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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.

No Nonsense
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The Rocket fan
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    Are you specifically interested in NASA and crewed spaceflight, or in any agencies and also uncrewed spaceflight? – gerrit Apr 26 '22 at 12:55
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    This sounds like a direct ascent trajectory as contemplated for Apollo. – Alex Hajnal Apr 26 '22 at 13:46
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    If it goes to the moon it can't be in any way considered suborbital. – GdD Apr 26 '22 at 13:47
  • I don't understand your question. Are you talking about a one-way trip or does the payload include a module that allows a return from the moon? Are you asking if a direct-ascent rocket is possible, or why one wasn't used for Apollo? – GdD Apr 26 '22 at 14:13
  • @AlexHajnal the "direct ascent" of the direct ascent trajectory is as opposed to assembly in LEO or lunar orbit rendezvous. It still involved an orbital launch. – Christopher James Huff Apr 26 '22 at 14:33
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    @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 – Anton Hengst Apr 26 '22 at 14:38
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    @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. – GdD Apr 26 '22 at 15:02
  • What I am trying to say is that you would fly straight up on big suborbital flight. You wouldn't bother changing your trajectory so you would be in an orbit. I mean that the rocket would fly and go on a trajectory going so far that it reaches the moon but that it didn't go in an orbit. If it misses the moon it would fall straight back to earth and not fly again – The Rocket fan Apr 26 '22 at 15:09
  • @AlexHajnal yes I am talking about a Direct ascent trajectory. – The Rocket fan Apr 26 '22 at 15:13
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    This answer could help – BrendanLuke15 Apr 26 '22 at 15:16
  • Comments are not for extended discussion; this conversation has been moved to chat. – called2voyage Apr 26 '22 at 17:37

2 Answers2

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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.)

Loren Pechtel
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    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. – The Rocket fan May 18 '22 at 04:37
  • @TheRocketfan The way you did it most certainly cost you more fuel than the standard approach would have. Probably quite a bit more. – Loren Pechtel May 18 '22 at 04:54
  • Actually it really didn’t – The Rocket fan May 18 '22 at 04:55
  • I was shocked how little fuel it took – The Rocket fan May 18 '22 at 04:56
  • do you have any reason why it would take more fuel – The Rocket fan May 18 '22 at 04:57
  • @TheRocketfan You incurred more gravity loss by not building orbital velocity as soon as feasible and you certainly didn't get a clean insertion at Mun on that trajectory. Trying to compare "fuel" doesn't work very well, any real comparison needs to be in Δv. The by-the-book answer is 5150m/s to the surface of Mun. – Loren Pechtel May 18 '22 at 05:01
  • We could compare it in delta V. The amount of delta V I used was far less then what would be needed for an orbital flight. The reason that I didn’t have to reach the orbital speed to get to the orbit. Yes indeed I went as fast as you would go in an orbit just that my direction was different. I wasn’t going around Kerban but instead was flying away from it with that speed. – The Rocket fan May 18 '22 at 05:05
  • To have the maximum effect from the change of delta v you would need to turn on the engines at the lowest part of your orbit. The further away you are the more delta v you would need. When you go on a suborbital flight to the moon you have to get to the speed with a certain amount of time before it becomes it inefficient. – The Rocket fan May 18 '22 at 05:13
  • @TheRocketfan Of course. That's what that 5150m/s assumes--all burns as close to the bodies as reasonable. – Loren Pechtel May 18 '22 at 23:42
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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.

The Rocket fan
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