Why wouldn't this space travel strategy work?

I know that a lot of the fuel in a rocket is used to get out of the atmosphere. Why couldn’t we send up an unmanned satalitte full of fuel so that when we send up the manned flight they can top off their tanks and have plenty of fuel for a trip to Mars? Is it merely cost in sending up the fuel or is there a technical reason why refueling in orbit won’t work?

Something like that may indeed be used if we were to use chemical rockets to make a trip for mars. It’s not cheaper - you still need to haul all that mass to space, and I’m guessing multiple launches is more expensive. But it allows you to top off and replace the fuel you used to get to LEO.

A mars craft, depending on the size, may come in several pieces to be assembled in space anyway.

I’m not very smart about these things but if you could develop a rocket that could escape earth’s gravity and still have that much fuel left, why not just make that the exploration ship?

Because in order to maximise the fuel payload, you have to skimp on everything else - it’ all comes down to weight - we can either deliver a big lump of fuel into orbit, or a big lump of habitat and life support etc, or some combination of the two, by total weight - or do it in more than one lift, as the OP proposes (and is probably the way it would be done)

Lifting humans is more expensive than lifting cargo, because of the extra safety requirements. If a cargo load of fuel blows up on launch, so what fuel’s cheap.

Well, presumably your human launch vehicle is already safety rated for humans, so that cost is still there.

It occurs to me that this issue is sort of like how they use aerial refueling for some military planes - they can actually carry more than payload than they can take off with, so they launch with a full weapons load but partially empty fuel tanks, and then get them filled up quickly once they’re airborne. Similarly an orbital refueling could get a bigger payload into space than is practical to lift up until that point.

For something like a trip to Mars, it would almost certainly be necessary to assemble and fuel the spacecraft in Earth orbit; the total mass would be impractically large to launch in one go from Earth’s surface.

As to why no one has experimented with in-orbit fuel transfer… well, that’s a pretty good question. Despite the decades that have passed, there are still a lot of very basic things that no one has tried in orbit yet, and I couldn’t tell you why. No one has tried even a basic experiment with rotational gravity (except for, I believe, a cargo of mice). No one has tried to actually construct (as in welding metal together) anything in space. No one has tried to keep a tank of cryogenic fuel refrigerated for long periods. No one has tried to keep a crew supplied with oxygen from algae or other plants. Virtually none of the engineering challenges that would have to be done for any large long-term manned presence in space.

It’s getting a little dated but the Mars Direct program side steps the problem by throwing a return vehicle and in-situ fueling machinery to Mars first and then follows up with a crew launch.

  1. Large amount of HW on site for exploration/research
  2. No need to drag return fuel with you to Mars
  3. No need for a Mars mother ship to return to and then return home
  4. Heavy lift vehicle in the Saturn V range with no need for in orbit assembly

The biggest problem is the need for a sufficiently compact power source to transform 4 tons of gelled hydrogen into methane, oxygen and water. You’d need a nuclear reactor for that.

And transmutation.

I suppose the problem is this: the only long-distance large payload missions were the Apollo flights to the moon. At the time they were in a hurry and wanted everything done at once; they didn’t want to risk failure because one half of the launch didn’t make it; so they launched it all at once and threw away pieces as they went on.

Since then, all the shuttle and Mir and ISS missions are low earth orbit. Once you’re up there, you don’t need a lot of fuel. I’m sure they WILL do what you describe for a Mars mission, simply because the payload requirements are such that even the living quarters - let alone lander, supplies, and fuel - probably needs to be assembled from multiple pieces, multiple launches.

IANARS (Rocket Scientist), but… The devil is in the details; the shuttle tank, for example is filled with liquid hydrogen and liquid oxygen; not fun things to handle, and they need to be heavily refrigerated. Probably not something you can pop up into orbit and forget about for a few weeks. We need a simple, easy tostore but powerful rocket fuel. LOX and H2 are ideal for earth launch because you can have a huge facility nearby to make and store them, and they are efficient and simple.

I’ve never understood why they haven’t, for example, made an orbit-to-orbit tugboat to service and maintain those massive communication staellites in geostationary orbit. This would allow them to assemble ISS-sized units piece by piece and repair or upgrade them as needed. Instead, each one is a single-payload, work-or-die unit.


To expand on that look at Sabatier reaction or In-Situ Resource utilization for more details.

No Chemistry that I know of is going to convert Hydrogen into Oxygen (or Carbon).

CO[sub]2[/sub] + 4H[sub]2[/sub] → CH[sub]4[/sub] + 2H[sub]2[/sub]O
CO[sub]2[/sub] + H[sub]2[/sub] → CO + H[sub]2[/sub]O
2CO[sub]2[/sub] (+ energy) → 2CO + O[sub]2[/sub]

Where does the CO2 come from?

The atmosphere.

Orbital refueling is probably more trouble than it’s worth, but it should be easy enough to launch a fully-fueled rocket stage into LEO, and then dock that to whatever payload you want sent to Mars. I doubt it would be easy to simply refill a rocket that’s already been fired; these things aren’t designed to be used repeatedly. Docking objects in LEO is something that we have a lot of experience with.

If you have that kind of energy available, why bother to lug all that hydrogen along? Just get it from the atmosphere (or soil) too.

Sounds like in less than 24 hours we Dopers came up with some (relatively) low-cost space exploration missions that even NASA could handle yet would be invaluable from a research perspective.

Add to that a reusable space ferry from LEO to lunar orbit then a reusable landing craft (yes, just like Heinlein wrote about) along with the fuel scenerio in the OP and we may actually have practical moon travel.

Straight Dope FTW!!!

Now all we have to do is build it. How hard can that be?

Well it seems to me that it simplifies the equipment needed to be sent for the initial missions. If you have to start harvesting water from regolith and then pass it through the same reactions to create the fuel, oxygen and water it adds mass and complexity to the overall mission. What happens when you can’t harvest water or not enough on your first few missions?

For your first run why not take 4 tons to prime the pump? Then as sufficient infrastructure is placed in Mars begin to process of harvesting further resources to become more independent. To my view it’s a lot like going camping and taking a fishing pole. You could craft a pole and hook once you got to where you were going but it makes it a lot more likely you’ll have fish if you take one with you by default.

I wouldn’t do it that way.
If I was already committed to hauling an energy source AND 4 tons of hydrogen all the way to Mars, I would just make my energy source 2 tons more powerful, and then use that energy to power 2 tons of robots to go out and harvest the ice needed.

But, I’m not in charge.