Charlie Mote
United States
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Phil posted this recently at the High Frontier Yahoo Group and when I asked if he was planning to repost here, he suggested I should do so in his stead:

A short essay on a Fuel Depot in LEO, such as the one in High Frontier, written for the Tucson Space Society by Phil Eklund January 17, '16

A Fuel Depot in LEO is, as Heinlein quipped, is halfway to anywhere, at least as far as delta-v is concerned. And a fuel depot is the only way to address the problem that LEO is drier than any desert. And overall resource poor, in the one commodity that spacefarers need most: mass.

The problem for getting around in space has never been energy. Space has plenty of that. The problem is mass. Every vehicle ever built needs both (1) momentum exchange with mass, plus (2) the energy to effect this.
On Earth, (1) is easy, and (2) is hard. In space, its the other way around.

Such a depot could store fuel at first, but to conquer space it will have to store propellant. The difference between fuel and propellant is simply that fuel stores the energy, and propellant stores the mass. As mentioned, energy is cheap and mass is expensive in space. A typical interplanetary nuclear spaceship will need a kilogram of fuel, but tens or even hundreds of tons of propellant.

As O'Neil wrote, space will never be conquered by upporting propellant from the bottom of Earth's gravity well. So, looking ahead a bit, the only practical source of propellant (in circumterrestrial space and beyond) is Luna or the asteroids. This requires ISRU technology. (In Situ Resource Utilization).

There are essentially three types of ISRU propellant that could be stored at the LEO 'fuel depot'.
(1) Cryofuels. Either hydrogen or hydrogen plus oxygen.
(2) Water.
(3) Regolith (i.e. rock dust).

Each of these implies a particular rocket type. There are three proven rocket types:
Chemical rockets (very low Isp) need (1).
Nuclear fission rockets need (1) for low Isp, or (2) for very low Isp.
Electric rockets need (2) or (3) for low Isp, (1) for medium Isp, and special propellants for high Isp. These special propellants are not readily available in space, with the possible exception of potassium.
Electric rockets running on (1) include VASIMR, on (2) include MET, and (3) include mass drivers.

Isp (stands for specific impulse) is a measure of rocket fuel economy (or, better said, propellant economy). Very low Isp is 500 seconds, low Isp is 1000 seconds, medium Isp is 2000 seconds, and high Isp is 4000 seconds. Because mass is so expensive in space, very low Isp systems are going to be so expensive and heavy that they will never provide a positive return in any entrepreneurial venture.

You can store cryofuels in water form at a depot, and thus making propellant type (1) on demand from propellant type (2). The advantage of this is that water is much easier to store than cryofuels, and no need for ZBO or refrigeration. And tank size is far smaller.

A serious drawback is that most cryofuel rockets run best on hydrogen alone. Only a chemical rocket like the SSME must accept both hydrogen and oxygen. Because water is eight parts oxygen for every part hydrogen, you will be wasting hundreds of tons of mass in liquid oxygen.

Well, not entirely wasted. Both nuclear fission and electric rockets could run at very low Isp on liquid oxygen. I can imagine low delta-v missions running on 'cheap' liquid oxygen, and high delta-v missions running on liquid hydrogen. For a high delta-v mission, every gram of propellant compounds itself many times over, so you need the highest Isp available.

What about the idea to use a solar power satellite to make spacecraft propellant from lunar or asteroid material?

My feeling is that all processing of lunar or asteroidal material will have to be done in situ, that is, at the site it was mined. This will be true whether the mine is on Earth, Luna, or an asteroid. Therefore, I believe there will never be a processing plant in LEO or in Lagrange points. The processing plant will always be where the mass is. In both the near and the distant future.

The reason is the tyranny of the rocket equation. Every gram of material you carry from the mine will need many grams of propellant to carry it.
Therefore that gram of material will need to be 'pure', i.e. refined material. If the ultimate product is iron what you want to carry is iron, not rocks. If the ultimate product is hydrogen you want to carry hydrogen, not water. All processing 'in situ'.

A specific example to drive home the point:

Let's assume a MET rocket (microwave electrothermal rocket, a type of electric rocket using water propellant and uranium fuel) starts at the LEO fuel depot and travels to a NEO carbonaceous chrondrite asteroid.
The mission is to bring back water to the depot. The dry mass of the rocket is 100 tonnes. (Dry mass is the mass of the rocket without propellant). The delta-v to the asteroid is 7.5 km/sec there, and another 7.5 km/sec return.

To get there, the rocket equation gives a wet mass = dry mass * e^(delta-v/(Isp/98)) = 212 tonnes. In other words, it takes 112 tonnes of water (supplied at LEO) to get a 100 tonne rocket with an Isp of 1000 to the water-bearing asteroid.

Assuming it picks up 1000 tonnes of ice at the asteroid (so the new dry mass is 1100 tonnes), the wet mass for return is 2328 tonnes, of which 1229 tonnes is expended underway. So for an investment at LEO of 112 tonnes of water, the return is 1000 tonnes. Net profit is 888 tonnes of water.

The BIG assumption here is the mining spacecraft did ISRU processing. In other words, it mined and processed ore at the site so that it (1) refined the 1229 tonnes of water needed for the return trip, and (2) brought back the cargo as pure ice.

Without ISRU processing, 3830 tonnes of water (supplied at LEO) would have been necessary for the trip, instead of 112 tonnes. What is worse, the 1000 tonne cargo returned is not ice but ore, which perhaps has only a few percent water. Expending 3830 tonnes of water at LEO to bring back 20 tonnes of water is a dead loss.

Using a 40 tonne 'Zuppero Iceship' (a nuclear fission rocket using water ice propellant) for the same mission would require 118 tonnes of ice for the trip there, and 2018 tonnes for the trip back, assuming ISRU. This still delivers about the same profit (since the water used in the return trip did not come from LEO), and uses a much smaller and simpler rocket.

The two take-home facts of space are (1) mass is so invaluable in space that you must obtain it by ISRU wherever you can find it, and (2) you can't dodge the rocket equation. These harsh facts will remain true no matter how far into the future we look, and no matter what technology we develop. Even photon sails are not really an exception. The laws of physics, as many a Star Trek hero has affirmed, can't be changed.

For these reasons, promoting a 'mass depot in space' is a great idea for those who are looking ahead.

Phil Eklund
Sierra Madre Games
Birkenwaeldle 30, 76228 Karlsruhe, Germany
+49 721 94302477 home
+49 17663 843779 cell
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