Tuesday, 22 December 2009

Atomic Rockets

I'm going all hard science fiction crazy at the moment with this blog, I know. Bear with me. I just wanted to link to the excellent site Atomic Rockets, an attempt to create a lay person's guide to rocketry and the science behind hard science fiction. (Aside from its contents, one thing you have to love about the site is its eccentric layout. As with all internet sites created by amateurs and which have been around for a long time, it truly feels as if it harks from a bygone era. I wonder if one day there will be an Antiques Roadshow of old websites?)

Anyway, browse through it and enjoy. Some particular favourite passages of mine, from the "common misconceptions" section:

Rockets Got Wings

If your rocket has a multi-megawatt power plant, an absurdly high thrust thermal rocket propulsion system, or directed energy weapons it will need huge heat radiators to purge all the waste heat. Otherwise the rocket will melt or even vaporize. Radiators look like large wings or arrays of panels. The necessity of radiators a real problem for warships since radiators are pathetically vulnerable to hostile weapons fire.


Rockets Don't Got Windows

Spacecraft have no need of windows or portholes, for much the same reason as a submarine. (No, the Seaview doesn't count. Strictly science fiction. There are no panoramic picture windows on a Trident submarine). Windows represent structural weakness, and there really isn't much to see in any event. Unless the spacecraft is orbiting a planet or docking with another ship, the only thing visible is the depths of space and the eye-searing sun. And unlike submarines, windows on a spacecraft also let in deadly radiation.

Star Trek, Star Wars, and Battlestar Galactica to the contrary, space battles will NOT be fought at a range of a few feet. Directed energy weapons will force ranges such that the enemy ships will only be visible through a telescope. Watching a space battle through a port hole, you will either see nothing because the enemy ships are too far away, or you will see nothing because a reflected laser beam or nuclear explosion has permanently robbed you of your eyesight.

The navigation room might have an astrodome for emergency navigation. But for the most part windows will be omitted in favor of radar, telescopic TV cameras, and similar sensors.

Fuel Is Not Propellant

In a rocket, there is a difference between "fuel" and "reaction mass." Rockets use Newton's third law of Action and Reaction in order to move. Mass is violently thrown away in the form of the rocket's exhaust and the reaction accelerates the rocket forward. This mass is of course the "reaction mass." It is sometimes also called "remass" or "propellant."

The "fuel" is what is burned or whatever to generated the energy to expel the reaction mass. For example, in a classic atomic rocket, the fuel is the uranium-235 rods in the nuclear reactor, the reaction mass is the hydrogen gas heated in the reactor and expelled from the exhaust nozzle.

There are only a few confusing cases where the fuel and the reaction mass are the same thing. This is the case with chemical rockets such as the Space Shuttle and the Saturn 5, which is how the misconception started in the first place.

Automobiles, airplanes, and boats are sizable vehicles with relatively small fuel tanks. Not so rockets. An incredibly powerful rocket might approach having half its mass composed of reaction mass and the other half structure, hull plates, crew members, and everything else. But it is more likely that 75% of the mass will be reaction mass. Or worse. Most rockets are huge propellant tanks with a rocket engine stuck on the tail and a tiny crew habitat stuck on the top.


  1. Very interesting reading. Thanks for the link!

  2. Simmon's Hyperion and Hamilton's Reality Dysfunction had some excellent 'quasi-realistic' sci-fi battles. The tactics used (shoot-and-scoot at maximum sensor range) had more in common with submarine hunting than with traditional naval broadside battles.