Post by JayUtah on Feb 17, 2007 15:54:41 GMT -4
35+ years after the fact this technology is still under research and development and it still cant be done.
Misleading.
You imply that we've been doggedly pursuing this problem for decades and can't make any headway. You imply the reason for this abhorrent lack of progress is the inherent difficulty of the dynamic instability problem.
That's hogwash. Research into landing on Earth using rocket-powered vehicles isn't really being pursued. You can't wave your hands at "35+ years" of supposed research and from that try to argue that the problem must be inherently difficult. If nobody's really interested in researching something, then it doesn't matter how much time passes in disinterest.
Why the lack of interest? Because there are much better ways of landing on Earth. Earth has air, and the air provides advantages.
Consider propulsion. Jet propulsion and rocket propulsion work according to the same principles, but jet propulsion requires less of a fuel load. The reaction mass in rocket propulsion has to be carried entirely in the rocket. In a jet, the air serves as part of the reaction mass. So a jet-propelled vehicle can be built with a more favorable mass ratio. If lift is to come from reaction propulsion, the jet is the obvious choice.
We consider rocket engines only when there is some other reason to use them. If a vehicle is meant to operate both in space and in an atmosphere, it's worth experimenting whether the rocket propulsion intended for space can also facilitate landing on Earth. If it works out, we can build a ship with only one kind of propulsion system that works in both places, and simplifies the vehicle design.
And it works -- it just isn't the best method of providing lift if you have an atmosphere to work with.
So consider lift independent of propulsion. We have fixed-wing and rotary-wing methods of producing lift in an atmosphere, and both have been used successfully to land spacecraft. Since they require no energy input, they are very favorable in an overall vehicle design. You pay for that efficiency in terms of flexibility -- you have to land within a certain radius of your atmospheric entry point. If you need flexibility, have your rocket produce forward velocity, which can induce lift via an airfoil. That is a very efficient and flexible way of producing lift. If energy is at a premium and flexibility isn't important, aeroshells and parachutes are the natural choice. They require only a sufficiently dense atmosphere.
A very fun method is the hybrid approach pioneered by the Russians. They use a combination of parachutes and rocket engines to deliver heavy payloads to Earth landing. And we've adapted the same method to land spacecraft on Mars.
The parachute delivers the payload to within a certain distance of the ground, albeit at a fairly good clip. At a certain altitude, a set of rocket engines fires and slows the vehicle dramatically just before it hits the ground. Of course rotational stability in this arrangement is largely a non-issue because there isn't much time for problems to arise, and the elements are mechanically decoupled and don't have a single rotational reference frame. You'd be right not to call this a controlled descent in the same vein as the LM. But think about how it might apply to the LLRV stability problem.
Misleading.
You imply that we've been doggedly pursuing this problem for decades and can't make any headway. You imply the reason for this abhorrent lack of progress is the inherent difficulty of the dynamic instability problem.
That's hogwash. Research into landing on Earth using rocket-powered vehicles isn't really being pursued. You can't wave your hands at "35+ years" of supposed research and from that try to argue that the problem must be inherently difficult. If nobody's really interested in researching something, then it doesn't matter how much time passes in disinterest.
Why the lack of interest? Because there are much better ways of landing on Earth. Earth has air, and the air provides advantages.
Consider propulsion. Jet propulsion and rocket propulsion work according to the same principles, but jet propulsion requires less of a fuel load. The reaction mass in rocket propulsion has to be carried entirely in the rocket. In a jet, the air serves as part of the reaction mass. So a jet-propelled vehicle can be built with a more favorable mass ratio. If lift is to come from reaction propulsion, the jet is the obvious choice.
We consider rocket engines only when there is some other reason to use them. If a vehicle is meant to operate both in space and in an atmosphere, it's worth experimenting whether the rocket propulsion intended for space can also facilitate landing on Earth. If it works out, we can build a ship with only one kind of propulsion system that works in both places, and simplifies the vehicle design.
And it works -- it just isn't the best method of providing lift if you have an atmosphere to work with.
So consider lift independent of propulsion. We have fixed-wing and rotary-wing methods of producing lift in an atmosphere, and both have been used successfully to land spacecraft. Since they require no energy input, they are very favorable in an overall vehicle design. You pay for that efficiency in terms of flexibility -- you have to land within a certain radius of your atmospheric entry point. If you need flexibility, have your rocket produce forward velocity, which can induce lift via an airfoil. That is a very efficient and flexible way of producing lift. If energy is at a premium and flexibility isn't important, aeroshells and parachutes are the natural choice. They require only a sufficiently dense atmosphere.
A very fun method is the hybrid approach pioneered by the Russians. They use a combination of parachutes and rocket engines to deliver heavy payloads to Earth landing. And we've adapted the same method to land spacecraft on Mars.
The parachute delivers the payload to within a certain distance of the ground, albeit at a fairly good clip. At a certain altitude, a set of rocket engines fires and slows the vehicle dramatically just before it hits the ground. Of course rotational stability in this arrangement is largely a non-issue because there isn't much time for problems to arise, and the elements are mechanically decoupled and don't have a single rotational reference frame. You'd be right not to call this a controlled descent in the same vein as the LM. But think about how it might apply to the LLRV stability problem.