# Could an astronaut with a jump and a "jet pack" "jump" off of the Moon?

The initial naïve question was:
"Could an astronaut with a jump and a "jet pack" "jump" off of the Moon?" :)

For fun, let's assume the optimal circumstances:
Gemini era AMU ("jet pack") with a capability ($$\Delta v$$ I guess?) of about 250 feet per second (76.2 meters per second).

I can't find any weight for it but I assume it would, in the first place, be to heavy for a man to carry on the Moon - but let's ignore that.

$$\Delta v$$ for Moon ascent seems to be in the range of $$1600 \frac{\text{m}}{\text{s}}$$ to $$2600 \frac{\text{m}}{\text{s}}$$ - depending on the acceleration.
These are calculated for around $$15 \frac{\text{m}}{\text{s}^2}$$ acceleration I read.

So I conclude:
a high jump and the strongest, most dangerous jet pack ever built wouldn't be nearly enough to ascent from the Moon to the lowest possible orbit for an astronaut, would it? :)

Did I miss something?

EDIT: I learned a lot from other discussions and an online $$\Delta v$$ calculator:
The astronaut (90kg) with empty jetpack (50kg) would weigh fueled (100kg) 240kg on Earth.

The distinction between mass and weight seems unimportant as the $$\Delta v$$ calculator states:
"Mass units are arbitrary; use whatever you like, as long as you're consistent."
It seems to depend only on the ratio.
This is backed by the fact that I get the same $$\Delta v$$ values, regardless of which I use: mass or weight.

The best online sources I can find for $$v_{exhaust}$$ is $$1800 \frac{\text{m}}{\text{s}^2}$$ (which equals $$183\ \text{I}_{sp}$$). But these seem to be values from Earth.
$$\text{I}_{sp}$$ seems different if there is no atmosphere.

So the information to find is: What would be the $$\text{I}_{sp}$$ / $$v_{exhaust}$$ of the jetpack on the Moon?
Maybe I can find a more popular vehicles information and interpolate the difference.

• This somewhat compares with a short range ballistic missile of 300-400km range with a warhead of 100kg. Commented Mar 1, 2022 at 13:42
• Even neglecting delta-V, the MMUs are made for use in microgravity, and I suspect they don't have enough thrust to noticeably affect the hangtime of an astronaut jumping on the lunar surface. Commented Mar 1, 2022 at 16:35
• Yes, you missed that acceleration is m/s/s. That 15m/s really adds up over the total flight time. A jetpack capable of 15m/s (easy, since we can easily hover with jetpacks in earths 9m/s/s gravity) requires just 3mins flight time to exceed 2500m/s. Most modern jetpacks could carry an astronaut into lunar orbit, assuming you can adapt it to working in a vacuum. Commented Mar 1, 2022 at 21:18
• Oh, yes, I missed a /s dammit Commented Mar 1, 2022 at 21:27
• Thanks @BrendanLuke15 I think so too. Most Delta-V maps I find are between 1700 and 2200 so for now I take 1700. Commented Mar 2, 2022 at 15:45

With a certain flexibility on "jet pack", this makes me think of the proposed LESS lunar escape system - an emergency backup to the LEM. This would have been capable of taking two astronauts in Apollo suits into a lunar orbit for retrieval by a CSM. Let's assume it would have worked, and see if we can turn it into a "jetpack".

A basic 1970 design looks to have been about the size of a quad-bike, with a dry mass of 364.5 lbs and 1160 lbs of propellant (which would be scavenged from the LM ascent stage) = total around 690kg plus the mass of the two astronauts + suits.

A bit big for a "jetpack" even in lunar gravity, but ... not completely implausible. Halve the size to fly only one person, 345kg, shave a bit off if you are willing to forgo any guidance equipment, say 330kg. Add 90kg for the suit itself, and you are looking at a total mass burden of 420kg. This sounds unworkable, but is equivalent to a less horrendous ~70kg in earth gravity terms; I wouldn't want to do it, but it's not impossible.

It then becomes a question of bulk. The LESS expected you would sit on it, rather than wear it as a "jetpack", so it would need redesigned to be carried this way - not so much to save weight as to distribute the mass more evenly, as otherwise it would be very hard to balance. Perhaps a harness-type arrangement with the rocket at the back and fuel on the front/sides? I am assuming you do not want the rocket exhaust to pass too close to anything delicate like - well, anything - which would be an added complication.

However, on the very back-of-an-envelope calculation, it suggests you could potentially build a suit + propulsion unit combination that someone could successfully walk around in and then use to fly into orbit, which feels like it fits the spirit of "jetpack". Of course, they might not be able to do much walking in it, they had definitely better not fall over, and they wouldn't be able to do much when they got to orbit...

• Spent ages trying to find an old "externally piloted" rocket, so +1 for tracking that down. Commented Mar 2, 2022 at 16:45
• They could rendezvous with an orbital rescue craft that couldn't land. It's more of a lunar surface escape pod, as you say. Commented Jun 1, 2022 at 16:03

The correct sort of jet pack to use for comparison is not a Gemini APU, but rather an earth-based jet pack of the sort that has been built over the decades. They generally use hydrogen peroxide as a monopropellant and can hover under 1G conditions for about 30 seconds. Ignoring lunar gravity, that's a total $$\Delta v$$ of around 300 m/s, not enough for lunar orbit. But it's off by less than a factor of 10, so with a better propellent and larger tanks, then, well, maybe? But you have to remember that the astronaut already has a life support system backpack, so once you add fuel tanks, perhaps on the chest and above the head, then you almost just have a small spacecraft with the astronaut's legs as the landing gear.

So I'm going to go with "Not really."

It somewhat depends on how practical you want your jetpack to be.

For a classic, sci-fi style, bare-minimum jetpack with no automation/navigation/communication what-so-ever, 50kg dry and 150kg fueled can indeed get you off the Moon (and reach the velocity of 1600$$\frac{\text{m}}{\text{s}}$$, according to this rocket calculator, assuming Isp=320s).

Quite promising, until...

if you want to get to your target lunar orbit with some level of precision and certainty, i.e. not just "off the Moon" but actually to somewhere, then the weight of extra hardware can go easily past what a person could carry, even with reduced gravity.

I came to this conclusion based on the most lunar sample-return mission, Chang'e-5.

Chang'e-5 ascent stage reached 1.6$$\frac{\text{km}}{\text{s}}$$ in 6min using a 3000N main engine, with a liftoff mass of 560kg and in-orbit mass of 300-400kg (with quite a bit of leftover fuel, dry mass is probably 280kg, targeting a maximum $$\Delta v$$ of 2200$$\frac{\text{m}}{\text{s}}$$ just like Apollo).

Adding 100kg of an astronaut to this vehicle you need a 70kg more fuel to get to the same velocity (from 560kg-330kg-1600$$\frac{\text{m}}{\text{s}}$$ to 730-430-1660, assuming Isp=320s). That's just too bulky and heavy for a person to carry as a "pack".

(I will not any references here because finding reliable and accurate data for a Chinese spacecraft is almost impossible. You are welcome to improve upon my very rough estimate, e.g. maybe study Soviet lunar-sample-return missions.)

• So with astronaut weight lets say 90kg (incl suit etc.) it would be 140kg dry and 240kg fueled. That is ~1700 delta-v according to the calculator you posted. But the graphic on the right side states 2200 m/s delta-v necessary for lunar ascent. Don´t know if they chose lowest orbit possible. Hmm I probably have to find out if 2200 or 1600 really. Thank your for your detailed explanations! Commented Mar 2, 2022 at 10:31
• Also is it possible that assuming an isp of 320 is curently unrealistic? Rocket belts tested seem to have a vex of 1800 (isp ~180). Can´t find any darn numbers for AMU / MMU. Commented Mar 2, 2022 at 10:38
• I cannot recreate the 1600 m/s value in the linked "rocket calculator," what are the exact inputs? Commented Mar 2, 2022 at 14:15
• @BrendanLuke15 560-330-320 get you 1659.58. You need to select "specific impulse" on the 3rd row. Commented Mar 2, 2022 at 15:53
• @MartinEckleben bipropellant or mono? Commented Mar 2, 2022 at 15:58