Thanks for the thoughtful remarks Isaac. The way you express the math is very clear.
As for elliptical orbits versus LaGrange points: I don't see why you think the "wait time" is going to be a big deal--after all, EML1 or 2 is only a week or so from Earth. Since Mars gravity is so weak compared to the Sun, the Sun-Mars L1 point is going to be fairly close to Mars. Also, Lagrange points are ideal locations for propellant depots. (Not to mention that EML1/2 is close to Lunar propellant sources ).
For a given Isp (I don't care if it's 50 or 5000) and in-space trajectory change (orbit change, maneuvers, escapes), does high thrust or low thrust end up with less propellant required?
Quote from: Antares on 08/01/2011 02:29 pmFor a given Isp (I don't care if it's 50 or 5000) and in-space trajectory change (orbit change, maneuvers, escapes), does high thrust or low thrust end up with less propellant required?It really depends... Given that rocket engines are limited in their T/W, a "really high"-thrust stage will take more propellant than a "moderately high"-thrust, since the dry mass that needs to be put through your delta-v will be higher for the high-thrust stage. Also, higher thrust (if higher than launch accelerations, etc) can also mean higher structural mass requirements in both the rocket stage and payload, thus increasing total propellant needed.And, of course, if you have too low thrust, your long burn means you can't burn all the necessary propellant deep in the gravity well, thus you need more propellant for a given escape velocity, etc.But with multiple passes and multiple burns (raising apogee, not perigee, say for escape), it can be possible to "simulate" a higher thrust rocket engine with a much lower thrust rocket engine...Akin's Law #8:"8. In nature, the optimum is almost always in the middle somewhere. Distrust assertions that the optimum is at an extreme point."http://spacecraft.ssl.umd.edu/akins_laws.html
...A VASIMR rocket is not bothered as much by multiple restarts as a gridded ion engine is....
Quote from: Patchouli on 08/02/2011 05:02 am...A VASIMR rocket is not bothered as much by multiple restarts as a gridded ion engine is.......And the main reason is because VASIMR has never flown. I actually kind of doubt this. Have you seen the flight profile of Dawn (which uses a gridded ion engine)? It's all chopped up... Very many starts and stops.
I believe this actually was done to save a com sat using the RCS because the apogee engine failed but it's also an advantage VASIMR has over ion engines.A VASIMR rocket is not bothered as much by multiple restarts as a gridded ion engine is.Might add a another law to the list if a change only nets an extra 3% performance gain but increases the cost by 15% it's best not to implement it.I'll call it Korolev's law as Russians LVs often tend just accept a lower payload mass fraction if a given design element is lower cost or more robust.Russian LVs tend to be simple and robust while western LVs tend to be highly tuned hotrods.
One rookie question: Do rocket stages using hypergolic fuel have the danger of fuel/oxidizer sloshing? Why?
Let's try to assume a realistic rocket that uses methane as a propellant. No oxygen for this rocket.How much methane is needed to raise 100 kilograms of cargo onto LEO (400 kilometers above the surface)?How much power and energy is needed to heat up that methane and produce enough thrust to raise that mass of 100 kilograms onto LEO (400 km)?
Is it possible to build a Solar Thermal Rocket with thrust-to-weight ratio > 1?
Might add a another law to the list if a change only nets an extra 3% performance gain but increases the cost by 15% it's best not to implement it.I'll call it Korolev's law as Russians LVs often tend just accept a lower payload mass fraction if a given design element is lower cost or more robust.Russian LVs tend to be simple and robust while western LVs tend to be highly tuned hotrods.
The sun provides about 1000W/m^2 of energy flux near Earth.
Quote from: strangequark on 08/19/2011 05:12 amThe sun provides about 1000W/m^2 of energy flux near Earth.1366W/m^2, actually.
If you'd have a tank pressurized over the triple point of a liquid (let's use H2 for the example), what would happen if you start with a full liquid charge?Would it allow better density? (disregarding the tank's mass, of course)And if you went beyond the critical point?