I’m guessing that it wouldn’t take as much energy to accelerate it to 25K mph, since it wouldn’t have atmospheric drag to consider–but would it have been any really noticable amount?
Of course, with no atmosphere, we have a lot of other problems that aren’t really part of this thought experiment.
The launch trajectory for vehicles intended to orbit the earth (this includes the initial phase of each Apollo mission) is to first move on a path that’s nearly vertical so as to get above the densest part of the atmosphere ASAP; as they get above most of the atmosphere, they gradually redirect to a path that’s closer to horizontal to build up lateral speed and achieve orbit. Moreover, the space shuttle has a phase of its ascent where they limit their acceleration so as to not subject the vehicle to excessive aerodynamic pressures. I don’t know whether the Saturn V rockets had the same max-Q problem (and a requirement for temporarily limiting acceleration) that the space shuttle did.
Not only does the atmosphere bleed off kinetic energy via aerodynamic drag, but rocket engines are less efficient (in terms of mechanical work imparted to the vehicle) when the vehicle is moving more slowly. If you want an efficient rocket-powered vehicle, your goal is to build up a lot of speed early on. In the absence of an atmosphere, you might see a flight path that turns into something close to horizontal very soon after liftoff. And since you wouldn’t have any concerns about aerodynamic stresses, you wouldn’t have to deliberately limit your velocity while at lower altitudes (as the space shuttle did). Presto: you are free to build up your velocity as rapidly as you can, maximizing engine efficiency with respect to vehicle speed.
Also, rocket engines behave differently in environments with different ambient pressure. The design of any rocket that must operate within a wide range of ambient pressures (e.g. one that will achieve orbit from sea level) is a compromise; see here for an explanation of why). If there’s no atmosphere, the rocket engine design can be optimized for operation in a vacuum, maximizing its efficiency with respect to the operating environment (subject to constraints of engine bell size).
So yes, in the absence of an atmosphere, theoretically you should be able to put the same payload into orbit with smaller, more efficient engines and less fuel. Given the feedback loop that’s at work there - smaller engines and less fuel means a lighter vehicle, which means you can redesign again with even smaller engines and even less fuel - I would bet the effect is pretty significant.
Related, would a hovering rocket (zero velocity) be more efficient in a atmosphere due to ‘backpressure’? (Along with most likely a negligible buoyancy effect. )
But you lose all your savings back, with ruinous interest, by now having to carry a descent motor and fuel for it.
I recall something about rockets being less efficient in an atmosphere, but that’s based on fixed nozzle shape. Don’t know what would happen if you had a nozzle optimized for a vacuum.
Check the link in my previous post…
Ok, but I don’t see a clear answer (maybe it’s my eyes). Given different nozzles optimized for vacuum and atmosphere, is there a difference in efficiency in their approriate environments? I realize that you would have to optimize the combustion chamber, pumps, injection system, and maybe cooling as well. But is there some general principal that would give an advantage to operating in either environment.
Yes. Vacuum-optimized engines have slightly higher ISP than sea level optimized engines, all other things being equal. Rocket engines are basically gaining thrust by the work done by the expansion of the rocket exhaust. At sea level, you can only expand the exhaust to sea level air pressure. In a vacuum, you can harness the expansion of the exhaust further, to much closer to zero pressure. You can’t go all the way to zero, as that would take an infinitely large exhaust nozzle, and you will have a slightly heavier engine due to having a larger exhaust nozzle, but you will get more thrust for the same amount of fuel burned.
Thank you AndrewL. Makes perfect sense too.
We can see a real-life example of this in the Merlin 1 engine used in the SpaceX Falcon 9 rocket. The Falcon 9 uses essentially the same rocket in the first and second stages. The first-stage engine is optimized for sea level operation, and generates 125,000 pounds of thrust. The second stage, vacuum-optimized engine generates 138,000 pounds of thrust for the same fuel use.
So, our hypothetical vacuum-optimized Saturn V can have its first stage engines fitted with larger nozzles and gain a slight performance increase. I don’t know offhand how much, but it will probably be comparable to the gain the Merlin 1 gets, since they’re running on the same fuel and have a very similar design. While you’re at it, you can remove the fins and aerodynamic farings around the engines, since they aren’t doing you any good in a vacuum. This is all first-stage performance benefits, which doesn’t help you as much as bonuses in later stages would.
You can also remove the parachutes and heat shield from the capsule, since without an atmosphere they won’t do anything. Remove the escape system while you’re at it, since without a parachute it won’t be any use. I hope your astronauts are okay with not landing, ever.