Exhaust velocity?

[dennisp]

Being that focus fusion is already a rocket, I’m wondering: what is the exhaust velocity?

Beyond that, has anyone figured specific impulse and thrust? Would it make sense to simply mix additional reaction mass into the exhaust stream for a higher-thrust application?

There’s some information out there on the performance characteristics of various polywell-based rocket designs, and they’re quite impressive. A focus fusion rocket would seem to be simpler and cheaper, even with the extra plumbing for a high thrust application.

[Phil’s Dad]

I think you’ve just come full circle. Space thrusters were how this got started. Then someone thought “What if…”

Welcome :cheese:

[Aeronaut]

Phil’s Dad wrote: I think you’ve just come full circle. Space thrusters were how this got started. Then someone thought “What if…”

Welcome :cheese:

Welcome to the FFS.

Yes, we have a thread around here somewhere where we determined (ok- speculated) that the best way to get thrust was to use FF to make the power for a specialized ion thruster, probably a VASIMIR.

[dennisp]

Why thank you 🙂 …Didn’t realize that was how it started, but I did know that people were well aware of the rocketry application. Just haven’t found the specs anywhere. How fast can we get to Mars, what’s the highest speed we could get to, what would be the launch cost per pound…I’m guessing it would be in the same range as polywell designs, with cheaper hardware. (Polywell rocket specs here: http://nextbigfuture.com/2007/11/fusion-propulsion-if-bussard-iec-fusion.html) Then again, cost per watt is way cheaper with focus fusion, maybe launch costs would be somewhat cheaper as well.

I saw a design the other day for an implosion-based fusion rocket that could hit 20% lightspeed with two stages. Exhaust velocity is 6.3% lightspeed, about twice the Daedalus design. Here is is: http://nextbigfuture.com/2009/10/winterberg-fusion-rocket-could-go-20-of.html

So, just wondering how focus fusion compares to all this.

[Aeronaut]

dennisp wrote: Why thank you 🙂 …Didn’t realize that was how it started, but I did know that people were well aware of the rocketry application. Just haven’t found the specs anywhere. How fast can we get to Mars, what’s the highest speed we could get to, what would be the launch cost per pound…I’m guessing it would be in the same range as polywell designs, with cheaper hardware. (Polywell rocket specs here: http://nextbigfuture.com/2007/11/fusion-propulsion-if-bussard-iec-fusion.html) Then again, cost per watt is way cheaper with focus fusion, maybe launch costs would be somewhat cheaper as well.

I saw a design the other day for an implosion-based fusion rocket that could hit 20% lightspeed with two stages. Exhaust velocity is 6.3% lightspeed, about twice the Daedalus design. Here is is: http://nextbigfuture.com/2009/10/winterberg-fusion-rocket-could-go-20-of.html

So, just wondering how focus fusion compares to all this.

Glad to help, Dennis. Look up Jolly Roger’s threads for the details and alternatives.

The best way to bring launch cost down is to separate the payload at the end of a 69,000 mile space elevator tether to eliminate the chemical rockets while maximizing initial velocity. Our cousins at PolyWell are always going to have to have to drag the extra mass and energy overhead of their electromagnets with them, so there really is no direct comparison.

From what I’ve seen over there, their Navy funding is getting in the way of transparency and possibly results. Far as I know, I’m betting on FF to deliver on fusion’s long overdue promise the firstest, with the mostest. :coolsmile:

[KeithPickering]

dennisp wrote: Being that focus fusion is already a rocket, I’m wondering: what is the exhaust velocity?

Beyond that, has anyone figured specific impulse and thrust? Would it make sense to simply mix additional reaction mass into the exhaust stream for a higher-thrust application?

I compute it as follows:
1. The energy released in a single reaction (p+11B -> 3He) is 8.7 MeV, or 1.3939e-12 Joules.
2. I will assume here that ALL of this energy is kinetic energy carried away by the He ions. In actual fact, they tend to run into electrons in the plasma (which slows them down) and that energy is emitted by the electrons as x-rays. But let’s assume best-case scenario for now.
3. The mass of a 4He nucleus (2 protons, 2 neutrons) is 6.7e-27 kg, and three of those have a combined mass of 2.01e-26 kg.
4. Since e = 1/2 m v², we can compute v = √ (2e / m)
5. From this, the velocity of the three helium nuclei, in the best case scenario, would be 11781 km per second, about 4% of c.

Specific impulse and thrust depend on how much and how rapidly fuel is burned. I don’t believe we know the answers to those questions yet.

[KeithPickering]

KeithPickering wrote:
Specific impulse and thrust depend on how much and how rapidly fuel is burned. I don’t believe we know the answers to those questions yet.

Amending my own post: specific impulse is 1/g times the exhaust velocity. At 11781 km per second, or 11,781,000 meters per second, specific impulse would be about 1,201,334 seconds. But of course, that’s still very much a best-case scenario.

[vansig]

dennisp wrote: Being that focus fusion is already a rocket, I’m wondering: what is the exhaust velocity?

Beyond that, has anyone figured specific impulse and thrust? Would it make sense to simply mix additional reaction mass into the exhaust stream for a higher-thrust application?

I asked a similar question awhile back, which was “what is the speed of the alphas?”, and got the reply ~0.028 c, (8400 km/s).
http://answers.yahoo.com/question/index?qid=20090402091850AAAnih4

exit velocity, and therefore specific impulse, will depend on what these carry with them as they leave the reaction chamber. yes, you can mix additional reaction mass for a higher thrust application.

thrust will be very low, since amount of material expended per pinch is very low, but FF becomes a viable form of propulsion even before it reaches break-even.

[KeithPickering]

vansig wrote:

thrust will be very low, since amount of material expended per pinch is very low, but FF becomes a viable form of propulsion even before it reaches break-even.

This last is doubtful, since power is always at a premium aboard a spacecraft, and if the engine is an energy hog, the spacecraft will have inherent difficulties. Theoretically possible, but sooo much easier if you’re beyond breakeven, and hopefully well beyond.

[QuantumDot]

How much would it help to increase the temperature to ~300keV and the density to about 1 atm or 760 torr?
and would it still be stable at these levels?

[vansig]

The current alternative is VASIMR, which already consumes lots of electricity; can a below-unity FF reactor achieve greater specific impulse than VASIMR on the same input power? maybe it can.

[KeithPickering]

vansig wrote: The current alternative is VASIMR, which already consumes lots of electricity; can a below-unity FF reactor achieve greater specific impulse than VASIMR on the same input power? maybe it can.

VASIMR’s specific implulse is about 5000 seconds optimally, according to wikipedia, compared to about 1.2 million seconds (optimally) from p-B11 reactions. Thrust from p-B11 will be low, like other forms of ion drive. VASIMR’s advantage is higher thrust.

[Tulse]

KeithPickering wrote: VASIMR’s advantage is higher thrust.

Higher and variable, as full name (Variable Specific Impulse Magnetoplasma Rocket) implies — unlike most other “ion-drive” type devices, it can trade off thrust for Isp, and thus is far more versatile than typical drives. I don’t know how easy it would be to do this with a FF propulsion system.

Another advantage is that the plasma never comes in contact with the electrodes, so there is no electrode erosion — I’d guess this would be an issue for long-term use of FF-based space propulsion.

[vansig]

You could reduce the exhaust velocity of the alphas by bleeding off energy from them with the coil, but that doesn’t trade off Isp for thrust in the same way that VASIMR does. The other way is to collide the alphas with some of the unburnt plasma.

[Augustine]

Well the drawback to VASIMR is that it requires a power source and that the power source will have mass. The best bets are solar with a stretched lens array that provides something like 250-500W/kg or nuclear power. Solar isn’t useful past the orbit of Mars and to get to Mars in a reasonable time period you would want a square kilometer of solar cells. Nuclear requires shielding and radiators, preferably to be used with a high temperature nuclear reactor (which can be used to minimize radiator mass).

I think that a DPF will have less mass than either option. I’m worried about the VASIMR magnetic field(s) mucking with the magnetic field of the DPF, and I am worried about how a ship does station keeping when its power source also produces some thrust (perhaps a bank of ion thrusters to counteract the thrust generated by the alphas?).

If you believe in Polywell (and I am pulling for them) then a Polywell reactor has the benefit of scaling to produce gigawatts of power, much of it as heat and alphas. You can dump the heat and current into water/hydrogen to produce enormous thrust which is handy for getting mass to low earth orbit.

[Author]

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