From: snidely.too@gmail.com   
      
   Remember  Monday, when  Alain Fournier asked plaintively:   
   > 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.   
      
   The RVACs are known (by design and test results so far) to not tear   
   when used in the atmosphere.  They can be used at sea level.  This is,   
   of course, made possible by trade-offs ... some vacuum efficiency is   
   sacraficed to provide more stability down low.  I believe this was   
   discussed during Tim Dodd's tour, as well as being mentioned in other   
   places.   
      
   > 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.   
      
   /dps   
      
   --   
   Who, me?  And what lacuna?   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)