From: alain245@videotron.ca   
      
   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.   
      
      
   Alain Fournier   
      
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