You are not logged in.
Pages: 1
I posted a thread about this many years ago. It's probably here in Interplanetary transportation somewhere. But let's start again.
As GW Johnson recently posted, the 1950s project Orion proposed using nuclear bombs for propulsion. That would deliberately detonate a nuclear bomb, with the spacecraft within the blast radius. A heavy concussion plate would prevent destruction of the spacecraft, with shock absorbers to even out the thrust. Mathematical calculations showed it should produce extreme Isp. Of course it had several problems. To start with, you deliberately detonate a nuclear bomb with yourself inside the blast radius. Calling that unsafe is an understatement. Second, imagine the radiation. Third, and the reason this project was cancelled, it means the spacecraft carries hundreds of nuclear bombs. If just one drops on a city on Earth, boom! The UN would not allow any nuclear bombs in space. So the whole thing was cancelled. I understand NASA was against it too; it was a project of the Air Force.
Ok, so is there a way to make this safe? No bomb, instead a rocket that uses nuclear combustion? If the nuclear explosion provides such extreme Isp, how can we use the nuclear reaction as combustion? A piston engine captures a gasoline/air explosion, turning it into mechanical force. But a jet engine uses continuous combustion to provide thrust. How do we create a continuous nuclear rocket engine?
Is the issue simply temperature? The hotter your exhaust, the higher your exhaust velocity, consequently higher Isp. Is extreme temperature from a bomb the key? If so, how do we make a nuclear thermal rocket with extreme temperature?
One interesting technology is the plasma window. This can contain 1.5 atmospheres of pressure against hard vacuum. Yup, a force field that can hold in air. I was amazed when I read about this, I didn't think it was possible. Yet, Ady Hershcovitch at the Brookhaven National Laboratory did it, patented in 1995. The initial application was for an electron beam welder. Creating an electron beam requires vacuum, but if the beam is strong enough to weld with, how do you get the beam out of a vacuum chamber without melting the walls of your chamber? His solution was a plasma window. It would keep air out, yet because it's plasma, heat from the electron beam just helps. Currently the plasma window only works in a circular opening, only produces a flat plasma window, and consumes 20,000 watts per inch diameter. Plasma has to be at least 12,000°K to form the thick viscous plasma required to hold back air; currently researchers generally use 14,000 or 15,000°K.
Why is this applicable to propulsion? Extreme heat from a nuclear rocket would melt titanium or inconel. But what if hot propellant is contained by a plasma window? How hot could we get it? Use a shaped plasma window for the reaction chamber and exhaust nozzle. VASIMR uses a magnetic vacuum bottle to do something similar, but that requires extremely thin and low pressure propellant. A plasma window could hold much greater pressure.
So how do we shape a plasma window? Would a static charge on a metal plate behind a flat circular plasma window pull it into a convex shape? The inverse square rule indicates the plasma would be drawn more strongly as it got closer to the metal plate, causing contact. We don't want contact. We want something that repels, to prevent contact. But we need something to draw the plasma toward the plate. Hmm.
To reduce power consumption, we really want something to reduce radiant heat loss from the plasma. I suspect the 20,000 watts per inch diameter are due to radiant heat loss. Can we create a layer of something that reflects specific electromagnetic frequencies emitted by hot plasma? Such as a gas? Laminar layers of gas to prevent radiative and convective heat loss?
Offline
Like button can go here
To reduce power consumption, we really want something to reduce radiant heat loss from the plasma. I suspect the 20,000 watts per inch diameter are due to radiant heat loss. Can we create a layer of something that reflects specific electromagnetic frequencies emitted by hot plasma? Such as a gas? Laminar layers of gas to prevent radiative and convective heat loss?
I suspect plasma can be tuned to become reflective at some frequencies using Langmuir waves: a possibility may be a plasma sandwich with an hot positive core between two cold negative layer tuned to be reflective at the frequencies emitted by hot layer.
But, instead of using plasma windows as the walls of the chamber that contain the reaction, why not using the plasma window as a reaction support?
I imagine a stack of windows of uranium exafluoride plasma, mantained at 14000° K by nuclear reactions, with hydrogen or water steam propellant that flows between the windows, gaining an exaust velocity of 27 km/s for hydrogen or 11 km/s for water.
Last edited by Quaoar (2014-04-09 15:13:41)
Offline
Like button can go here
Pages: 1