FireStar Fusion Drive

[K^2]

RocketStar has made discussion on this forum a couple of times, but with some of their older projects. This one's spicier. I just randomly came across an article effectively repeating RocketStar's press release, and I kind of wanted to take this one apart a little bit.

To get the tl;dr out of the way, the claim is that they are boosting their 1U pulsed plasma drive with fusion to give it an extra kick "for free". How much of a kick isn't really specified.

There is no point of going over an article, so lets just go to the source with RocketStar's Press Release. Note that they are referencing real experiments that have confirmed the reaction and talk about the upcoming flights of their current FireStar drive, but don't say much about the fusion results beyond detection of gamma and alpha radiation. Which is cool in itself, but doesn't say much for the practical side of things.

They do specifically clarify in the press release that the "fusion boosted" really means "fusion-triggered-fission-boosted," which is something that needs to be taken apart a bit. Specifically, the claim is that by introducing boron to the water, which is used as main propellant, they are getting boron-proton fusion, which results in an unstable state of carbon, which decays to alpha particles.

The release does not specify the isotopes used, but boron does come in ¹⁰B and ¹¹B, with the latter being more abundant and more interesting of the two. The process ¹¹B + p -> ¹²C is of interest here. As we all, know, ¹²C is stable, however, there is a rather well known Hoyle State of carbon which isn't. The natural way in which Hoyle State comes up is when stars burn helium in their cores. The process involves two ⁴He nuclei coming together to form a highly unstable ⁸Be nucleus, which has just enough time to collide with a third ⁴He nucleus to form the Hoyle State of ¹²C. The latter is highly unstable, and is almost like the 3 alpha particles bouncing around in a common strong force potential than a real nucleus. Consequently, most of the time it decays back to three alpha particles, and the star has to try again. Occasionally, however, the ¹²C^** state emits a gamma, decays to a lower energy ¹²C^* state and finally to ground state ¹²C. The Hoyle State has 7.7MeV above carbon's ground state, which is a lot. The full proton-proton cycle is about 24MeV. So we're talking a good fraction of typical nuclear energies.

On paper, this works. The total binding energy of ¹¹B is 76.21MeV and ground state ¹²C is 92.15MeV. That gives ¹¹B + p plenty of energy to form Hoyle State, so long as protons are hot enough to overcome the repulsion barrier. It is much lower for proton-proton or even D-T fusion, since ¹¹B has the charge much more distributed through the larger nucleus, but I don't really have any numbers to attach to it at this time. I'll try to find either something from the cited SBIR experiments or other publications at a later point to try and get a back-of-the-envelope estimate for how much fusion/fission we are actually expecting in a chamber of a plasma drive.

 

What we have that's probably pretty reliable are the specs of the current generation FireStar M 1.4, which does not involve any fusion/fission processes. Just a conventional pulsed plasma propulsion unit, utilizing water, ionizing arc, and an accelerator cavity. Since this is a commercial product, and validation experiments are apparently available under the NDA, I have no reason to suspect them from being far off. Cited directly from RocketStar's website.

Quote

• Propellant: H2O (Filled as liquid. Liquid or solid during storage and use)
• Envelope: 95x95x95 mm (i.e., 1U with 5mm margin)
• Wet Mass: 980 grams
• Propellant Load: 250 grams of liquid, pure water
• System Power: 6 Watts (Typical flow rate with all thrusters active)
• Max System Power: 18 Watts (Maximum flow rate with all thrusters active)
• Thermal Draw:11.5 Watts (Heater on, no thrusters active)
• Temperature Range: -25C to +49C (Valves heated to 4C)
• Survival Temperature: -40C to 49C
• Input Voltage: 5V logic, 12V thrust
• Thrust:17.2 mN (all thrusters active)
• ISP: 7,300 sec
• Total Impulse: 17,800 Ns
• Delta V:   4,620 m/s for 4kg 3U
  2,273 m/s for 8kg 6U
• Thrust/Power: 2.9 mN/Watt
• Thrust/Mass: 17.6 mN/kg
• Lifetime: >= 4x propellant load (based on ongoing experiments)

The I_SP cited is consistent with the total impulse with 250g of propellant and implies mean exhaust velocity of about 71km/s. If the protons are accelerated by the field of the same strength, they can be potentially traveling about twice as fast, resulting in effective plasma temperatures of up to low mega-Kelvins on collisions. That feels like ballpark reasonable for what it'd take for ¹¹B + p fusion to take place. But again, I don't have good estimates for that at this time.

 

On the net, interesting. I want to see the numbers attached to the claimed efficiency boost. Also, the fact that they're calling it a fusion rocket when it's really more of a fission rocket feels a little scummy, but maybe they were trying to not scare people with neutron radiation associated with fission? This is, if real, an aneutronic process, so it would be reasonably safe for the applications for which this is meant. Either way, a real nuclear-boosted drive that's commercially available is really exciting, even if the performance benefits are going to be marginal.

Finally, even the base model, without fusion/fission is a really nifty example of how far the commercial plasma propulsion has gone. A 1U propulsion unit that can provide a 3U sat with 4.6km/s of delta-V? Yeah, that's not bad. With the right planning, you can take your 3U from LEO all the way to Mars utilizing a boost from Luna. Neat.

[Nuke]

sounds like the perfect little drive for shotgun space exploration.