Date: Tue May 8 17:42:35 2001
Posted By: Jason Goodman, Postdoctoral Fellow, Department of Geosciences
Area of science: Engineering
ID: 988658108.Eg
Message:
The fuel savings is important, but not huge. As I've said elsewhere
in the MadSci Network, a rocket must go fast to enter orbit:
going high is less important and less difficult. The main
benefits of launching from high altitude (in rough order of importance)
are:
- Reduced drag losses: The lower atmosphere is thick, and hard to
move through at high speed. By launching at high altitude, air drag is
reduced.
- Increased engine expansion A rocket works by allowing hot,
high-pressure gas to expand. As it expands, its pressure and temperature
drop and its speed and thrust increase. Exhaust gas can't leave the rocket
at a pressure lower than the surrounding air pressure. A rocket at high
altitude can allow its exhaust gas to expand more completely, and so
operate more efficiently.
- Reduced gravity losses: Being farther from the Earth, and
launching to orbit more quickly, slightly reduces the amount of work you
have to do to fight Earth's gravity.
I've done some very rough calculations based on information cited below
which suggest that the fuel savings from launching at 4 kilometers altitude
will amount to very roughly 10%. I've assumed a single-stage to orbit
rocket with an exhaust velocity of about 3.5 km/s. The savings will vary a
great deal depending on the type of rocket, the altitude it's launched
from, and how well it's been designed to take advantage of a high-altitude
launch.
Note that the fuel savings doesn't translate directly into increased
payload. A typical large rocket might weigh 1000 tonnes at launch and
carry 15 tonnes of payload. Launching from high altitude might save 100
tonnes of fuel, but that does not mean you can carry 115 tonnes of
payload. The best you could do is about 16.5 tonnes of payload.
Sources:
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