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I'm working on a space elevator design using kerosene-burning jet engines and dirigibles supporting 1km stages of pipeline and cable stretching up to the end of the stratosphere (~50km), then using hydrogen and oxygen-burning rocket engines to provide support and corrective thrust in the mesosphere, and again hydrogen and oxygen rocket engines for a series of 10km stages of pipeline and cable stretching from the Karman line out to roughly the beginning of LEO, where the end station of the elevator will be anchored.

In this design, I'm planning on pumping liquid hydrogen and oxygen along fuel lines running the length of the structure, utilising the Venturi effect generated by the vacuum of space to minimise energy used in pumping the fuel. I'm aware that the ISS (orbiting at ~400km from Earth's surface) consumes about 7000kg of fuel per year; I'm envisaging that this structure will first of all weight more than the ISS and will be anchoring itself lower in LEO than the ISS, and as such will use more fuel. Still, per kg of payload, I expect the payload cost in terms of propellant to be significantly lower than with rocket.

My question is this: what major challenges do you see with the design specifically so far as the piping and burning of hydrogen propellant is concerned? I'm keen to remain on the technical side of this question, and am less interested for the time being in the economics of this problem. The very cold temperatures of space and the upper atmosphere made piping hydrogen seem like an attractive prospect to me; the engines also, in the mesosphere and space, would be kept nice and cool; and I'm aware also that NASA did some research on piping hydrogen using composites and this could be used to save on weight. What would you have said the biggest challenges are with this hypothetical notion? also, is it better to pump up gaseous hydrogen/oxygen, and then burn those (especially since I expect this structure won't need to burn as much as rockets)?

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  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Space Exploration Meta, or in Space Exploration Chat. Comments continuing discussion may be removed. $\endgroup$
    – Rory Alsop
    Oct 9, 2023 at 17:32
  • $\begingroup$ Additionally, I have added the final close vote to this. There are many reasons we could have closed, not just lack of focus. The problems are as the community have tried to point out - if you have a theoretical new method for getting things to space, you really need to do your research before pitching it here. There are reasons why we use rockets, despite thousands of folks trying to find other ways. $\endgroup$
    – Rory Alsop
    Oct 9, 2023 at 17:35

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Maybe it will be easier to fit all the problems with this in an answer instead of a comment. Some of that includes short answers to the (too many) questions you asked. I'm going to turn on community wiki for this so others can contribute as well. It's a stretch of the StackExchange system, but maybe with it all written out then somebody else can read this if they have a similar idea in the future.

What's the point of a space elevator in the first place?

It allows you to move material to orbit without the extreme power consumption of a rocket. If you can operate multiple cars on a single cable you can also reduce the energy costs some by using them as counterweights.

Adding rockets to a space elevator should make anyone immediately suspicious. It's missing the point.

Why is what you proposed not a space elevator?

To deposit payloads into space, some point on the elevator needs to be moving at orbital velocity for its altitude. If the bottom of your device is attached to the ground, then that orbit must have zero average groundspeed (i.e. be in geostationary or geosynchronous orbit). If you're allowing the device to move along the ground somehow (it's a megastructure, why not also build a railroad track all the way around the world or something) then there are limits to how fast that can move without incurring huge drag penalties and/or causing damage to the neighbors from sonic booms.

If no point is moving at orbital velocity, whatever you deploy from the elevator will fall, as Organic Marble mentioned in a comment. There are fun and useful things to do in sub-orbit too, because you can get minutes of freefall out of it, but those trajectories are also relatively cheap and easy to obtain already. A megastructure isn't necessary to do that.

LEO, with its roughly 90-minute period, is totally unsuited to being the top end of a space elevator. The bottom would also have to be chasing the top around the Earth every 90 minutes (causing aerothermal problems and incurring large speeding fines), otherwise the structure has come apart and there are even more problems.

If the top is suborbital, as you mentioned in a comment, then you have to launch a rocket from the top for the payload to continue into orbit. You can find out how roughly much energy this arrangement saves by comparing the square of the velocity at the top of the elevator to the square of the needed orbital velocity. Given the constraints on how fast the bottom of the elevator can go, the savings are inevitably negligible.

"What major challenges do you see with the design specifically so far as the piping and burning of hydrogen propellant is concerned?"

You're trying to pump hydrogen at minimum up hundreds of kilometers of vertical column. If it's not impossible to do, it will be extremely costly to do so. The amount of hydrogen you're trying to pump is also some absurdly high multiple of how much liquid hydrogen the world produces.

"I suppose it would use the negative pressure created by space's vacuum to draw the fluid along the fuel lines like a Venturi pump would."

You're trying to make a column of fluid hundreds of kilometers high. You need A LOT of pump. Space can't save you here; the pressure differential between LEO and the ground is only 14.7 psi.

"the ISS (orbiting at ~400km from Earth's surface) consumes about 7000kg of fuel per year"

The ISS is in orbit (and a large amount of propellant was consumed putting it there) and uses its fuel to maneuver and to overcome the tiny amount of atmospheric drag present in LEO. It maintains altitude because it's moving so fast that it falls past the ground.

Your structure isn't in orbit. It's not moving fast enough to fall past the ground; if it falls, it hits the ground. So it has to expend energy (and power) to maintain altitude--i.e., it's hovering, like a helicopter (or a Starship test article). As multiple commenters already mentioned.

"What would you have said the biggest challenges are with this hypothetical notion?"

lol. You asked this; I don't understand why you've been so resistant to people pointing out that it's not going to work.

"also, is it better to pump up gaseous hydrogen/oxygen, and then burn those (especially since I expect this structure won't need to burn as much as rockets)?"

The rocket thrust equation depends on the amount of mass you're moving. Gas would technically be easier to pump to 400km altitude (maybe still impossible, but less dense!) but you need to pump the same amount of mass.

As already pointed out, you're wrong that it won't need to burn as much as rockets. It needs to burn much, much more because it doesn't use its propellant efficiently (for accelerating the payload), it uses it inefficiently (for holding up a megastructure).

"Come on dude, this opens up Mars, it opens up the outer solar system, it enables us to do a world of things in space we wouldn't really have considered using rockets alone."

It really doesn't. An actual space elevator might.

What other problems might this scheme have

Even the jet engines will have to be torn down for maintenance every couple thousand hours or so. I don't think anyone's ever considered how long the maintenance intervals should be for rockets that run continuously for hours, let alone days or longer. Even if this thing worked, which it wouldn't, or was helpful, which it isn't, you'd have to regularly lower and re-raise it to maintain the bits.

"The elevator would, I predict, be able to carry 4 Ariane 5 sized payloads to LEO every day"

I would love to know how you've predicted this.

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  • $\begingroup$ I'll address your headings one by one. 1. You've intentionally decided to miss the point entirely once again, and have ignored the central issue at fault with your economic criticisms: propellant is not all that expensive as a component in rocket launches. Space elevator economics is advantageous due to reusability NOT propellant costs. "Without the extreme power of rockets"?? Any space elevator design will use an enormous amount of energy whichever way you cut it; there's electricity as a minimum, there are certainly also maintenance costs. $\endgroup$
    – Sam Cottle
    Oct 12, 2023 at 20:25
  • $\begingroup$ 2. "What you proposed is not a space elevator" Whatever, you're splitting hairs. Fine, it's a 'space tower' with elevators fitted to it. You win. "To deposit payloads into space, some point on the elevator needs to be moving at orbital velocity for its altitude." I don't see why. You can drop something from the Karman line, it'll be in space for a little while, then descend through the atmosphere unless it has rockets. Likely whatever we'd deploy from a 36,000km 'elevator' would also need some system of propulsion (for the variety of reasons space craft need propulsion systems). $\endgroup$
    – Sam Cottle
    Oct 12, 2023 at 20:29
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    $\begingroup$ @SamCottle "No, sorry, I'm afraid it's you who are wrong since the structure is mostly not 'supported' by engines, rather it's mostly steered by them; the section in space is there to pull the section in the mesosphere taut. Go back and read what I originally posted" well, you posted "jet engines and dirigibles SUPPORTING 1km stages" and "rocket engines to provide SUPPORT". The upper end isn't at orbital velocity, so it isn't supporting itself or anything else. I'm not "splitting hairs" in the second section--all this is fundamental to your question. $\endgroup$
    – Erin Anne
    Oct 12, 2023 at 23:19
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    $\begingroup$ @SamCottle if you insist that all you want to know is if you can pump either liquid or gas up 400km, this isn't the site to ask. That's a physics question. The related engineering question is probably "what kind of megastructure would you use to move rocket propellant up 400km." They're probably going to ask why you want it there. Proposing a design for a space elevator--or whatever you want to call it--on THIS site and then expecting people not to pay attention to that, only to pay attention to the question of "can a pump move liquid uphill," is trolling. $\endgroup$
    – Erin Anne
    Oct 12, 2023 at 23:29
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    $\begingroup$ @SamCottle Repeatedly insisting that the fundamental problems with your proposal do not exist is not going to make them go away. The fact is, your "elevator" is just a 400 km tall tower with only a tiny fraction able to be supported using jet engines or "dirigibles", with the great bulk of it supported directly using the thrust of rocket engines. The claim that those engines will only need to "steer and correct" the structure is absolutely false, they will be directly supporting the weight of the structure against Earth's gravity. $\endgroup$ Oct 13, 2023 at 19:56

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