SpaceX Launch Successfully Delivers Satellite Into Orbit

[MSimon]

The limit for power is the size of radiator you need. Chemical rockets have an advantage with that. Of course they have a lot of other disadvantages.

[jmc]

Yes to accelerate a rocket you need to shoot momentum out the back and
p=sqrt(2E/m) so if you have a limited amount of mass you can carry on board your best bet is expel it at the maximum energy (fusion neutrons ion thrusters etc.), but the gain in momentum is only proportional to the square root of the increase in your energy. The catch is that what you kick outwards first has to bounce against your combustion chamber first, infact it is this bounce that gives the rocket its impulse.

But when the exhaust going backwards whacks against the combustion chamber it heats it up by an ammount proportional to its energy. This limits the ammount of power you can eject out the other end before your combustion chamber melts. And if you are limited by the ammount of kinetic energy you can expel then the best thing to do is distribute it over the largest possible mass. However this wastes propellent.

Its a catch-22 scenario, all efficient thrusters will inescapably have to have extremely low accelerations. While all high g thrusters will have to carry extremely large masses of fuel on board when compared with the payload.

A mass driver could be a solution to this on the moon. This is a basically a maglev railtrack capable of accelerating cargo to escape velocity while on the ground. There the exhaust mass is effectively the moon and all the kinetic energy goes into the payload. The only catch is the shear length of track required to accelerate masses at realistic accelerations.

[ravingdave]

Yes to accelerate a rocket you need to shoot momentum out the back and
p=sqrt(2E/m) so if you have a limited amount of mass you can carry on board your best bet is expel it at the maximum energy (fusion neutrons ion thrusters etc.), but the gain in momentum is only proportional to the square root of the increase in your energy. The catch is that what you kick outwards first has to bounce against your combustion chamber first, infact it is this bounce that gives the rocket its impulse.

But when the exhaust going backwards whacks against the combustion chamber it heats it up by an ammount proportional to its energy. This limits the ammount of power you can eject out the other end before your combustion chamber melts. And if you are limited by the ammount of kinetic energy you can expel then the best thing to do is distribute it over the largest possible mass. However this wastes propellent.

Its a catch-22 scenario, all efficient thrusters will inescapably have to have extremely low accelerations. While all high g thrusters will have to carry extremely large masses of fuel on board when compared with the payload.

My understanding of the VASIMIR is that it does not comport with this idea. The reaction mass is not "whacking" any of the engine components, it is held in confinement by a magnetic field such that it doesn't touch any of the engine parts. It's a lot like a polywell in this regard except that it is a thermalised plasma, and the confinement is designed to leak in one direction.

The plasma does impart force to the engine though. It pushes against the magnetic field which pushes against the magnets generating it, which are physically attached to the engine.

With this in mind, your energy level is limited by how much you can pump in, (in the form of microwaves) and by how strong of a field you can maintain.


Also, for the above mentioned reasons, since there is no thermal transfer from reaction mass to the engine itself, there is likewise no need for any radiators to cool anything but the power source cold sink.

edit:
Scratch some of the above. There is probably an issue with bremsstrahlung radiation which would heat the engine components, but it's not nearly as much as if the plasma were in direct contact with the physical parts of the engine.

Vasimir is an awesome idea, and at the moment, i'm not aware of anything better.


David

[TDPerk]

The quicker your propellant, the less efficient is the flight.

I'm sure I'm misunderstanding your statement. Rockets propulsion is always more efficient the faster the velocity of the exiting fuel is, and it never matters to Isp how quickly you go through your fuel.

Vexhaust X ln(fueled/empty) = delta V

And in practive it's not hard to realize most of that theoretic delta V.

Now it's inefficient if you have a high Isp system that shoves it's fuel out the back in too short a time to get to large delta V changes, but that's because you've made your crew a fine paste on the front of a rear bulkhead...

Yours, TDP, ml, msl, & pfpp

[chrismb]

I'm sure I'm misunderstanding your statement. Rockets propulsion is always more efficient the faster the velocity of the exiting fuel is, and it never matters to Isp how quickly you go through your fuel.

Not sure what you're saying there. Roket propulsion may be more powerful the faster &c., but not more efficient. The amount of energy converted by the combustion of that fuel, or acceleration of ions for that matter, to get a given amount of reactive momentum is always more if you use more mass at lower exit speed. This is hare and tortoise stuff. Momentum gained is mv, energy used to accelerate to said mv is mv^2/2. So to accelerate a half as much substance at twice the velocity takes twice the energy to do so, but results in only the same thrust.

[Helius]

The limit for power is the size of radiator you need. Chemical rockets have an advantage with that. Of course they have a lot of other disadvantages.

Your point led my imagination to a strange place.

There's a solution as defined by the Acronym: RMATV
R: Radiate Mass At the Thrust Vector
M: Mass
A: at the
T: Thrust
V: Vector

Directed radiative mass would itself need a radiator; So direct it...

[MSimon]

With efficient direct conversion we can afford to run the radiators at much higher temperatures. T^4 really helps.