787 compared to Saturn V

Some people ask "why not make rockets reusable like airliners?" Then they don't wait around for the answer, because they want to keep repeating the "question", which is not a real question but a statement of belief.

• 1 - Fuel, Fuel, Fuel! Or more accurately, propellant, fuel+oxidizer. Rockets aiming for orbit are 95% propellant by weight; they must be, to achieve the high speeds necessary. Just the first stage of a rocket goes 9 times faster than an airliner; a 3 stage stack can go 50 times faster. Because the rocket must carry its own oxidizer, and must lift its own mass (wings aren't much use at high mach in thin atmosphere), the propellant is 4.4 times heavier per energy out. So the Saturn V first stage carries 31 times the propellant of a 787, and burns it 12 thousand times more quickly.

• 2 - Airplanes don't need staging, rockets must stage so that most of the dry structure weight doesn't have to accelerate to orbital speeds.
• 3 - Rockets are big masses of propellant wrapped with a thin skin of aluminum-lithium alloy. The propellant is expended, so at best a "reusable" rocket system is only 5% reusable.
• 4 - Reusability implies perfect structural integrity and control not only though launch, but also through glide and soft landing. A single-use rocket stage doesn't need control after separation, and falls into dense atmosphere at very high mach numbers. Surviving that with zero damage, then arriving at the ground at near-zero velocity, requires aerodynamics structures such as nose cones, and deceleration systems such as parachutes and terminal thrust rockets, and stout non-deforming structure, a lot of extra weight.
• 5 - Every gram added to a rocket stage dry mass subtracts from the weight of the stages above it, ultimately subtracting from the payload mass, which defeats the purpose of the launch. A kilogram in orbit is $5000; formed rocket tank aluminum is $5/kg or so. So if we increase dry mass by 10% in the quest for reusability, and cut payload by half, we have increased the fuel per payload kilogram by about 2.2x.
• 6 - Systems designed for the long-term must tolerate wear and accumulated microfractures - that means thicker and heavier.
• 7 - If you recover all your stages, are you going to send them up again without detailed inspection? The recovery/disassembly/inspection/reassembly process will probably require more work than the original assembly.

Rockets are not airplanes. They have far different missions, go much faster, are subjected to far more stress, and guzzle far more fuel. If we want to improve cost/performance, we should focus on automated manufacturing, logistics, and reliability. Tankage and rocket bells costs can be driven towards raw materials cost. Perhaps we can work on recycling the returning stages as materials, perhaps even reusing the avionics, turbopumps, and thrust chambers after detailed inspection. But making the pop can holding the fuel reusable? We don't even do that for soda pop.

It might be possible to burn the tank aluminum for more thrust. The Isp approaches RP-1. I haven't a clue how to turn a tank into powder and burn it, though. Once in orbit, tank skin is usable as ballast mass for server sky thinsats.

MIR spent 5519 days in orbit, and travelled 2.27 billion miles. 747 cruising speed is 555mph, and 747s are retired before 130,000 hours. So MIR travelled 30 times farther in its lifetime than any 747 ever will. 787 long term durability is unknown - some fear the composites will delaminate sooner than aluminum fatigues. I hope 787s last much longer, but time will tell. Airlines buy 787s for fuel performance, not durability, and rocketeers should use the same metric.

Rockets are hard. We and the Soviets together spent 8 trillion dollars developing them. I think SpaceX will do better for less, but not a lot better. If getting to space with chemical energy was easy, the earth would have not retained an atmosphere for four billion years; both launch and atmospheric boiloff are subject to the same constraints. Humans got to space because we use staging, and are fiendishly clever; if we are airheads, we stay grounded.

[1] http://en.wikipedia.org/wiki/Boeing_787_Dreamliner [2] http://en.wikipedia.org/wiki/Saturn_V