Why do we need ion to power a ion-engined spacecraft?

[yaohua2000]

If we use photon other than ion, because photon also has momentum, we can make a powerful laser taillight, such a spacecraft don't need any ion propellant, right?

[crosscountry]

If we use photon other than ion, because photon also has momentum, we can make a powerful laser taillight, such a spacecraft don't need any ion propellant, right?

Right, but I calculated once that it would take 14.2 years to move a 150kg person 1meter/second with a 100 Watt laser.


14 years with 100 watts. that's not an acceptable time frame.

[John Kierein]

You need to burn energy to shine the laser. This is actually pretty efficient Isp but such a small thrust you need a huge laser. Best way would be to shine a ground-based laser on a perfect mirror, if you could keep it pointed at the craft all the way. But decelerating might take some clever engineering.

[sts60]

If your main drive is a big honkin' laser, than the "flight" and "fight" reflexes become one and the same. Larry Niven had a short story that involved some unfriendly aliens learning this lesson the hard way. 8)

[Ilya]

If we use photon other than ion, because photon also has momentum, we can make a powerful laser taillight, such a spacecraft don't need any ion propellant, right?

Right, but I calculated once that it would take 14.2 years to move a 150kg person 1meter/second with a 100 Watt laser.

I think you mean "accelerate a 150 kg person to 1 m/sec".

14 years with 100 watts. that's not an acceptable time frame.

Of course not, but I seriously doubt Yaohua2000 meant 100 Watt when he wrote "powerful laser." Assuming your calculation is correct, with 100 MW laser same acceleration would take less than 10 minutes.

If the energy for that laser comes from something on board the spacecraft, you are still stuck with the basic problem of all rockets -- you must expend energy to accelerate the unused fuel. I did the calculations on this matter years ago, and came to the conclusion that no matter how great your Isp and/or thrust, the maximum practically attainable velocity for any rocket, expressed as a fraction of c, is about the fraction of fuel's mass being converted into energy. IOW, since nuclear fission converts about 0.1% of uranium's mass into energy, 0.1% c is about the fastest a nuclear-electric propulsion (or fission-powered laser rocket) could ever reach. And that's assuming your rocket is practically all fuel, and you throw away all fission byproducts as soon as they are produced.

[frogesque]

It might not be very user friendly but would it be possible to ionise your nuclear waste and blast it out the back as well?

[Ilya]

It might not be very user friendly but would it be possible to ionise your nuclear waste and blast it out the back as well?

Certainly, but then you end up with lower Isp (and higher thrust) than using a laser.

[eburacum45]

I did the calculations on this matter years ago, and came to the conclusion that no matter how great your Isp and/or thrust, the maximum practically attainable velocity for any rocket, expressed as a fraction of c, is about the fraction of fuel's mass being converted into energy

So this antimatter driven starship I have imagined up would need to convert approximately 10% of it's fuel to energy, in order to reach its cruising speed of 0.1c? Whoo- that makes the antimatter containment problem interesting...

Other strategies include beaming the energy to the ship; this means you don't have to accelerate the fuel. Robert Forward's Starwisp is one good design; the acceleration due to laser light is low, but the starwisp can be very light- if you don't want to decelerate at the other end.
Similar to the starwisp is the Beamrider concept; this uses tiny particles with mass; (my image of a far-future beamrider here)
but this really need a beam station at the destination system to work.

How about the Seeded Ramscoop system? Fuel pellets are accelerated into the path of a ramscoop type ship; this removes the need for the ship to carry its own fuel- the pellets are accelerated using particle beam technology (which seems to have remarkable potential).

How about a ship with a rectenna for gathering broadcast microwaves, and converting that to thrust in an ion drive? This fuel-less ship still requires propellant, but there might be some advantage in broadcast power (er... this one is mostly my idea, so it could do with some informed criticism). This Microwave Powerbeam ship would be limited to interplanetary use, I expect...

[Launch window]

If we use photon other than ion, because photon also has momentum, we can make a powerful laser taillight, such a spacecraft don't need any ion propellant, right?

Right, but I calculated once that it would take 14.2 years to move a 150kg person 1meter/second with a 100 Watt laser.


14 years with 100 watts. that's not an acceptable time frame.

yikes, that's not very good

[publiusr]

Anti-matter photon drives are probably the only way to do it right. They outclass everything--with the Orion max, internal confine. fusion and an NSWR (80% solution) close behind.