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

787 compared to Saturn V

Saturn V
First Stage

Boeing 787-9

ratio

Length

m

42

63

Top speed

m/s

2300

265

8.7

Max Range

km

560

15700

Max Altitude

km

112

13

Max Thrust

kN

34000

640

53

Takeoff accel

gee

1.3

0.26

5.0

Max accel

gee

4.0

0.26

15

Running time

sec

168

62000

Total Weight

kg

2800k

251k

11

Payload Weight

kg

500k

63k

7.9

includes fueled 2nd and 3rd stages + LEM + CM fuel

Dry Weight

kg

131k

118k

1.11

Fuel Weight

kg

490k

70k

O₂ Weight

kg

1680k

0

Fuel+O₂ Weight

kg

2170k

70k

31

Fuel+O₂ rate

kg/s

13000

1.13

12000

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, since it is actually 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? Probably, the recovery/disassembly/inspection/reassembly process will 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.

Indeed, it might be possible to burn the tank aluminum for more thrust. The I_SP_ 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.

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

787SV (last edited 2013-06-04 17:52:37 by KeithLofstrom)