After trying it out in Kerbal Space Program for a while, I was wondering why NASA doesn't fly on suborbital trajectories to the Moon. When I tested this idea in Kerbal Space Program I found out that you would save fuel, time and money by building a rocket that would fly to the Moon. The flight would take about 20 hours instead of 3 to 7 days and the rocket would have 2 stages, but I did manage to do it with a single staged rocket. The first stage would separate and then land with a parachute, while the second stage would go on to fly to the Moon. I have done my own research and found no article saying anything about taking a rocket to the Moon on a suborbital flight. I am hoping that possibly I might find the answer here.
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asked Apr 26, 2022 at 12:31
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1$\begingroup$ Are you specifically interested in NASA and crewed spaceflight, or in any agencies and also uncrewed spaceflight? $\endgroup$– gerritApr 26, 2022 at 12:55
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4$\begingroup$ This sounds like a direct ascent trajectory as contemplated for Apollo. $\endgroup$– Alex HajnalApr 26, 2022 at 13:46
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9$\begingroup$ If it goes to the moon it can't be in any way considered suborbital. $\endgroup$– GdDApr 26, 2022 at 13:47
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1$\begingroup$ @GdG that's simply incorrect, you could launch out of the cape straight over the south pole on a technically-suborbital trajectory & intersect the lunar position, with the right timing $\endgroup$– Anton HengstApr 26, 2022 at 14:38
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3$\begingroup$ @AntonHengst, my understanding is that one of the definitions of a suborbital spaceflight is that it does not have the velocity to reach orbit, that would not be the case if a spacecraft had the velocity to reach the moon. $\endgroup$– GdDApr 26, 2022 at 15:02
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2 Answers
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A "suborbital" trajectory to the moon could be done--simply never raise your periapsis above 0. However, there's no reason to. By the time you're at the moon the difference between a periapsis on the surface vs a periapsis in low orbit comes down to 5 m/s.
To save that 5 m/s you have to accept an instantaneous launch window (no waiting around in orbit to get in the right spot) and you'll probably waste more in not having as accurate measurement of your position.
I rather suspect what you saved in KSP is even less and I'm sure you used more trying to do a manual burn than if you had used MechJeb to control the burn. (MechJeb has finer control than the keyboard.)
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1$\begingroup$ When I was in KSP I waited on the ground until the moon was about 80 degrees away from the Station (Doesn‘t have to exactly 80 degrees. Just has to be a bit less than 90). Then I launched my rocket upwards until the Apogee was where the moon would be. After that I just time warped ahead and finally I landed on the moon the normal way like if you started from an earth orbit. I didn’t spend any time or fuel trying to get to the right spot because when I launched, I flew straight up on the correct trajectory not needing to correct my trajectory. $\endgroup$ May 18, 2022 at 4:37
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$\begingroup$ @TheRocketfan The way you did it most certainly cost you more fuel than the standard approach would have. Probably quite a bit more. $\endgroup$ May 18, 2022 at 4:54
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$\begingroup$ I was shocked how little fuel it took $\endgroup$ May 18, 2022 at 4:56
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$\begingroup$ do you have any reason why it would take more fuel $\endgroup$ May 18, 2022 at 4:57
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The reason that NASA does not fly on suborbital flights to the moon is because of one major factor. If you somehow got the needed delta V within a second then the delta V needed would be less. The issue is the Hohmann effect. Since the rocket is flying straight up from the earth the effect from the delta V is getting smaller. At a certain point you would need more delta V to fly on a suborbital flight to the moon. Since NASA rockets like the SLS or the Saturn V do not accelerate fast enough, it would not make sense to fly on a suborbital trajectory. If you had a model rocket that could produce the delta V needed and maintane around 5G acceleration then it would be more fuel efficent to fly on a suborbital flight to the moon.
answered Jun 28, 2022 at 12:03