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ToSeek
2006-Jun-02, 05:10 PM
Orbiting gas stations key to interplanetary exploration (http://www.newscientistspace.com/article.ns?id=dn9259&feedId=online-news_rss20)


An orbiting "gas station" is an important requirement for the long-term future of human interplanetary exploration, a study by NASA engineers suggests.

For missions to Mars, spacecraft would make pit stops at fuelling stations orbiting the Earth or Moon to fill their tanks with liquid hydrogen and liquid oxygen generated in space.

Larry Jacks
2006-Jun-02, 08:53 PM
From an energy perspective, it makes no sense at all to fly to Earth's orbit, fly to the moon, enter lunar orbit, refuel, then fly on to Mars. The energy required to fly directly from Earth orbit to a Mars escape trajectory is pretty close to the energy required to fly to the moon. That isn't even counting the energy required to slow down enough to enter lunar orbit, then accelerate enough to fly from the moon to Mars. The idea of a lunar refueling station for Mars trips is silly. A lunar refueling station for refueling trips between the Earth and the moon is much more reasonable.

Now, an Earth orbit refueling station makes more sense, assuming the following:

1. Harvestable water ice exists on the moon,
2. Getting that ice and launching it to Earth orbit (perhaps using aerocapture/aerobraking) can be done in a reasonably economical manner, and
3. You can develop the facilities to process the ice into H2/O2 (not hard) and store it for long periods (not easy) can be developed.

An Earth orbit refueling station could be used for a space tug capability to ferry payloads between LEO and GEO, for example, potentially lowering the cost of launching high altitude satellites considerably.

JonClarke
2006-Jun-02, 11:31 PM
You don't need lunar water, LOX production on the Moon from the rocks would itself be valuable. However a study I have just read suggests that launch costs are a key component. If launch costs from earth drop to a tenth of their present level then lunar LOX becomes marginal at best.

It's also worth noting that this idea has been round for decades, perhaps longer. And it is a long term prospect at best.

Jon

Warren Platts
2006-Jun-03, 01:55 AM
Really, you'd want to preposition an unmanned gas station around Mars, to refuel a manned flight so they can get back to Earth.

JonClarke
2006-Jun-03, 02:55 AM
Really, you'd want to preposition an unmanned gas station around Mars, to refuel a manned flight so they can get back to Earth.


That's the whole point of Martian ISPP. You can manufacture the propellant for the return journey, or at least the ascent to orbit. But you store it on the surface, not in orbit. Much easier

Jon

ASEI
2006-Jun-03, 04:38 AM
The difference between these refueling stations and just launching more propellant stages/tanks into orbit escapes me for all situations not involving propellant production off Earth. It seems it's only useful if you can make your propellant somewhere else than Earth.

Getting propellant from the moon to low Earth orbit for the sake of a mars mission seems more expensive to me than going to the moon to get it, then continuing to mars. Once you're in lunar orbit, escaping earth takes far less energy. Taking the propellant from the moon to the earth, then back out of earth's low orbiting gravity well seems at first glance roughly twice as hard dv wise. Correct me if I'm too far wrong.

Larry Jacks
2006-Jun-03, 03:30 PM
The reason why it makes no sense to fly to lunar orbit before preceeding to Mars is one of delta-v, which takes propellant. Here are rough numbers:

1. Low Earth Orbit (LEO) orbital velocity: ~17,500 MPH
2. Translunar injection (TLI) velocity: ~25,000 MPH

Delta-v required to fly from the Earth to the moon: ~7,500 MPH

3. Velocity when you arrive at the moon: ~6,000 MPH
4. Lunar orbit velocity: ~4,000 MPH

Delta-v required to enter lunar orbit: ~2,000 MPH

5. Velocity required to fly from the moon to Mars: Don't know, but the Apollo missions had to accelerate to approximately 6,000 MPH to return to Earth. Suppose the number to fly to Mars from lunar orbit was approximately the same.

Delta-v required to fly from lunar orbit to Mars: ~2,000 MPH (big guess)

So, the total delta-v required to fly from LEO to the moon and then on to Mars:
7,500 MPH + 2,000 MPH + 2,000 MPH: ~11,500 MPH

Delta-v required to fly from LEO to Mars: not much more than to enter TLI, ~7,600 MPH.

Savings by flying directly from LEO to Mars: about 3,900 MPH. That equates to a lot of propellant.

Now, if you could harvest lunar materials (ice or other) and extract the H2/O2 and launch that to Earth orbit, it would also take a considerable amount of propellant. However, since your transfer vehicle would probably be a lot less massive than a manned spacecraft bound for Mars, the total mass of propellant could be much less. If you could use aerocapture/aerobraking at the Earth orbit end, you could save a lot of energy. You'd only need to expend energy to fly the lunar materials from the surface to Earth, ~6,000 MPH of delta-v plus whatever orbital corrections necessary to get the materials into the final desired Earth orbit.