Suppose I am a developing nation wishing to enter the 21st century with telecommunication sattelites, manned missions, etc. but don’t have a near-equator launch facility, nor powerful rockets capable of lifting a payload to orbit.
True, I could just pay NASA, ESA, The Russians, the Chinese, etc., but I wish to be self-sufficient.
So I have a simple launcher with a simple ramjet first stage and a traditional rocket engine second stage…and attach this launcher to a maglev pallet within a high vaccuum tunnel approx 100 miles long…starting horizontally and curving up a gentle gradient to almost vertical up the side of a mountain. The vaccuum would be maintained by air pumps so that magnetic accelleration can speed up the launcher to several mach numbers without aerodynamic drag. The exit would have a giant camera-like iris which would snap open at the last possible second.
As soon as the launcher cleared the maglev/railgun/coilgun tube, the engines would ignite.
So…If I have a payload going, say, mach three starting a mile above sea level, how much of a fuel weight savings do I have over a more traditional launch at sea level from a standing start?
What makes you think there will be any energy savings?
You launch your ship a mile high at a very high mach number. The kinetic energy didn’t come out of thin air. Some things you can’t change about physics and conservation of energy is one of them. Electricity isn’t free and I think your magnetic rail gun is going to draw a hell of a lot of amps driving a mass to high speed. Yes, it will be easier in a vacuum but what runs those vacuum pumps? Invislble underpants gnomes on a treadmill?
He’s not talking about net energy savings, he’s talking about fuel weight savings. Since the vehicle doesn’t have to expend additional energy to lift the weight of its own fuel, there’s more room for payload.
The biggest problem with the setup described is the transition from the vacuum of the launch tube to the outside air. At several times the speed of sound, this would be like hitting a brick wall. AT least all the energy you saved by accelerating the paylod in a vacuum will be lost when it hits the air anyway, so there’s no point in the added expense of engineering in the vacuum system and all the airseals and whatnot to make it work.
I think there’s too many variables here to be quite that certain!
A lot depends on how high the mountain is. If your exit is at the top of Kilimanjaro for example, the pressure is ~500 millibar so you’re halfway out of the atmosphere as far as drag is concerned, if not altitude. That’s a help.
Ramjets start to work at about Mach 2.5 and stop working at about Mach 6, if my not entirely reliable memory serves. So a 100 mile long evacuated maglev launcher is inappropriate in combination with a ramjet. If you want to use a ramjet, built a shorter non-evacuated coilgun on the top of your mountain with an exit velocity of Mach 2.5. Otherwise, leave it out and cut straight to the rocket. Any savings involved depend upon how fast you’re actually going at the time.
At simplest, you are eliminating the first and possibly second stages of a multistage rocket, replacing them with a massive engineering undertaking. The economics of this depend on how much you want to launch - if you’re building a set of space stations requiring tens of thousands of launches, it may just be worth it. If you want to get into the satellite business, it’ll never pay for itself.
Somewhat more interesting is if your maglav laucher alone gets you above the atmosphere at orbital speeds, and the rocket is only used for fine control. This is theoretically possible but you’re going to have to exit from the launcher at one hell of a speed to compensate for the velocity you’re going to lose to drag. A very old NewScientist (mid 80’s) ran an article on this suggesting an exit velocity of 30 km/s was needed at ground level - that is almost three times escape velocity! Exiting at the top of a high mountain might save you a bit. But you’re going to need a heatshield on the way up considerably more effective than the ones currently needed for re-entry! The technical difficulties might make this approach impractical.
Assuming that once you’ve built this monster you can fire it up all day long without too much wear and tear, then your costs per launch can be very low compared with rockets. However, again you’re going to have to launch a tremendous amount for the savings to be worth building the thing in the first place. That may be the point however - once built, we may find things to do in space that justify that sort of launch capacity.
By the way - curving from the horizontal to vertical requires a pretty big radius to avoid centripetal forces crushing your passengers. I wouldn’t recommend sending people up in this thing!