Supersonic Man

May 10, 2017

no Apollo

Filed under: Hobbyism and Nerdry,thoughtful handwaving — Supersonic Man @ 9:21 am

If NASA had not been hurried into building the Apollo mission by the “space race” against the USSR, how might we have arrived at the Moon? Space development might have proceeded a good deal more slowly and less expensively, building on the X-15 rocket plane experiments. I think that program would eventually have arrived at something fairly close to the Space Shuttle. If you solve all the problems of the X-15 one by one to make it orbit-worthy, it would have had to be much larger and blunter, because any adequate heat shield is going to be around four inches thick, and that doesn’t scale down for something skinny or pointy. That sounds a lot like the shuttle to me.

So let’s say we were trying to send a mission to the moon using space shuttles. The shuttle itself can’t go there even in you fill the cargo bay with fuel, and that would be wasteful anyway, as you don’t need most of its bulk. So I think the bits that actually go to the moon would be much as they historically were in Apollo: a lunar module, command module, and service module. Why not just stick those into a shuttle bay?

The shuttle’s cargo bay is 60 feet long and 15 feet across, though for a cylindrical cargo the cross section needs to be a bit smaller, as the space isn’t fully round. The mass limit for a flight to low orbit is a hair over 30 English tons, or 27.5 metric tons. (I don’t think any real flight ever exceeded 83% of that capacity.) What can we work out based on these limits?

You can’t fit all three modules into one shuttle-load, but they’ll go in two loads, if you make the lander a bit less broad and gangly. One would be the command module and lunar module, and the service module would be the other. And we might have to trim a bit of weight from the service module, like maybe take out the heavy batteries and put them in the other load. This means the service module would have to be mounted to the command module by shuttle astronauts in space suits, which would be inconvenient, but doable. Alternately, you might cram the three modules into one flight all preassembled, if their fuel were in another. This would mean at least six operations of astronauts pumping dangerous fluids into various tanks spread throughout the modules. It might also mean assembling the lander’s legs from some inconveniently compact from.

Now you need a rocket to send the set toward the moon — one rather like the S-IVB third stage of Apollo, which used the majority of its fuel to lift the three modules out of low orbit and fling them toward the moon. This rocket was a bit too large to fit into a shuttle bay, but we can reduce its size by at least 25%. Its weight is no problem, if it’s empty. But the fuel would take three additional shuttle loads. Historically this rocket weighed 10 metric tons empty, and pushed a 45 ton payload. The required delta-V is 3.1 km/s. It burned around 75 tons of hydrogen and oxygen to accomplish this. It used about 30 tons more to finish lifting Apollo into low orbit around Earth during launch, which would not be needed in this case.

So the mission would require six shuttle launches, starting with one to put up the booster with maybe the first splash of fuel in it, and three more to fill it up. Then the service module would be brought up, and attached to the booster. The command and lunar modules would come up last, along with the astronauts who will ride in them. That last shuttle could stay in orbit for a couple of weeks to await their return.

It might be better to bring the fuel up in the tanks that will be used instead of needing to pump it from one tank to another, so maybe the booster would just be a framework that fuel tanks would be bolted into. Such a framework might be folded smaller for transport. This would require additional assembly in space, possibly employing double digit numbers of shuttle astronauts over several flights.  But if everything were prepared well on the ground, the task should not be difficult or dangerous. And if the orbits were well planned, the booster stage could be recovered into Earth orbit, and either refueled for another mission, or if necessary flown back down for refurbishment. As SpaceX has demonstrated with their Falcon landings, once a booster is detached from its payload and has mostly empty tanks, a small amount of remaining fuel can accomplish quite a lot of maneuvering, so I don’t think it’s implausible that its engine could return it to low orbit with the last of its fuel, especially if it discards some dead weight such as empty tanks.

The command module might not need to splash down into the ocean. But it might still need a heat shield, just to brake in Earth’s atmosphere enough to slow down into an Earth orbit, so a shuttle can pick it up. Or, this somewhat risky air-braking might be avoidable by making the service module larger and giving it more fuel. (Perhaps it also could use bolt-in tanks. Add at least one more fuel-hauling flight to the schedule in this case.) An ocean splashdown might be the emergency backup option if the rendezvous fails. But if it succeeds, they could even have the option of recovering the upper stage of the lunar module, and flying both modules down with the astronauts in one shuttle landing.

I’m sure this sounds a lot more awkward and inconvenient than the Apollo’s comparatively simple process of just launching one big rocket, but it would have been vastly less expensive. Most of the parts would be reusable instead of disposable. The only part that absolutely could not be reused is the bottom stage of the lunar module. Apollo cost us at least $20 billion per landing, in today’s money; this would cost perhaps a quarter of that — and I’m sure if we made this a continuing operation, we would have found ways to lower the costs further. Instead of just six trips to the moon, we might have continued doing dozens. We might never have stopped.

However, I do worry that this process might have exposed astronauts to greater risks. Lots of opportunities for something to go wrong up in orbit, and lots more shuttle flights. As we have seen, those shuttles were not the safest things to fly in.

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1 Comment »

  1. Pretty soon it should get a lot easier than this. If SpaceX’s “BFR” plans work out anything like they hope, a moon mission should become pretty darn easy. Launch one BFR, power it up to a fairly high orbit, launch another one as a tanker, pump fuel from the second to the first, then fly that first BFR to the moon, down, up, back to Earth, and down to a propulsive landing. SpaceX intends to fly exactly this mission sometime in the next decade, just to show they can.

    Comment by Supersonic Man — October 5, 2017 @ 11:51 am | Reply


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