5 ms·

You're eliminating the first part of the flight, so the rocket just has to provide the delta-V for orbital insertion, not lift the fuel to do that from sea leve

by zelos 12y ago

You're eliminating the first part of the flight, so the rocket just has to provide the delta-V for orbital insertion, not lift the fuel to do that from sea level. The Tsiolkovsky equation suggests that's going to be a big saving: http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation And yes, my only qualification is Kerbal as well ;-)

lmm 12y ago
> You're eliminating the first part of the flight, so the rocket just has to provide the delta-V for orbital insertion, not lift the fuel to do that from sea level. Sure, but the altitude gain is almost "free" - you need to accelerate by something like 6 km/s laterally (number pulled out of somewhere, it's 2.3 in kerbal space program but the planets are smaller there) and 1 km/s vertically (depending on ascent profile, 0 for an instantaneous ascent and infinite for a slow hover upwards, but it ends up something like that), so by Pythagoras it's sqrt-37 = really not a lot more than the 6 kms/s you'd have to spend if you could start at 100km altitude. Intuitively we assume rockets go "upwards" partly because that's the part we see, but actually the gravity turn occurs very early and most of the acceleration is closer to horizontal. E.g. the less-crazy of the space shuttle abort plans if something went wrong early in flight was to land at Shannon.
- berntb 12y ago
  It is an hour, someone much more knowledgeable than me should have answered? OK, I'll give it a go. Rocket engine nozzle designs are optimized depending on the external pressure. If the rocket engines can start at a higher altitude, it is a nice win. http://en.wikipedia.org/wiki/Rocket_engine_nozzle#Atmospheric_use http://en.wikipedia.org/wiki/Rocket_engine_nozzle#Atmospheri... Also, it is quite a lot of extra drag at sea level, so starting from higher up is another benefit in that way. It is not obvious if those advantages is larger than the weight of the wings and the weight of robust wheels etc to start horizontally with all that propellant weight. This is an alternative design: http://www.ai.mit.edu/projects/im/magnus/bh/analog.html http://www.ai.mit.edu/projects/im/magnus/bh/analog.html
pjc50 12y ago
Another KSP observation: air resistance is much more of an issue at sea level, and it lowers specific impulse.

by zelos 12y ago

lmm 12y ago

> You're eliminating the first part of the flight, so the rocket just has to provide the delta-V for orbital insertion, not lift the fuel to do that from sea level. Sure, but the altitude gain is almost "free" - you need to accelerate by something like 6 km/s laterally (number pulled out of somewhere, it's 2.3 in kerbal space program but the planets are smaller there) and 1 km/s vertically (depending on ascent profile, 0 for an instantaneous ascent and infinite for a slow hover upwards, but it ends up something like that), so by Pythagoras it's sqrt-37 = really not a lot more than the 6 kms/s you'd have to spend if you could start at 100km altitude. Intuitively we assume rockets go "upwards" partly because that's the part we see, but actually the gravity turn occurs very early and most of the acceleration is closer to horizontal. E.g. the less-crazy of the space shuttle abort plans if something went wrong early in flight was to land at Shannon.

berntb 12y ago
It is an hour, someone much more knowledgeable than me should have answered? OK, I'll give it a go. Rocket engine nozzle designs are optimized depending on the external pressure. If the rocket engines can start at a higher altitude, it is a nice win. http://en.wikipedia.org/wiki/Rocket_engine_nozzle#Atmospheric_use http://en.wikipedia.org/wiki/Rocket_engine_nozzle#Atmospheri... Also, it is quite a lot of extra drag at sea level, so starting from higher up is another benefit in that way. It is not obvious if those advantages is larger than the weight of the wings and the weight of robust wheels etc to start horizontally with all that propellant weight. This is an alternative design: http://www.ai.mit.edu/projects/im/magnus/bh/analog.html http://www.ai.mit.edu/projects/im/magnus/bh/analog.html

pjc50 12y ago

Another KSP observation: air resistance is much more of an issue at sea level, and it lowers specific impulse.