This question is inspired by this interview with John Carmack, developer of Doom and Quake, one of many people working to win the X-Prize, a $10 million reward for the first private venture sending paying passengers into space.
Carmack mentions that they’re working with various combinations of propellants: hydrogen peroxide, kerosine, etc. My question is why they’re bothering with rockets at all to get to space.
My understanding of rocketry is that the largest portion of fuel is spent just getting the rocket off the ground and through the atmosphere. Why not design a helicopter platform to take the vehicle to the edge of the atmosphere, and then use rockets to leave the atmosphere? Is it a “sweet spot” problem, where a helicopter can’t be built to be powerful enough to carry the required fuel to use rockets for going further? Aren’t there other alternatives for getting through the atmosphere that are less dramatic but more efficient (a glider, say)?
Probably because a helicopter can’t get that high. As the air gets thinner, any aerodynamic vehicle (including planes) need to generate more thrust in order to get a high enough pressure under the wing to maintain lift. Gliders have an even bigger problem.
Using helicopters, or gliders, or what-have-you, you would save at best a tiny fraction of fuel, and add needless millions (billions?) to the total cost of the operation. Just go with what works… make your rocket 2% bigger for added fuel and just shunt it up through the atmosphere via brute force (or finesse, if you can build a successful ramjet that big, but even those need a big, fuel-thirsty nudge to get started).
Dammit, SPOOFE! I’m the helicopter guy around here! (Well, one of them…)
Anyway, SPOOFE is correct that helicopters can’t fly that high. The air pressure is less as you ascend, so you need to move a larger volume of air to maintain your altitude the higher you go. In an airplane this can be done by increasing the speed, but a helicopter can have “retreating blade stall” (which isn’t a good thing). In forward flight the advancing blade has a higher airspeed than the retreating blade. (Actually, this happens in flight of any direction; but you know what I mean.) At some point the retreating blade’s airspeed will not be great enough to generate lift and it will stall. Can you say, “Tailboom chop”? So as the helicopter flies higher, it must reduce its forward speed until at some altitde it is in a hover and can climb no higher.
Something being launched into space needs to reach “escape velocity”. This is hard to do when your aerial launch platform is stationary. Also, helicopters are not very efficient machines. With a 150hp engine you can have a two-seat helicopter that flies 100 mph. With the same engine you can build a two-seat airplane (such as the Rutan Vari-Eze) that approaches 200 mph, or a four-seat airplane (such as the Cessna 172) that can fly at 140 mph. A helicopter would just not be the right machine, as it would not just have to lift itself and a heavy payload.
The Antisatellite Missile was carried aloft by an F-15. When the F-15 got as high as it could, the pilot would launch the missile which would use its solid rocket motor to fly into space and destroy a satellite. We could develop space vehicles that used a “mothership” to take it aloft, but it would be very expensive and the operation would be complex.
Oneof the X-Prize team’s is using a balloon to hoist their rocket aloft. Once the balloon reaches a certain altitude, the rocket’s cut loose and flies into space, while the balloon sinks back down to Earth.
Depends on if you’re just trying to get out of the atmosphere, or trying to get into a stable orbit. For getting into standard low earth orbit, something like 90% of the energy you have to spend is just to accelerate to orbital velocity. Getting to altitude is the other 10%. So starting off from a stationary platform high up doesn’t save you much.
A number of the X-Prize teams (see them all here) ARE however, using traditional aircraft to boost the spacecraft to a higher altitude before launching with rocket engines to get out of the atmosphere.
At some point you have to go to a rocket instead of a system that requires air to stay up. It can be much simpler, more reliable, and less expensive to use rocket power all the way from the ground, although air-launching does have some advantages.
There’s also the Pegasus launcher built by Orbital Sciences. It’s launched from a Lockheed L-1011 airliner at an altitude of 40,000 feet.
This method has been proposed for larger spacecraft but none of them took off. I’d guess it’s because rockets science is an extremely conservative field. When you’re dealing with machines which have to work perfectly the first time, and risk hundreds of millions of dollars (or even human lives) if it they don’t, you learn to use only the tried and proven methods.
Squink: The movie wouldn’t play for me, either on Netscape or Explorer. But I did see something about this vehicle on The Discovery Channel. While the test vehicle is designed to fly up to 6,000 feet, the actual vehicle would have used the rotor during descent, not launch.