# Low cost launch technology

PhilKravitz
Why is there no low cost space launch technology? The cost to buy the energy (in the form of electricity) required is very low (don't have the number on me).

## Answers and Replies

russ_watters
Mentor
How do you define "low cost" and what does electricity have to do with it? The problem of launching an object into orbit is pretty straightforward and to do better than chemical rockets would require something pretty exotic.

PhilKravitz
How do you define "low cost" and what does electricity have to do with it? The problem of launching an object into orbit is pretty straightforward and to do better than chemical rockets would require something pretty exotic.

If one buy the joules per kilogram required to reach orbit from the electric company in the form of electricity it is well let see

1 kilo to say 1e4 m/sec using 1/2*m*v^2 is 5E7 joules

so 1 kilowatt hour at 20 cents from con edison is 3.6e6 joules for 20 cents. So 5E7 joules would cost about $3. Of course there are always losses in any system but even with only 10% eff. that would be$30 per kilo to orbit as far as energy costs are concerned. But rockets cost roughly $10,000 per kilo. I do like the beamed power solutions I have been seeing articles about recently. Beam power to the "rocket" in the form of microwaves or as laser light. russ_watters Mentor The fundamental problem with chemical rockets is that you aren't just lifting the payload but are also lifting the fuel. Finding a technology that keeps the fuel on the ground hasn't been very easy. If one buy the joules per kilogram required to reach orbit from the electric company in the form of electricity it is well let see 1 kilo to say 1e4 m/sec using 1/2*m*v^2 is 5E7 joules so 1 kilowatt hour at 20 cents from con edison is 3.6e6 joules for 20 cents. So 5E7 joules would cost about$3. Of course there are always losses in any system but even with only 10% eff. that would be $30 per kilo to orbit as far as energy costs are concerned. But rockets cost roughly$10,000 per kilo.

I do like the beamed power solutions I have been seeing articles about recently. Beam power to the "rocket" in the form of microwaves or as laser light.
There is a lot more than just energy required to launch a rocket. The force required to accelerate a rocket is based on momentum transfer from the rocket to the exhaust gases. Momentum transfer requires mass (photons are the exception*). Because mass is transferred to the exhaust from the rocket, the rocket loses mass, so the rocket has to carry this mass in addition to the energy needed to accelerate the expelled mass. Also, the acceleration rate has to be limited to a few g's (10?) in order not to crush the astronauts inside. See the rocket equations in

http://www.braeunig.us/space/propuls.htm

Bob S

*photons are an extremely poor source of momentum transfer, in terms of thrust per unit energy.

Gold Member
Not to mention that power on the grid is generated by giant machines. For a rocket, you have to compact all that required energy and mass into a tiny space, which is generally super expensive.

I do like the beamed power solutions I have been seeing articles about recently. Beam power to the "rocket" in the form of microwaves or as laser light.
Kinetic energy may be written (Newtoninan classical mechanics)

Eclass = ½mv2 = ½(mv)2/m = p2/(2m)

So the momentum p is, for a given classical energy Eclass

p = sqrt(2mEclass)

So classically, photons have no mass and therefore can transfer no momentum.

Relativistically, however

p = Etotal/c

where Etotal = [STRIKE]Eclass + mc2[/STRIKE] = EPlanck, and c = speed of light.

So there is a very small momentum transfer with photons.

Bob S

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mheslep
Gold Member
There must have been some prior looks at some kind of pre-launch acceleration tower for heavy lift vehicles, to save on launch fuel? It appears that a 3g (?) acceleration along, say, a 1000M 'elevator' tower takes the vehicle to ~250 m/s = sqrt(2 * 3g * height) before it needs to burn its own fuel. This would be the heavy lift version of what is already done with air launched rockets such as Pegasus.

Edit: Listing problems/comments here as they occur to me:
• A launch failure might destroy not only the vehicle but the significant investment in the vehicle pre-launch elevator.
• Drag incurred at low altitude (250 m/s) forces the vehicle to give back some energy gained by using the elevator. (Meaning the launch needs to move from the Cape to Pike's Peak :tongue:)
• Fuel savings: ~2kg of LOX per metric ton of payload pre-accelerated to 250 m/s [using a savings of 15 MJ combustion energy per kg of LOX to reach 13 kJ of kinetic energy per kg of payload at launch velocity]. So on the shuttle for instance at 2000 mt launch weight, this would save 4 mt of LOX, not counting mechanical structure savings, etc.
• Mechanical complexity of the elevator. Lifting a 2000 mt vehicle slowly is achievable with current technology. Lifting and accelerating something to 150 m/s is achievable with current technology. Doing both things together might be quite difficult - rapidly changing large moments along the tower, etc

I suppose what I'm looking for here are the right terms for which to google, as nothing pops up under the terms I have used above.

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