srw
|
|
Scramjets
| 
Mar 15 20:52 UTC 1999 |
There's an interesting article in a Scientific American I was perusing
(I think it's fairly recent) about scramjets.
The real topic is how to find a cheaper way to lift paylod into earth
orbit. Right now we use rockets, and the process is very expensive.
Rockets carry their own oxidant, usually lox. and it is heavy, boosting
the weight of the vehicle enormously, and resulting in the need for
multiple stages, resulting in great waste and expense.
What is needed is a jet-based propulsion system, so that the oxygen
in air can be used to oxidize the fuel. This would cut down on all of
this waste. It's an engineering problem, though.
Our most advance jets are turbine-based jets. The parts in these fail at
high speeds (over mach 2) and much higher speeds are needed. Ramjets are
designed to work without such turbines, so that they can run at higher
speeds. Ramjets compress the incoming air, a process necessary to slow
down the air and heat it up. The fuel is injected where the air is
moving subsonicly, and the mix ignites producing thrust. Ramjets don't
work well at slow speeds bu can go much faster than turbo jets.
But they still can't go fast enough. At some point (mach 20?) the
compression of the air generates so much heat that the product of the
combustion (water) breaks down. This is bad.
So this article discusses new technology being investigated, called
Supersonic-comustion Ramjets (Scramjets). In these, the air is not
compressed so much, but this means that the fuel is injected at
supersonic speeds. It has only a millisecond or so to ignite. It's very
tricky stuff, but has the potential to reach greater speeds. Ultimately,
the vehicle gets too high, and a rocket is needed to reach orbit, but
this could perhaps be done much less expensively this way.
Of course it's all just on the drawing board, but I thought it was
interesting.
|
russ
|
|
response 1 of 1:
|
Mar 17 04:47 UTC 1999 |
Scramjets are a very interesting scientific exercise. However, as a
practical method of getting to orbit they fall a bit short.
Consider the last several serious space-vehicle development efforts.
DC-X (prototype for the DC-1): used rockets. The VentureStar: uses
rockets. Roton: uses rockets. Black Horse (a small aerospace plane):
uses rockets. See any trends here?
A scramjet promises to be very fuel-efficient route to orbit. However,
fuel isn't a particularly expensive part of the trip. AAMOF, fuel is
downright cheap; even at $10/gallon, the fuel for a Shuttle launch is
only a few million out of the roughly half-billion dollar total.
The costly part of getting to space is the hardware, both R&D and recurring
costs. It's here that scramjets are tough.
Rockets have the disadvantage that they have to carry their own oxygen.
This adds up to bigger tanks and more weight. However, dispensing with
the need to breathe air lets rockets do most of their accelerating outside
all but the faintest wisps of atmosphere; the engineer is free to choose
shapes and materials which would be utterly impossible to fly through air
at the necessary speeds.
Contrast the scramjet. Such a machine has to do most of its accelerating
in the atmosphere, if it's going to get much benefit out of the weight of
the airframe and engines. This means flying through air at very high Mach
numbers. And there's the problem; it's really, really hard to find
materials which will hold together under a sustained assault of glowing
oxy-nitrogen plasma. Building an airframe out of these materials is
even harder. Making that airframe propel itself with an air-breathing
engine is a further set of difficulties, and squeezing all the required
fuel plus a payload into that airframe piles on even more constraints.
Satisfying all these constraints is a long, expensive learning process.
The upshot is, we have neither the money nor the need to build a vehicle
like that. We do need to get to orbit, but there are other ways. (The
materials and aerodynamics research is good for things like ballistic-
missile warheads, which may be the primary use of such research.)
Space travel seems to be going in the opposite direction. DC-X was a
fat, blunt lifting body; it was designed to leave the atmosphere quickly
on the way up, and do most of its decelerating up high on the way down.
Roton: another design that's largely empty tankage on re-entry, so
pretty much the same where it counts. Ditto VentureStar and Black Horse.
The only place I see even ramjets making a difference is in vehicles
like Pegasus, which is air-launched from a carrier 'plane; since the
launch weight is limited by the carrier vehicle, the weight advantage
of a ramjet over a rocket motor would allow a much bigger payload to
be sent up. This may be a win, but it's still iffy. I don't see anything
using scramjets in the next 20 years.
Oh, check out the Roton homepage: www.rotaryrocket.com.
|