[ytturbed]

>Currently, all our human rated rocket engines use chemical reactions (combustion of a fuel and oxidizer) to produce the energy.

Yes, however, for completeness: an explanation of why we must limit designs to chemical rockets ought to include an explanation of why the dozen or so fusion projects underway around the world will all fail, i.e. let's inject some rational optimism. Note that the Apollo programme began before its tech was ready.

[pjc50]

The fusion projects have been running for a long time with little success. Apollo was built on scaling tech that already worked (1940s rocketry could reach space although not achieve orbit)

[DennisP]

If you consider exponential progress since 1970 or so to be "little success," you're right. Fusion has a very high threshold before it becomes useful, but we've come a long way, and we're not that far from the breakeven point now.

NASA is currently working with John Slough's company on a fusion rocket for interplanetary travel.

http://www.nasa.gov/directorates/spacetech/niac/2012_phaseII...

http://www.washington.edu/news/2013/04/04/rocket-powered-by-...

[ytturbed]

To address my claim you would need an explanation of why that trend will continue, including for the several new projects. Btw, Apollo used a novel alloy.

[shalmanese]

Fusion devices aren't rockets and are thus, not subject to the Tyranny of the rocket equation.

[Sharlin]

Anything that pushes stuff out from a small hole in order to accelerate in the opposite direction is a rocket, by definition, and subject to the rocket equation. If a chemical reaction is used to create the energy to push the stuff out, it's a chemical rocket. If a nuclear reactor is used to heat the stuff so it's pushed out, it's a nuclear thermal rocket. If a nuclear reactor or solar panels are used to generate electricity that's used to push the stuff out, it's a nuclear electric or solar electric rocket respectively.

[jordanthoms]

How about http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsi... ?

[dalke]

The rocket equation applies to all momentum machines which carry its own "fuel", that is, the mass to eject.

Orion gets its momentum transfer from the plasma debris of the nuclear explosion, and the plasma speed comes from the explosion itself. Orion carries the source of the plasma, hence it is limited by the rocket equation.

[nyrath]

Agreed, but to be pedantic, it applies to all momentum machines which carry its own "propellant", not "fuel".

For instance, in a NERVA rocket, the "fuel" is the uranium rods in the reactor, the "propellant" is the liquid hydrogen that is heated and shot out the exhaust bell.

[chii]

is there a form of transport that is _not_ limited by the rocket equation?

[nyrath]

Solar Sail, Laser Sail, Electric Sail, Magnetic Sail, Mini-magnetospheric plasma sail, MagBeam, Plasma Magnet Sail, Photon Drive, Bussard Ramjet, Ram-Augmented Interstellar Rocket.

Basically any spacecraft that does not carry its own propellant. Most of these have either pathetic thrust or are way beyond our current state-of-the-art.

[shalmanese]

Solar sails, bussard ramjets, space elevators.

[Symmetry]

The rocket equation still applies, but the propellent velocity is so high that you don't a large mass fraction.

[ytturbed]

I was addressing the statement 'Currently, all our human rated rocket engines use chemical reactions', and I agreed with it. But it's not good enough if you consider the lengthy design cycle of propulsion systems. We ought to be creating additional designs which assume that fusion sources of various sizes and masses will be available.

[mcguire]

How much does your fusion plant weigh?

[ytturbed]

We ought to design for a range of masses.