the RVacs can be fixed, because the RCs with TVC are still part of the picture.

If you took an MVac and removed TVC, you would have pitch and yaw problems. But the roll problems would still be comparatively low, because the moment it isn't 'distance behind CoM' x angular misalignment x thrust, it's only 'positional misalignment' x angular misalignment x thrust. (a single engine is nominally positioned right on the roll axis)
« Last Edit: 11/26/2023 09:36 pm by Brigantine »

Offline catdlr

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Re: Starship performance improvements
« Reply #45 on: 11/26/2023 11:26 pm »
Cross post from https://forum.nasaspaceflight.com/index.php?topic=59871.msg2544151#msg2544151

Scott on his viewpoint on IFT-2 starship second stage starting at 11:00 ending at 15:00 (after reporting all the other worldwide launches, IFT-2 first stage, and hot stagging)

« Last Edit: 11/27/2023 01:06 am by catdlr »
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Offline Jim

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Re: Starship performance improvements
« Reply #46 on: 11/27/2023 01:20 am »
Yes, as Brigantine said, roll control on single engine upper stages is controlled by RCS. But it works fine in this case because there's only one source of thrust and its alignment is always straight through the center of mass. With multiple fixed engines not thrusting through the center of mass (Raptor vacuum engines) but rather around the periphery, there will always be slight misalignments and a roll motion will be induced. This doesn't happen with single engine upper stage because the thrust always pointing through the center of mass. And relying of differential thrust as your only yaw and pitch control isn't optimal either since turbopump engines don't react as fast as a gimballing engine does.
Your assuming perfect alignment for a single engine, but imperfect alignment for multiple engines.  Something is wrong with that picture.


A single engine has to be able to gimbal.



« Last Edit: 11/27/2023 01:20 am by Jim »

Offline chopsticks

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Re: Starship performance improvements
« Reply #47 on: 11/27/2023 03:07 am »
Yes, as Brigantine said, roll control on single engine upper stages is controlled by RCS. But it works fine in this case because there's only one source of thrust and its alignment is always straight through the center of mass. With multiple fixed engines not thrusting through the center of mass (Raptor vacuum engines) but rather around the periphery, there will always be slight misalignments and a roll motion will be induced. This doesn't happen with single engine upper stage because the thrust always pointing through the center of mass. And relying of differential thrust as your only yaw and pitch control isn't optimal either since turbopump engines don't react as fast as a gimballing engine does.
Your assuming perfect alignment for a single engine, but imperfect alignment for multiple engines.  Something is wrong with that picture.
No, I'm also assuming imperfect alignment on a single engine as well. But it doesn't matter because the single point of thrust (gimballing) is always going to be pointing through the COM and no roll motion will be imparted.

Offline Twark_Main

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Re: Starship performance improvements
« Reply #48 on: 11/27/2023 04:05 am »
Yes, as Brigantine said, roll control on single engine upper stages is controlled by RCS. But it works fine in this case because there's only one source of thrust and its alignment is always straight through the center of mass. With multiple fixed engines not thrusting through the center of mass (Raptor vacuum engines) but rather around the periphery, there will always be slight misalignments and a roll motion will be induced. This doesn't happen with single engine upper stage because the thrust always pointing through the center of mass. And relying of differential thrust as your only yaw and pitch control isn't optimal either since turbopump engines don't react as fast as a gimballing engine does.
Your assuming perfect alignment for a single engine, but imperfect alignment for multiple engines.  Something is wrong with that picture.
No, I'm also assuming imperfect alignment on a single engine as well. But it doesn't matter because the single point of thrust (gimballing) is always going to be pointing through the COM and no roll motion will be imparted.


This assumes exactly zero angular momentum ("swirl") in the exhaust plume gases, which generally is not a valid assumption.
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Offline chopsticks

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Re: Starship performance improvements
« Reply #49 on: 11/27/2023 04:12 am »
Sorry, I should have said minimal then. I think you knew what I meant.

Offline chopsticks

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Re: Starship performance improvements
« Reply #50 on: 11/27/2023 04:14 am »
Point is, whatever roll motion imparted by a single engine is going to much less than multiple engines mounted at the periphery.

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