This week, Musk tweeted that:
Booster will have 33 Raptor V2.0 engines
and
Starship will have 3 sea level gimballing engines, and 6 fixed vaccum engines. (so moving from 6 to 9 engines) with increased propellant load.
For starship, would "increased propellant load" mean that that structure itself will grow taller? or are they growing tanks by reducing payload volume? If the ship itself grows taller, is that a major change in
strcture? or did they realize that their current design/steel has the strength to grow taller?
If Raptor V2.0 is much better, I can understand seeing Booster with more
umph and thus able to lift heavier Sharship, hece ability to lift more
fuel. But why the exra vaccuum engines in Sharship?
I was always under the impression that once dropped by a stage 1, a
stage 2 has luxury of time to accelerate to orbit. Does the addition of
3 engines to Starship mean that in the end, they don't have that luxury
and with the mass it has, it needs to finish acceleration to orbital
speed at faster rate?
Since Raptor 2.0 is supposed to be the "new and improved" version, once
would have expected the need for fewer engines to achieve same thrust
instead of need to add more.
Since only sea level engines will gimbal, is it fair to state that they
will be fired up after stage 1 separation to help push Starship to
orbital speed? or will they sue differential thrust on the 6 fixed
vaccum to achieve directional control?
In the case of the Moon shuttle, since Starship woll operate excusively
in vaccuum, can we expect it to have a different mix of engines with a sportion of vaccum engines having gimbals?
This week, Musk tweeted that:
Booster will have 33 Raptor V2.0 engines
and
Starship will have 3 sea level gimballing engines, and 6 fixed vaccum engines. (so moving from 6 to 9 engines) with increased propellant load.
For starship, would "increased propellant load" mean that that structure itself will grow taller? or are they growing tanks by reducing payload volume? If the ship itself grows taller, is that a major change in
strcture? or did they realize that their current design/steel has the strength to grow taller?
If Raptor V2.0 is much better, I can understand seeing Booster with more
umph and thus able to lift heavier Sharship, hece ability to lift more
fuel. But why the exra vaccuum engines in Sharship?
I was always under the impression that once dropped by a stage 1, a
stage 2 has luxury of time to accelerate to orbit. Does the addition of
3 engines to Starship mean that in the end, they don't have that luxury
and with the mass it has, it needs to finish acceleration to orbital
speed at faster rate?
Since Raptor 2.0 is supposed to be the "new and improved" version, once
would have expected the need for fewer engines to achieve same thrust
instead of need to add more.
Since only sea level engines will gimbal, is it fair to state that they
will be fired up after stage 1 separation to help push Starship to
orbital speed? or will they sue differential thrust on the 6 fixed
vaccum to achieve directional control?
In the case of the Moon shuttle, since Starship woll operate excusively
in vaccuum, can we expect it to have a different mix of engines with a sportion of vaccum engines having gimbals?
On Dec/18/2021 at 19:06, JF Mezei wrote :
This week, Musk tweeted that:
Booster will have 33 Raptor V2.0 engines
and
Starship will have 3 sea level gimballing engines, and 6 fixed vaccum
engines. (so moving from 6 to 9 engines) with increased propellant load.
For starship, would "increased propellant load" mean that that structure
itself will grow taller? or are they growing tanks by reducing payload
volume? If the ship itself grows taller, is that a major change in
strcture? or did they realize that their current design/steel has the
strength to grow taller?
If Raptor V2.0 is much better, I can understand seeing Booster with more
umph and thus able to lift heavier Sharship, hece ability to lift more
fuel. But why the exra vaccuum engines in Sharship?
I was always under the impression that once dropped by a stage 1, a
stage 2 has luxury of time to accelerate to orbit. Does the addition of
3 engines to Starship mean that in the end, they don't have that luxury
and with the mass it has, it needs to finish acceleration to orbital
speed at faster rate?
Since Raptor 2.0 is supposed to be the "new and improved" version, once
would have expected the need for fewer engines to achieve same thrust
instead of need to add more.
Since only sea level engines will gimbal, is it fair to state that they
will be fired up after stage 1 separation to help push Starship to
orbital speed? or will they sue differential thrust on the 6 fixed
vaccum to achieve directional control?
In the case of the Moon shuttle, since Starship woll operate excusively
in vaccuum, can we expect it to have a different mix of engines with a
sportion of vaccum engines having gimbals?
I don't have answers to your questions. But I can give you some general information about sea level vs vacuum engines and about engine counts.
The difference between sea level engines and vacuum engines is mainly the length of the nozzle. Longer nozzle make more efficient engines but can make the engine unstable in an atmosphere. That means that sea level engines will work fine in a vacuum but will be less efficient than vacuum engines. On the other hand, vacuum engines will tend to tear apart if used in an atmosphere.
A higher engine count (for engines with similar power) will make you accelerate faster, which will save you on gravity loss. But when you have reached orbit you basically no longer suffer gravity loss. If you haven't yet reached orbital speed but you are close you will have some gravity loss, but it will be small. So once near orbital speed extra engines tend to be not very useful extra weight. Therefore the last stages of a multistage rocket will usually have a lower fraction of its mass in engines. Once you are in orbit to go higher up or to escape, it is more efficient to have longer burn times with less engines.
So, a "traditional" rocket that you want to use to send stuff very high or escape Earth's gravity, usually has a smaller fraction of its mass for engines in upper stages. On the other hand if you want to optimise the rocket for LEO, your engines are used before reaching orbit while you can suffer gravity loss, and you might want to have more engines to avoid gravity loss.
So why would SpaceX put extra engines on Starship which they want to use to go to Mars? Well Starship is not a traditional rocket. For trips to Mars, they want to refuel it in LEO. So when Starship is used to launch satellites in LEO, you want more power to avoid gravity loss. And when it is used to go to Mars, well you are really using it to go to LEO first and you are willing to use extra mass to avoid gravity loss. Then you refill, at that point, the extra engines are not very useful to goto Mars but you still have them because you needed them to avoid gravity loss at launch and they will be useful to avoid gravity loss when you will launch from Mars.
The extra engines are not the sea level engines, they are the 3
additional vacuum engines (RVACs). You don't need sea level engines
in LEO, you only need them during launch and landing. But they are
still useful in orbit, despite reduced efficiency.
On 2021-12-20 16:19, Snidely wrote:
The extra engines are not the sea level engines, they are the 3
additional vacuum engines (RVACs). You don't need sea level engines
in LEO, you only need them during launch and landing. But they are
still useful in orbit, despite reduced efficiency.
Does adding mass for 3 additional engines matter much in the grand
scheme of things? I recall criticism of Space Shuttle having to
accelerate its SSMEs to orbital speed as dead weight since it was OMS
that did the work.
I gather this is some fancy equation of accelerating with lighter mass
(6 engines) vs accelerating with higher mass ( 9 engines) but greater
thrust. But those extra 3 engines still have to be lifted from ground
before they kick in.
Do second stages typically have to throttle back to maintain comfortable
3G when there are humans on-board?
Curious to see if the existing 6
engiunes can reach that 3G easily after being dropped off in semi orbit
bu the booster, or whether they really need those extra 3 to reach 3G.
In terms of lunar lander, I was under the impression that the "main"
engines would be used for orbital ops, and the "side mounted" thrusters
would be used in proximity of lunar ground (last stage of landingk first stage of taking off).
BTW, if the Raptors can't be used to land on the Moon where regolith
obeys laws of physics (pushed sideways) could they be used to land on
Mars where the atmosphere causes the dust/rocks to behave more
unpredictably?
Just this Saturday, JF Mezei explained that ...
This week, Musk tweeted that:
Booster will have 33 Raptor V2.0 engines
and
Starship will have 3 sea level gimballing engines, and 6 fixed vaccum
engines. (so moving from 6 to 9 engines) with increased propellant load.
For starship, would "increased propellant load" mean that that structure
itself will grow taller? or are they growing tanks by reducing payload
volume? If the ship itself grows taller, is that a major change in
strcture? or did they realize that their current design/steel has the
strength to grow taller?
If Raptor V2.0 is much better, I can understand seeing Booster with more
umph and thus able to lift heavier Sharship, hece ability to lift more
fuel. But why the exra vaccuum engines in Sharship?
I was always under the impression that once dropped by a stage 1, a
stage 2 has luxury of time to accelerate to orbit. Does the addition of
3 engines to Starship mean that in the end, they don't have that luxury
and with the mass it has, it needs to finish acceleration to orbital
speed at faster rate?
Since Raptor 2.0 is supposed to be the "new and improved" version, once
would have expected the need for fewer engines to achieve same thrust
instead of need to add more.
Since only sea level engines will gimbal, is it fair to state that they
will be fired up after stage 1 separation to help push Starship to
orbital speed? or will they sue differential thrust on the 6 fixed
vaccum to achieve directional control?
In the 3+3 configuration, it was certainly expected that all the engines would fire on the way to orbit. If the purpose of the engine count revision is to allow bigger payloads, then I would expect the sea level engines to be asked to contribute some kick. But it is probably too early for anyone not working on Starship to answer.
| Sysop: | Keyop |
|---|---|
| Location: | Huddersfield, West Yorkshire, UK |
| Users: | 296 |
| Nodes: | 16 (2 / 14) |
| Uptime: | 89:31:36 |
| Calls: | 6,658 |
| Files: | 12,203 |
| Messages: | 5,334,031 |