Author Topic: Starship performance improvements  (Read 14383 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.


Offline 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.

Offline 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.
"The search for a universal design which suits all sites, people, and situations is obviously impossible. What is possible is well designed examples of the application of universal principles." ~~ David Holmgren

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.

Online 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.

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