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Thread: Would it be possible to jetpack back to Earth from the Moon?

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    Would it be possible to jetpack back to Earth from the Moon?

    I read that NASA had considered using jetpacks on their 60s-70s exploration of the Moon.

    Would it be possible to get back to Earth from the Moon with some kind of jetpack system?

    Or would that take too much fuel, and take too long?

    And then there is the problem of the heat from re-entry.
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    Quote Originally Posted by WaxRubiks View Post
    I read that NASA had considered using jetpacks on their 60s-70s exploration of the Moon.

    Would it be possible to get back to Earth from the Moon with some kind of jetpack system?
    The problem is “some kind of.” I think it depends on how loosely you use it. It would have to be quite a big Jetpack I imagine. And like you said, the teen try would be tricky. I think it would be preferable to be inside.


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    Quote Originally Posted by Jens View Post
    the teen try would be tricky.
    I had to read that three times, then the OP again, before I figured out that some auto-correct probably ruined your "re-entry". But I had a fun time trying to put a meaning to tricky teen tries.
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    Quote Originally Posted by slang View Post
    I had to read that three times, then the OP again, before I figured out that some auto-correct probably ruined your "re-entry". But I had a fun time trying to put a meaning to tricky teen tries.
    Sorry, yes, that was sent from my iPhone. Funny creature. I guess it's a tricky teen itself. But probably it was a mixture of typo and auto-correct. The keyboard is really small and it's easy to hit the wrong key.
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    Quote Originally Posted by Jens View Post
    Sorry, yes, that was sent from my iPhone. Funny creature. I guess it's a tricky teen itself. But probably it was a mixture of typo and auto-correct. The keyboard is really small and it's easy to hit the wrong key.
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    Many people are confused by escape velocity. They think it means you cannot escape a planet without exceeding its escape velocity.

    This is not true.

    You cannot escape a planet without exceeding escape velocity by the time you run out of fuel.

    If Some Kind Of Jetpack(TM. WaxRubiks) had sufficient fuel (and at least 1.1g of thrust), you could toddle all the way to the Moon at 1mph.


    Same with teen try.

    Heating during re-entry is because it is using unpowered atmospheric braking from an orbital velocity of 25,000mph.

    With sufficient fuel, you could bypass orbit completely and happily descend from the Moon, through the atmosphere, straight down to Earth at a leisurely 1mph.


    Escape velocity and re-entry heating (and orbits, while we're at it) are entirely artifacts of limited fuel.
    Last edited by DaveC426913; 2019-Feb-03 at 07:24 PM.

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    Quote Originally Posted by DaveC426913 View Post

    Same with teen try.
    Teen try strikes again!!

    Also a wonderful point about escape velocities only being an issue with our limited fuel rockets that we use, I hadn't thought of that.

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    Escape energy is what's important. Escape velocity is just what happens if all the energy is delivered at the start of the trajectory.

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    Quote Originally Posted by DaveC426913 View Post

    You cannot escape a planet without exceeding escape velocity by the time you run out of fuel.

    If Some Kind Of Jetpack(TM. WaxRubiks) had sufficient fuel (and at least 1.1g of thrust), you could toddle all the way to the Moon at 1mph.
    But I think that's precisely what the discussion is about (I don't think anybody brought up escape velocity). How much fuel would you need to get back to the earth, and could you get it into a jetpack? It seems that Apollo used about 4 tons of fuel to get back, and that was with three astronauts, so the weight would be a little less and I suppose you would need less fuel. But still, maybe a few tons. Now, would you call a device strapped to you with 2 tons of fuel a jetpack?


    Quote Originally Posted by DaveC426913 View Post
    Same with teen try.
    And yes, teen try would be hard with a jetpack. You certainly wouldn't want to be strapped to the front of the device! I think it would be preferable to be inside the "jetpack".
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    Teenage Ninja Astronauts, springs to mind...
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    Quote Originally Posted by Jens View Post
    But I think that's precisely what the discussion is about (I don't think anybody brought up escape velocity). How much fuel would you need to get back to the earth, and could you get it into a jetpack? It seems that Apollo used about 4 tons of fuel to get back, and that was with three astronauts, so the weight would be a little less and I suppose you would need less fuel. But still, maybe a few tons. Now, would you call a device strapped to you with 2 tons of fuel a jetpack?




    And yes, teen try would be hard with a jetpack. You certainly wouldn't want to be strapped to the front of the device! I think it would be preferable to be inside the "jetpack".
    using 4tonnes of fuel for return: most of that would be to bring back the various component stages, I would guess, so to get back one person with a jetpack should be a lot less.

    I just read the lunar module:

    The dry mass of the ascent stage was 2445 kg and it held 2376 kg of propellant.
    https://nssdc.gsfc.nasa.gov/nmc/spac...n?id=1969-059C

    So that's around 2kg of fuel for every kg of ascent stage mass..

    say a jetpack with fuel plus astronaut weighed 320kgs, that doesn't seem to be too much, for a return to a command module type thing.

    Maybe some future explorations could just involve having a command module and jetpacks to go down to the lunar surface, and return to the CM,,,when I say 'jetpack', I just mean some kind of small one manned vehicle....when the astronaut got to the surface, there could be a secondary jetpack to fly around the surface.
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    Would it be possible to jetpack back to Earth from the Moon?

    But those Lunar lander weight numbers only take into account two astronauts and the ascent stage to rendezvous with the command and service module. Once docked and the astronauts back onboard the CSM, the LEM was discarded. The crew then still needed to burn a lot of fuel - firing for 180 seconds - to break free from Lunar orbit and return to Earth.

    ETA: oops, never mind. I misread the last post. You were specifically talking about a jetpack for return to Lunar orbit, not Earth.

    You might like this article about a concept to provide astronauts with jet packs. On the moon.

    http://blogs.discovermagazine.com/vi.../#.XFfzbqROmEc

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    Last edited by schlaugh; 2019-Feb-04 at 08:12 AM.

  13. #13
    Quote Originally Posted by WaxRubiks View Post
    Maybe some future explorations could just involve having a command module and jetpacks to go down to the lunar surface, and return to the CM,,,when I say 'jetpack', I just mean some kind of small one manned vehicle....when the astronaut got to the surface, there could be a secondary jetpack to fly around the surface.
    That's not too far from something that was considered. There were some drawings of something looking like a couple of lawn chairs tied together with some aluminium tubing, and some rockets strapped on. The life support would have come from the astronauts' spacesuits, rather than from the vehicle. Not exactly a personal jetpack (or rocketpack, since it needs its own oxygen), but rather close. So they would descend from a vehicle in lunar orbit down to the surface in rocket-powered lawn chairs, and return the same way.

    For whatever reason, that's not what they did, using a lunar lander with pressurisation and full life support and all that.

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    Quote Originally Posted by Jens View Post
    But I think that's precisely what the discussion is about (I don't think anybody brought up escape velocity).
    I'm just pointing out that you are free to go anywhere, as slowly as you desire, provided you have the fuel for it.


    Quote Originally Posted by Jens View Post
    And yes, teen try would be hard with a jetpack. You certainly wouldn't want to be strapped to the front of the device! I think it would be preferable to be inside the "jetpack".
    Again only if have no choice but to use aerobraking. With sufficient fuel, you can simply slow down from orbital velocity until you are hovering above a stationary point on Earth - then descend straight down at a leisurely pace.

    Furthermore, if you have sufficient fuel, you do not need to orbit at all. Orbiting is nothing more than a way to save fuel.

    Sure, it's practically implausible to have unlimited fuel, I'm just trying to disabuse you of the notion that any of these maneuvers (escaping Earth, returning to Earth) are hard physical constraints. They are concessions to the limited fuel densities we currently have available.

    So, if you're going to posit sufficient fuel to go to the Moon and back in a jetpack, then you're already opening up that constraint.
    Last edited by DaveC426913; 2019-Feb-05 at 12:26 AM.

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    The creation of a machine which could slowly, very slowly, rise up out of the Earth's gravitational well was the premise of the American television show "Salvage 1", which aired in 1979.

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    Quote Originally Posted by DaveC426913 View Post
    Sure, it's practically implausible to have unlimited fuel, I'm just trying to disabuse you of the notion that any of these maneuvers (escaping Earth, returning to Earth) are hard physical constraints. They are concessions to the limited fuel densities we currently have available.
    I appreciate the effort, but actually I wasn't abused by that notion in the first place. I realize that you can do the whole maneuver by just using fuel and doing a soft landing if you want it. The question is, could you fit that much fuel into a jetpack, and would it still be a "jetpack" if you did that? I'm trying to answer the practical question of whether you could practically return to the earth from the moon with a jetpack. I'm not discussing physical possibilities. . .
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    Quote Originally Posted by WaxRubiks View Post
    Would it be possible to get back to Earth from the Moon with some kind of jetpack system?

    Or would that take too much fuel, and take too long?
    And I believe that I am answering the question that was originally posted. "Would it take too much fuel" implies to me that the question was about how much fuel it would take, not the assumption of unlimited fuel. Am I wrong?
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    Quote Originally Posted by StupendousMan View Post
    The creation of a machine which could slowly, very slowly, rise up out of the Earth's gravitational well was the premise of the American television show "Salvage 1", which aired in 1979.
    If only....how nice that would be.

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    Quote Originally Posted by Jens View Post
    I appreciate the effort, but actually I wasn't abused by that notion in the first place. I realize that you can do the whole maneuver by just using fuel and doing a soft landing if you want it. The question is, could you fit that much fuel into a jetpack, and would it still be a "jetpack" if you did that? I'm trying to answer the practical question of whether you could practically return to the earth from the moon with a jetpack. I'm not discussing physical possibilities. . .
    Well then no.

    We already know how much fuel it takes to get 3 men to the Moon - a Saturn V's worth. Getting one man there (sans Rover) will be less, but it's still going to take a rocket's worth. And almost all that fuel is spent just getting to orbit.

    So, if the constraints on fuel aren't relaxed for this discussion, then I'm not sure how the question wasn't already answered before it was even asked.

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    Actually I would argue that very little of the energy used to get to the Moon was used to get into orbit. The amount of energy needed to get from distance R from the center of the earth, to distance r, scales like 1-R/r times the energy to escape the Earth completely. The amount of energy to get into orbit around Earth at r is R/(2r) in those same units. Since R/r is here about 1/60, these numbers are 59/60 vs. 1/120. The energy to get into orbit around the Moon, once at the Moon, in these same units is about 1/42 (and this is actually energy you have to dissipate, but it's pretty much a time reversed situation and we're assuming extreme efficiency).

    So the scale of energy to get into orbit around the Moon from the surface of the Moon is also about 1/42 in these units, compared to the energy of 59/60 to get to the Moon, for the same mass. That's why it might conceivably be imagined for a jetpack. The energy to get from lunar orbit to the Earth is again 1/42 to escape the Moon, and then 1/120 to get back to Earth, all at extreme efficiency. So that's about again the energy to get into lunar orbit, and again might be conceivable for a very powerful jetpack.

    The problem is, you are basically getting to the Earth by falling. Hence, you arrive at some 11 km/s straight down! If you don't regard that as a problem, then you might be able to do it with a jetpack. But you have a pretty nasty re-entry problem there.

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    It's not a jetpack, but here are some of the issues that would arise if you wanted to slide down a fire pole from the Moon to Earth.
    Conserve energy. Commute with the Hamiltonian.

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    Quote Originally Posted by Ken G View Post
    Actually I would argue that very little of the energy used to get to the Moon was used to get into orbit.
    ?
    Surely this indicates otherwise.
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    Last edited by DaveC426913; 2019-Feb-06 at 11:20 PM.

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    The problem is not just one of moving the mass by burning fuel. You also need to consider travel time during which you need to sustain the humans. It was about a 4-day trip, one-way. The astronauts would need ways to eat drink, urinate, & defecate in their suits... and not go insane from drifting with no connection to anything else solid for a few days. Once you beef up the suit enough to handle all that (along with the fuel), it's not a suit anymore; it's a spacecraft. And then you might as well let him move around inside it instead of being strapped to a wall just so we can claim he's "wearing" the spacecraft.

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    Quote Originally Posted by DaveC426913 View Post
    ?
    Surely this indicates otherwise.
    Although it doesn't invalidate your general point, the Apollo translunar injection was actually done by the third stage. So the fuel for the third stage should be part green and part red (the third stage was also used to complete the orbital insertion; I'm not sure what fraction of the fuel was used for each part).
    Conserve energy. Commute with the Hamiltonian.

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    Quote Originally Posted by Grey View Post
    Although it doesn't invalidate your general point, the Apollo translunar injection was actually done by the third stage. So the fuel for the third stage should be part green and part red (the third stage was also used to complete the orbital insertion; I'm not sure what fraction of the fuel was used for each part).
    If my math is right most of the fuel was used for TLI; 150 seconds of thrust to enter Earth orbit and 320 seconds of thrust for TLI. Total burn time was 475 seconds, give or take.

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    The fuel for returning the three men from the Moon was moving several tons of hardware in addition to the men and their space suits. In a thought exercise we could have a man and his space suit, including the life support backpack, enclosed in a hypothetical lightweight cocoon that would somehow enable him to survive the reentry. We know how much delta V is needed to escape from the Moon and return to the reentry point, so with the rocket equation we could calculate how much fuel is needed for the job, and thus get a lower bound on the total mass of the jetpack.

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    Quote Originally Posted by DaveC426913 View Post
    ?
    Surely this indicates otherwise.
    So it does. I forgot that there is an efficiency issue for turning chemical energy into lifting energy. A rocket doesn't directly turn chemical energy into lift energy, it uses a burn rate along with an exhaust speed to generate momentum per second, over a given burn time. To turn that into lift energy, one has to multiply by the speed of the rocket in the Earth frame. So it's better to get the rocket going as fast as possible as soon as possible, which means it's better to point sideways and go into orbit than to go straight up. I had forgotten that. None of that really matters to my answer though, so just ignore that incorrect part. The energies give the comparative scale of the challenges, but how you actually achieve those energies depends on getting as fast as possible, so even the jetpack would want to take off at an angle and go into lunar orbit in order to generate as much speed as possible during the burn. Once it escapes the Moon the rest is pretty easy, it's not going very fast way out there so it doesn't need a lot of impulse to slow it enough to fall to Earth. A doubt the solar tide is a big issue either. My main point was that if you are allowed to let the Earth's atmosphere do the stopping, it takes way less energy to get back than it takes to get out.
    Last edited by Ken G; 2019-Feb-07 at 02:41 AM.

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    The big hold up is that while it wouldn't take much to launch directly from the Moon and return to Earth, a shorter hop to meet a spacecraft takes even less. Plus you can get back to an atmosphere you can breathe faster.

    This is about as small as you want to go
    https://en.wikipedia.org/wiki/LK_(sp...on_drawing.png

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    Don't forget re-entry, that's the dealbreaker.

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    Quote Originally Posted by Ken G View Post
    Don't forget re-entry, that's the dealbreaker.
    the MOOSE system might apply there....somehow surround the astronaut in a bag filled with heat resistant foam.
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