A survey of alternatives to the rocket for boosting into space, followed by a new (to me) method. Extant proposals require imaginary super-strength materials, or subject a payload to gun-like extreme g-forces. The new method is much closer to current state of the art. It may actually be doable, and g-forces are moderate.
Caveat: The topic is so popular, I would have expected to find evidence of prior art. I didn’t find any. This method is new to me.
Artemis represents the culmination of the disposable chemical rocket, unlikely to be exceeded unless metallic hydrogen becomes a practical fuel. Rockets are subject to the tyranny of the rocket equation, discovered independently by William Moore, Konstantin Tsiolkovsky, Robert Goddard and Hermann Oberth. Quoting from (NASA) Ideal Rocket Equation,
From the ideal rocket equation, 90% of the weight of a rocket going to orbit is propellant weight. The remaining 10% of the weight includes structure, engines, and payload. So given the current state-of-the-art, the payload accounts for only about 1% of the weight of an ideal rocket at launch. Rockets are terribly inefficient and expensive.
Though Artemis provides a gentle, hopefully reliable vehicle for humans and scientific exploration, the rocket equation prohibits a role in the commercial exploitation of space, or sustenance of colonies. With Artemis, a bulldozer on the moon becomes a billion dollar bulldozer. See also (NASA) The Tyranny of the Rocket Equation.
A rocket has to lift and accelerate its propellant. If a rocket could perform without the weight of propellant, we’d be vacationing on Mars. But there is no way around this; fuel is part and parcel of the concept. There are other concepts, which do not employ rockets, which avoid the rocket equation altogether:
- Tsiolkovsky‘s space tower, the ultimate high-rise, 22500 miles high, through which would run an elevator all the way to the top.
- Artsutanov’s space elevator, a cable dangling 22000+ miles from a synchronous orbit satellite, all the way to ground. Cars or crawlers would travel up and down.
- Momentum exchange tethers, a class of devices, some of which can serve as alternative boosts into orbit.
All of the above rely super-strength materials that do not exist, even in nanotechnology carbons. Carbon fiber was one dashed hope, and carbon nanotubules were actually a step backwards. Evolved graphenes are unlikely to reach the out-of-this world required strengths.
These methods show promise, but involve extremely high g-forces that would crush a spacecraft, let alone a human:
- Gerard Bull’s HARP., a very large artillery cannon.
- Electromagnetic catapult, also known as a mass driver.
- SpinLaunch, a compact centrifugal device that acts as a catapult.
Beam-powered propulsion is derivative of rocket. Fuel carried by a rocket is replaced by a narrow laser or microwave beam aimed at a ground-to-orbit craft, which uses the energy of the beam to heat and expand a reaction propellant. Freeman Dyson wrote (JASON via Wikipedia),
“Laser propulsion as an idea that may produce a revolution in space technology. A single laser facility on the ground can in theory launch single-stage vehicles into low or high earth orbit. The payload can be 20% or 30% of the vehicle take-off weight. It is far more economical in the use of mass and energy than chemical propulsion, and it is far more flexible in putting identical vehicles into a variety of orbits.”
Beam powered propulsion is very promising, but unrelated to the method described below.
If a rocket could perform without having to accelerate the weight of propellant, we’d be vacationing on Mars. What if this were possible? It is, by recombination of the old:
- Impulse engine, a rocket or jet.
- Centrifugal acceleration, analogous to SpinLaunch.
- Momentum exchange tether, 10 kilometers length, more or less. Inspired by but in contrast with the impossibly long tether of the space elevator, it is barely within reach of carbon, with substantial prospect of improvement.
The tether connects two separate objects to transfer momentum from one to the other:
- The propulsion body contains the fuel and reaction propulsion system, a jet or rocket motor.
- The space body may also contain motors and fuel, but these are not active while tethered to the propulsion body.
- Momentum is generated by the propulsion body, and transferred to the space body.
- The momentum of the propulsion body remains static within a small range. The momentum of the space body steadily increases until the tether is cut.
- The momentum of the fuel does not increase.
This is easy to visualize with a hardware nut and a length of string:
- Tie the nut, the space body, to a foot of string.
- Tie the other end to your finger, the propulsion body.
- Dangle the nut.
- Make small rhythmic movements with your finger/hand.
- Discover what movements cause the nut to swing in increasing circular arc.
- If the nut is swinging fast enough, the nut will orbit your finger in a nearly horizontal plane. This can be maintained, with almost no finger/hand movement.
- Momentum is transferred from your finger to the nut.
- Imagine how, if the string were cut, the nut would fly off fast, in a direction tangential to its orbit about your finger.
- A subsonic jet airplane “mothership”, with high-bypass turbofan engines, is the propulsion body, flying at a comfortable 500 miles/hour.
- The space body is suspended from the mothership via a winched carbon fiber or graphene tether.
- The mothership flies in an expanding circle, limited by maximum tether length. The path is precisely determined to maximize energy transfer to the space body, which can be caused to orbit the mothership at 10X the speed, 5,000 miles/hour, neglecting real-world impediments.
- Released at a precise moment, the space body flies off, having received the delta-v of a large booster from a reusable vehicle at a fraction of the fuel cost.
This is CTMT, centripetal tethered momentum transfer.
Next, some napkin calculations.