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In a recent discussion of cost estimates of hypothetical space elevators it was mentioned that a space elevator is (obviously) well suited to bring payloads to a geosynchronous orbit while reaching other orbits would still need major delta v. This restriction in easily available orbits is a major disadvantage of space elevators; it is not very well suited to serve the huge market of low Earth orbits.

But we are outside the atmosphere and have as much time as the payload permits; we can use high-efficiency, low-mass, low-thrust drives like ion propulsion. Such systems are already routinely used for orbital transfers of payloads launched by chemical rockets.

Would low Earth orbits, at various inclinations, be within reach of non-chemical propulsion for spacecraft released from a space elevator (probably from half-way up)?

Peter - Reinstate Monica
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  • Ironically, because a space elevator will need a counterweight above GEO, it's probably cheaper to reach orbits above GEO (by climbing all the way to the Counterweight and dropping off the 'bottom' of it) than to get to anything below it. In fact, if the counterweight station is at ~100,000 km, then you can get to the moon with almost no delta V by just letting go at the right time. – Darth Pseudonym Jan 18 '24 at 15:17
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    I'd note that a main reason why LEO is such a huge market is that it's what's easiest to get to with conventional rockets. Many of the satellites in LEO today could be replaced with satellites in some other orbit, if those were easier to reach. – leftaroundabout Jan 18 '24 at 22:26
  • @leftaroundabout Is that so? Starlink and other low-latency communication and control as well as surveillance critically depend on low altitudes, the lower the better. – Peter - Reinstate Monica Jan 18 '24 at 22:55
  • @Peter-ReinstateMonica a non-LEO orbit can still have a very low perigee. Sure that's not ideal in the sense that the satellite will spend most of its life at higher altitude and so you need more redundancy, but that could be offset if the satellites are mass-produced and launch cost is more of a limitation than the satellites themselves. Certainly you wouldn't do this for expensive spy satellites. I didn't say "most satellites", just "many". – leftaroundabout Jan 18 '24 at 23:07
  • @leftaroundabout Well, the majority of a huge market is likely still a huge market ;-). – Peter - Reinstate Monica Jan 18 '24 at 23:08
  • Another reason that LEO is popular is that it mostly avoids the Van Allen belts. GEO is situated within the outer belt. – PM 2Ring Jan 19 '24 at 17:41

2 Answers2

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Momentum exchange systems

I'm fond of momentum exchange tether systems, more generally known as "skyhook" to solve the challenge of bringing an object to orbital velocity. These systems can be used in various directions and manners. A skyhook system can affect orbit-to-orbit changes, and can be "naturally" maintained by symmetrical loading (upmass = downmass). Provided some engineering challenges can be solved, a skyhook system could even do direct surface-to-orbit transit.

Challenges of Space Elevator to orbit:

  • High thrust requirement. Unless you ride all the way up to GEO and release in what is effectively a zero-velocity orbital transfer, you will need to pile on a lot of velocity very quickly. For example, if I just rode up to 400km, I'd need to add quite a bit of horizontal velocity very quickly so that I enter orbit before hitting Earth.

  • GEO is a high-energy orbit. It's difficult to say where people will want to put their space things, but getting from GEO to LEO without aerobraking requires a bunch of fuel

To address these, I'd use a system of multiple skyhooks, at different orbital "shells" that enable easy and essentially thrust-less transfer through different orbits at a very low cost.

Advantages:

  • High ISP possible. A skyhook system flings a smaller spacecraft into a different orbit by transferring momentum. This momentum still needs to be "paid" for, but unlike the shuttle, it can be equipped with an extremely high ISP, low thrust engine that is running all the time. Instead of having to pay the enormous dv cost on the shuttle when you get off your space elevator at 400km, this cost gets passed on to the far more massive skyhook, which has many days or weeks to gradually pay back this cost with it's own hyperefficent thruster system

  • Low shuttle dv requirements. Your shuttle craft that ride on the space elevator will still need small high-thrust engines for orbital maneuvering and reaching skyhook pickup points, but beyond that, they don't need a lot of fuel or total dv. Whether hooking on directly at 400km, or using a series of hook transfers, a very low fuel shuttle could transfer basically everywhere in the Earth-Moon system with basically no fuel if there were a properly developed network of skyhooks.

Matija Nalis
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Dragongeek
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    why would you use a space elevator targeting an orbit of 400km and release from the elevator when that's your apogee instead of when that's your perigee? – Erin Anne Jan 18 '24 at 23:20
  • @ErinAnne Good point. If you ride up until your perigee is at 400km, you could use this to "regenerate" a skyhook to circularize the orbit. – Dragongeek Jan 19 '24 at 08:14
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Not necessarily going to be the answer you are looking for, but:

Lower orbits

Aerobraking

The payload can reach any lower orbit by climbing to an altitude that, when it is released, puts it into an orbit that just dips into the atmosphere. A small thruster would be needed to circularise the orbit once the desired apogee is obtained. For low earth orbit, this would be on the order of 100m/s.

The difficulty is that you want non-chemical propulsion. Ion thrusters would probably waste a lot of time and fuel pulling a satellite out of an aerobraking orbit. When using aerobraking, a (small) chemical system is likely to be optimal.

Higher orbits

As Darth Pseudonym noted in his comment, for higher orbits you can go above the GEO belt and release with extra energy. You will still need a propulsion system if you want to circularise, but the energy requirements will be comparable to circularising at LEO. In this case, a small chemical propulsion unit seems ideal again; however, you have so much more time than circularising in LEO that an electric engine could be used without major issues.

Even if you have other things like skyhooks, nuclear engines, or high-power electric engines, the space elevator does so much of the work for you that the simplicity of small chemical rockets outweighs any advantages you might gain from these more complex alternatives.

Infinite_Maelstrom
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