space engines should have larger bells

[jab136]

IRL engines designed for vacuum have much larger bells than those designed for atmosphere, this is because as velocity rises, pressure drops due to bernoulli's principle. Also, engine performance is highest when exit pressure matches that of the atmosphere around the craft, since this isn't possible in space, they try to bring the pressure as low as possible to get the highest exit velocity and therefore highest Isp with the limiting factor generally being weight of the engine. this is just something that has been bugging me, and I realize it is at least partly so that landing legs only work with certain engines, but still, if we are trying to properly apply thrust/Isp curves with atmosphere, we should be getting proper models.

[Coyote27]

What if there was a vacuum lander engine with a retractable bell? You could operate it with the bell extended for better ISP most of the time, and then retract it just before the ground when you lower the landing legs so it'll clear.

[jab136]

or you could just not have a retractable bell, because that would just add a ton of weight and be completely impractical, and instead, just build your landers in such a way as to not have an issue with getting the landing gear to the ground

[cfds]

Oh yes, so much this. There should be three "types" of engine:

- First stage engine, optimised for atmosphere: smallish bell, because of this a high TWR

- Vacuum engine: heavy high expansion bell -> high vacuum ISP, bad(ish) TWR

- Lander engine: Low profile, pays with both badish ISP and TWR

[nothingSpecial]

What about longer yet smaller bell for landers, 0.625 ones? High ISP, low TWR for Kerbin circularizing yet enough for landing on non-atmoshered bodies such as Mun and Minmus?

And larger bells for Terrier and Poodle, so Apollo-alikes can be done.

[NathanKell]

Ven's Stock Revamp is a good example of this, everything fits in the original sizes but the bells are sane.