
Originally Posted by
kojax
You don't think there might be any cheaper ways?
I'm thinking in terms of just how much thrust you need to get where you're trying to go. 100 kilometers straight up to get out of the atmosphere, and then you have to accelerate to mach 24.
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That's not how its done. You only go straight up for a short distance and then you start to turn "down range". As you climb, you slowly "flatten out" your trajectory, so that by the time you reach orbital height you are flying parallel to the ground. You then continue to fire your engines until you've established orbit. Essentially, the trajectory follows the reverse of that of a "gravity turn" landing. It is the most efficient trajectory to follow to establish orbit.
However long it takes to get to that point is how long you have to continuously generate thrust equal to gravity + air resistance. Or well, as you start approaching mach 24, I'm sure gravity resistance gets to be less and less important, and air resistance gets less and less the higher you go.
One advantage to an unmanned mission is that you can accelerate faster than 4 g's without worrying that you'll kill the crew.
In principle, it just seems like there must be an easier way than the ones that have been tried. I can't think of one off hand, but it seems like they must exist.
As stated above, we already use the most efficient method of launch in terms of trajectory. The only way to make it more efficient is to develop better rocket engines.
Suppose we use different rocket configurations to accomplish different purposes.
Part 1:
We know that wind resistance increases as the cube of velocity, but the amount of time needed to go a given distance only decreases linearly with additional velocity. So, getting to the 100 kilometer limit is best done with a slower, but longer sustained burn.
Part 2:
After hitting the 100 kilometer limit, we want to accelerate as much as possible as fast as possible, so a shorter & faster burn is called for.
Part 3:
Once we're in orbit, all we want is maximum specific impulse/ efficiency. An ion drive or something like it.
These stages kind of overlap in certain ways. Like the 100 kilometre limit isn't a discreet moment. As we get closer and closer to it, wind resistance gets less and less. As we get closer and closer to Mach 24, gravity becomes less and less important as well.
The gist of it is this:
The determining factor for reaching orbit is delta v. It takes a given amount of delta v to achieve orbit. How much delta v can be gain by a certain mass ratio (mass of fueled ship divided by mass of un-fueled ship) is determined by the exhaust speed generated by the engine. the relationship is as follows:
dv = ve * ln(MR)
dv = delta V
ve = exhaust velocity
MR = mass ratio.
We want that delta V to happen at different rates during the different stages if possible. It's just as important to minimize the influence of air and gravity as it is to maximize our thrust.
Here's the problem: Higher exhaust velocities mean high combustion chamber temps and pressures. At present, we can develop high efficiency engines only at the cost of thrust. But to lift payloads into orbit we need high thrust. So, for now, we with are stuck with the lower efficiency, high thrust chemical rockets for launch vehicles. Once in orbit you can use the higher efficiency engines(Ion engines for example). One drawback to the ion engine is that it is a bit more complicated than a chemical rocket, and has a grater up-front cost. So, for some operations, it is still more economical to use chemical rockets.
So what we want is:
Part 1: Exactly enough thrust to match gravity. In the first few seconds we need enough thrust to reach whatever we consider our cruising velocity for the trip to 100 kilometres.
Part 2: As much thrust as we can get as fast as we can get it without burning through our fuel before we can reach mach 24.
Part 3: Something akin to an ion engine.
So, if your soul intention is to land a small package on the Moon, you are not going to it cheaper than using existing launch systems, as the cost of developing a new launch system would exceed that cost greatly.
If on the other hand, you are looking at maintaining a presence in space, it could be to your advantage to develop your own launch system if you can. (And if you can develop a cheaper/better one, then you could probably re-coup your investment by hiring out launch services)
If I figured out it was possible..... well it's more an issue of having fun I guess. I could use that knowledge to predict the likelihood that someone will win.
There's an interesting requirement for the contest: only 10% of the money can come from any public source. So I'm not sure what that means for piggy backing on the space shuttle or whatever other similar options. It's probably ruled out.