I recall hearing that methane has a specific impulse of ~900 seconds, making it about twice as efficient as hydrogen. For a while, I accepted that as truth, until I made a vital connection: pioneering rocket scientist Konstantin (?) Tsiolkovsky stated that hydrogen is the most efficient chemical fuel there is. I know that the science of Tsiolkovsky’s day was less advanced than today’s, but it still made me doubt that methane was more efficient than hydrogen, specific impulse-wise.
Is there a more efficient chemical fuel than hydrogen? I know that ion and nuclear fission, and forseeably, fusion and antimatter engines have greater impulses, but I’m looking for a chemical fuel that’s more efficient than hydrogen.
Now, for something completely different…
I heard lovely things about magsails. What sort of power would I need to generate a magsail large enough to pull along, say, a ship with a mass in the area of a large airliner? I’m looking for speeds that would get me to Mars in a month, by the way, if that alters the required size of the magsail any (and I’m sure it would).
I’m guessing that the Mars Express would have to be huge (compared to comtemporary manned spacecraft), but how large? I’m looking to support about ten really rich, Mars-bound suckers for a month in reasonable comfort, with baggage. My guess is a ship with the physical size of a 747 should do the job, but I may be wrong, and I’m no good at estimating mass (I know mass is of prime importance in a weightless environment where it costs a boatload of money for the slightest bit of weight.).
Can anyone solve my problems, answer my questions, and possibly recommend any propulsion systems that could concievably be built in the next fifty years? I’d be forever grateful if anyone can help me.
Specific impulse is a measure of exhaust velocity. Rockets work by throwing stuff to the back - the faster you throw the stuff (propellant), the more push you get per pound of propellant.
Now, if you have two gases at the same temperature, the lighter gas is moving faster. Which means that if you design a rocket to burn fuel at 5000 degrees, hydrogen molecules (well, water molecules which result from combustion) are lighter than, say, something you get from burning carbon, so it goes out the nozzle faster. So you get more specific impulse with lighter molecules. So yes, hydrogen is more efficient than methane, if the engine temperature is the same.
The way to get even better specific impulse is to burn stuff at even higher temperatures, or find a way to heat or acclerate the exhaust even more. Nuclear engines can theoretically obtain higher temperatures, so they are more efficient. Ion engines use an electric field to acclerate charged particles and are also very efficient.
I don’t remember what a magsail is, can you remind me? The only type of “sail” that seems practical is a solar sail, which uses photon pressure from sunlight (or artificial light) for propulsion.
A magsail, if I recall correctly works as follows:
Solar winds have a noticable effect on magnetic fields; they resist them as if they were solid. That’s why we’re not burning to death under the onslaught of the Sun’s radiation; our magnetic field shields us.
Our magnetic field also pushes the Earth, infinitesimally, of course, outwards away from the Sun, due to the pressure exerted by the solar wind. That’s the principle behind magsails. You generate a magnetic field large enough around a ship, and you can use the solar wind to push it outwards. As an added bonus, you get built-in radiation shielding. To sweeten the deal further, the field increases in size the farther from the Sun you get, due to reduced solar wind pressure, so the acceleration doesn’t drop off as quickly as it would with a solar sail.
From what I hear, a magsail has to be huge, though. I know little about magnetic fields, so I don’t know how big a sail one’d need.
This might be your mag-sail. Someone has developed a solar sail (in theory) that uses magnetic fields to trap the solar winds then it uses those trapped solar wind particals to act as the solar sail. This way no structure is needed for the sail.
As for a month one way I would guess you will need nucular engines. Solar sails usally require a trip towards the sun 1st as the solar winds arn’t enough to really get it going.
I digged out several books on space travel in my closet, and read a little more on theoretical ways of moving about in outer space. I found another variation on the sail: the laser sail. Basically, you have a monstrous, ground-based laser on Earth (or the Moon, or Mars, or wherever you want it) locked onto your ship, and the laser pushes it. Problems: it’s pretty hard to keep a good aim on something THAT far away, so I scrapped that idea. Bussard ramscoops are also not too possible, unless one can utilize the fuel without having to decelerate it. Fusion engines are far-off, but seem pretty nifty.
So I guess I better start looking to the former Soviet Union to lend me a nice little fission reactor… So far, it seems to be the best (if not the least safe) choice available within 50 years.
k2dave:
Also, in that book, I read about a mission plan drawn up for a sail-powered vessel. It involves basically orbiting the Earth, and when you’re moving away from the Sun, your sail is fully perpendicular to the rays, when moving towards, the sail is perfectly parallel (to save momentum).
Continue until escape velocity is reached, and there you go. It won’t go to Mars in a month, but it’ll get there without needing to slingshot around the Sun.
