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Under what conditions would fuel depots in earth orbit make sense?

As it costs $X/kg to launch something into space, wouldn't it cost the same to launch fuel by itself as it would to launch it as part of the vehicle that's going to end up using it?

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The $X/kg is a simplification.

Rockets are only available in certain sizes; the launch vehicle with the greatest capacity that is presently operating is the Delta IV-H, delivering a little less than 28 metric tons to LEO. Even hypothetical rockets have limits due to the logistics of assembling and launching them.

If you want to launch a large and heavy mission - perhaps for the human exploration of Mars - you have to split up your mission into multiple launches. One way to do that is to establish a propellant depot in LEO, launch multiple tankers to it to top it off, and finally launch your main mission with empty or partially full tanks, fill up and depart for Mars.

There are alternative strategies - you could instead launch a fully fueled Earth Departure Stage, and separately launch the habitat and lander components, have them rendezvous in LEO and leave. That does put more time pressure on the launch and rendezvous since a full EDS can't hang around for too long before boiloff becomes a problem.

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  • $\begingroup$ +1, But why LEO? Why not a depot at the Earth-Moon L1 point? Better yet, why not have depots at both locales? $\endgroup$ – David Hammen Nov 7 '14 at 11:27
  • $\begingroup$ @DavidHammen because it is more efficient to transfer to Mars from LEO than first transfer to L1 and then to Mars due to the Oberth effect. $\endgroup$ – fibonatic Nov 9 '14 at 17:27
  • $\begingroup$ @fibonatic -- The Oberth effect works even better for a vehicle launched from the EML1 or EML2 point than from LEO. Only a tiny delta V from either point is needed to put a vehicle on a trajectory that brings it very close to Earth. This vehicle is moving almost 40% faster at perigee than a vehicle in a circular low Earth orbit. The Oberth effect is huge. Another tiny delta V at perigee does the trick. The total of these two burns is much smaller than the delta V needed from LEO to achieve a Mars transfer orbit. $\endgroup$ – David Hammen Nov 9 '14 at 18:31
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    $\begingroup$ @DavidHammen but have you also included the delta V needed to "park" at L1 and this does limit your opportunities for transferwindows to Mars, namely when the Moon is on the right side of Earth, if you directly transfer at first periapsis passage. And the longer travel time will also add the need for more food supplies. $\endgroup$ – fibonatic Nov 9 '14 at 19:08
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Humans like fast transit (less supplies, less radiation), so they want to use chemical propellant which is much less mass efficient than electric propulsion. One way to mitigate this and lower the overall mass required for human missions is to use electric propulsion to transport chemical propellant to high energy locations, such as high Earth orbit or at asteroid or Mars destinations. Then after you wait a few years to get the propellant to where it needs to be, the humans can zoot around the solar system using the stored propellant.

You would go ahead and store the engines with the chemical propellant, so there's no need to transfer fluids. Just grab a complete stage that has been stationed for you and go. The engines are a small fraction of the mass of a stage, so you might as well attach engines to your fuel tanks.

Normally LH2/LOX is the chemical propellant discussed for this sort of thing, but that will require technology development and demonstration of zero boil-off for the time spans required.

The other way "depots" makes sense is if the propellants didn't come from Earth. Asteroids and Phobos can be sources of water to make LH2 and LOX. High-Earth orbit is energetically much closer to those locations than Earth.

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  • $\begingroup$ Grab-and-push. A fuel depot together with an engine. Solves lots of problems with docking and fuel transfer. And the very idea of refueling, or pushing, left-over upper stages? Sounds like a great idea. Suddenly thousands of opportunities became available. Wow! Shouldn't there be more upper stages than today active satellites in GEO? Anyway, what a great idea! Artificial near Earth objects and space debris as a resource. $\endgroup$ – LocalFluff Nov 5 '14 at 19:32
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    $\begingroup$ @LocalFluff I find it funny that the only place upper-stage reuse is discussed is in a comment to a unrelated answer. The rationale is completely obvious, but I've rarely ever seen it brought up, so maybe there's some deal breaker? If it works, the benefit would be impressive. You're actually expanding what constitutes your "payload" to orbit by reusing part of the machinery used to get you to orbit. It won't reduce costs to orbit, but it will do wonders to get around the size constraint of LVs. $\endgroup$ – AlanSE Nov 6 '14 at 18:21
  • $\begingroup$ @AlanSE At least one should consider the idea of designing last stages for reuse in space. SpaceX seems to have given up their ambition to soft land their F9r upper stage on Earth. But it seems to me much easier to reuse it in space. Especially if one designs the hardware to be reusable in space, which really is the environment it is meant to work in. Why pay the cost of landing it on Earth? $\endgroup$ – LocalFluff Nov 6 '14 at 22:00
  • $\begingroup$ @LocalFluff Exactly, along that same line of thinking, if the Falcon 9 reusable is going to be two-stage, then you're either going to do something with the upper stage in space or you'll reenter. If you reenter your economics will suck because of the heat shield. But this might be backwards logic, because the same F9r should be lifting the propellant in the first place. So maybe you build a specific space booster vehicle where the 2nd stage only uses, say, half of its propellant to get to orbit, then meets the interplanetary playload and fires again. $\endgroup$ – AlanSE Nov 6 '14 at 22:23
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As it costs $X/kg to launch something into space, wouldn't it cost the same to launch fuel by itself as it would to launch it as part of the vehicle that's going to end up using it?

Why assume that the launch cost is the same?

Here are some hypothetical scenarios why storing fuel in space might be a feasible idea. (Assuming a need for fuel in space.)

  • Regularly scheduled, low priority rockets (in advance) might be cheaper than sending large quantities of fuel irregularly (as needed).
  • The rockets required to send fuel, may be inherently cheaper than rockets needed to transport other cargo (e.g. delicate/expensive satellite, humans).
  • We may be able to use non-rocket technologies to get fuel into orbit. Getting to orbit is actually quite cheap (few $/kilo according to my napkin), the problem we face is our technology is inefficient (rocket equation). Some alternative solutions (e.g. Mass driver) may not be suitable for all cargo (high G), but fuel could be a valid use.
  • Secondary Cargo - why pay to send 1000 kilo's to space (your rocket's optimal payload) when you only need to send a 500 kilo satellite? Take a load of fuel as well to get a subsidised price.
  • Cheaper source. If fuel can be made on the Moon for example, then placing it in earth orbit would be far cheaper than lifting it from earth, while conversely building complex electronics/shipping earthers may be hard to do from moon.
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  • $\begingroup$ The question is about orbital fuel depots, so could you please elaborate on the point that mass drivers could work at few $/kilo? You still need to circularize to stay in orbit, so I don't see that possible. And you can't simply bombard the depot with projectiles carrying fuel from the surface of the Earth either. Not if you want them to stay in orbit, at least. I.e. you'd still need powered resupply vehicles. Tho for other areas in near-Earth space such as L-points that might not be such a problem. But that's not what the question asks. Other points you make are good tho. $\endgroup$ – TildalWave Nov 5 '14 at 13:15
  • $\begingroup$ This is just brainstorming. Orbit velocity = ~8km/s = ~32MJ/kg = ~9kWh = ~18cents (2c/kWh, bulk nuclear generation). Mass driver is just one example of alternative approaches, not a serious considered proposal for building orbital depot. $\endgroup$ – NPSF3000 Nov 5 '14 at 14:14
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Launching fuel from Earth without a specific purpose, waiting for a need to turn up, can't be a good idea. I've heard of fuel depots in the context of producing fuel on the Moon. So that is in a vision of a much more developed space flight than what we have today. The benefit would come from the lesser energy needed to launch from the Moon's lower gravity. And once produced on the Moon, the fuel might as well be put in a waiting depot in space. At a Lagrange point or some lunar orbits, the large distance to Earth makes any conventional Earth orbit easy to reach such as polar satellites, geostationaries, GPS. From Earth orbit, it would require a lot of fuel to navigate to different orbital inclinations.

The market for refueling satellites is limited by the economic lifetime of satellites. One might want to replace a 15 years old communication satellite with new hardware technology anyway. And it might be cheaper to fuel it for 15 years of station keeping already before launch, rather than hassling with docking and refueling systems.

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