fission fragment propulsion from dusty plasmas

tahanson43206 · 2024-11-05 12:21:24

For SpaceNut ... we have a ** lot ** of topics in this Category with the word propulsion...

The title of this topic comes from a post by kdb512 in a topic about ion propulsion.

This appears to something a step above mere "ion propulsion"

Because the topic is so different, I looked through the entire list of topics with the word propulsion in the title and did not find a match.

I think this concept deserves it's own topic.

(th)

tahanson43206 · 2024-11-05 12:22:29

This post is reserved for an index to posts that may be contributed by NewMars members over time.

We'll begin with #3, which will be a quote of a post by kdb512 in another topic.

This post contains what looks to me like a significant contribution by Calliban:
https://newmars.com/forums/viewtopic.ph … 20#p227720
The post includes a vision of a (possible) way to improve performance.

(th)

tahanson43206 · 2024-11-05 12:23:56

kbd512 wrote:

Calliban,
I took a look at this the other day, but I also looked at fission fragment propulsion from dusty plasmas, and I think if the large ship already requires a nuclear power source for practical propulsion, then you're better off with a direct energy conversion to propulsive power.
Final Report: Concept Assessment of a Fission Fragment Rocket Engine (FFRE) Propelled Spacecraft
The Isp of these dusty plasma fission fragment devices ranges somewhere between 5,000s (gas injection "afterburner" for 4.5kN) and 1,000,000s (only 10s of Newtons of thrust).
NASA's AFFRE (a horseshoe-shaped reactor with two electromagnetic nozzles) has a 2,500MW reactor power output level and it generates 4,651N of thrust, with an Isp of 32,000s. Mass flow is up to 4oz per hour for the Uranium powderized "fuel" and 140lbs per hour for the LH2 gas injected into the engine's "afterburner". You could increase thrust and reduce Isp by a factor of 10 by injecting a denser gas than Hydrogen, which is probably what you should do to exit through the Van Allen Belts as quickly as possible. For example, CO2 should do the trick, producing a near-impulsive burn capable of pushing a heavy transport ship onto an interplanetary transfer trajectory in a matter of hours, minimizing crew and vehicle radiation exposure.
Total mass for the engine is 198,000kg / 198t. The engine itself will easily fit in the payload bay of a Starship. Starship 3 has the payload performance to carry the engine to orbit. 107t is the mass of the engine itself, including electromagnetic nozzle and reactor shielding, and 91t is the moderator oil, so the engine could be delivered in pieces if desired or required. Total mass for all components, including tankage for the LH2, was predicted to fall between 223t and 269t (mass margin growth allowance). Delivering the engine to orbit using Starship flights is a fairly trivial matter, though it will likely come in several large pieces so that everything easily fits and doesn't push any limits.
As I understand it, the engine is non-nuclear (contains no fissile inventory, unlike what we normally think of as a prototypical "reactor core") prior to first ignition and has very few moving parts. The nuclear fuel is a separately stored powder induced to fission in the core using a very thick / heavy moderator, so the fuel can be transferred and resupplied when desired, such that total nuclear / fissile inventory is not above what is required for a single flight. This is for the second iteration of the engine design that doesn't use massive radiators. Isp is far less spectacular than the first iteration with the gigantic radiators, well within the range of what VASIMR is capable of delivering, but the total mass / complexity / cooling requirements are much lower than other types of electric propulsion systems, while still incorporating the whiz-bang factor of superconducting electromagnets.
Even though we could feasibly reach Mars much faster, I say we forego faster transit times, as well as wear and tear on the engine and vehicle, in favor of greater total ship tonnage. Adding lots of mass margin and artificial gravity confers crew safety all by itself, because it means the ship can be equipped to handle contingencies. 6 months is a standard deployment for US Navy ships, and subjecting a crew provided with artificial gravity and generous amenities is not an overly burdensome voyage.
I think the dusty plasma fission fragment rocket engine still qualifies as a form of electromagnetic propulsion that uses high velocity fission fragments and a superheated gas ejected out of an electromagnetic nozzle to either generate lots of thrust or very high Isp thrust. The primary advantage, at least as I see it, is that the engine / reactor is a purpose-built propulsion system, rather than a nuclear reactor attached to a propulsion system. The second design iteration, unless I'm mistaken, does not require the large high temperature radiators of the first. Most of the generated thermal power is reflected back into the reaction chamber and the chamber is optimally shaped to avoid the thermal heating of the first. The first iteration absorbs far more heat into the device due to its sub-optimal core geometry (trying to shape it like a conventional reactor core or rocket engine combustion chamber).

Quoted from the topic: Electrostatic Thruster vs Magnetic Thruster

This propulsion method appears to be significantly different from pure "electromagnetic" or "electrostatic" thrusters that it deserves it's own topic.

(th)

tahanson43206 · 2024-11-05 12:28:07

For NewMars members ... if anyone has time, please look for and post additional details about this propulsion method.

For example, how highly enriched must the Uranium powder fuel be to achieve the objectives of the method?

(th)

kbd512 · 2024-11-05 21:58:45

The first discovery was that the corrected Monte Carlo model now pushed the core density to provide FF escape down from 0.10 g/cc to 0.01 g/cc. When the problem first surfaced that the FF had insufficient energy remaining to escape the core, the proposed solution was to simply scale the FFRE to a larger size, since neutrons go as the volume, but friction goes as the diameter. Even when the reactor power was made very large, the friction remained close to 99%. More dismally, the thrust per GW of power remained close to 0.1 N/GW, a value far from the “ideal” FFRE value of 120N/GW. By changing the ratio of the dimensions of the torus, the efficiency could be raised to 2%, a not very encouraging number. By changing from a torus to a “spherical torus” (similar in shape to a Tokomak reactor), the efficiency could be raised to produce perhaps 5N/GW, again not very encouraging number.
Since geometry was not the solution to the thrust problem, choosing a fissile fuel with larger neutron cross section would lower the mass and density required to make the reactor critical. Uranium-235 with about 500 barns of cross section could be replaced with Plutonium-239 with 720 barns for a small improvement. Thrust developed based on this design provided about 97% thermal and 3% fission fragment thrust, for about 5N per GW-thermal. On the other hand, Americium-242m (Am242m) with 7200 barns of cross section would provide nearly 40% fission fragment thrust with only 60% going into heat.

Every engine NASA throws money at these days is another pie-in-the-sky, "this one's gonna take us to the stars, boys". Well... No. It won't do that in any practical way. Absent a functional warp drive, we are simply not going to visit any other nearby stars over human lifespan timescales. We're a very long way from having that kind of engine technology. We're just barely scratching the surface of the new physics required to make warp drives. Initial results are very promising, but it'll take decades of work from a literal army of scientists. Before we can make that happen, a "higher-than-NERVA-Isp" combined with a "greater-than-ion-engine" thrust would be spectacularly useful for exploration within our own solar system. Researchers keep throwing out the term "game changer", without realizing that an affordable and simplistic high-Isp (greater than 2,000s) combined with moderate thrust (no more than the LOX/LH2 RL-10 Centaur upper stage engine) is "where the real game changer is at". 5,000s with 110kN of thrust would be nothing short of revolutionary for interplanetary transport. The mere fact that it's still woefully insufficient to visit another star is utterly irrelevant to any practical near-term space exploration goal. We need to cease and desist with trying to turn every electric engine into a "Star Shot" when a "Mars Shot" is all we really need. If we had affordable and reliable transportation within our own solar system, that would be a technological windfall on par with the invention of the combustion engine or microchip.

I actually view this "apparent problem" (to the people who worked on this particular AFFRE design) as the "actual solution". The people who designed this rocket engine were fixated on generating very high-Isp thrust from fission fragments. What they came up with generated stupendously high Isp, as high as 1,000,000s, but the end result from that ridiculously high Isp was ridiculously low thrust- on the order of single-digit Newtons per GigaWatt. That kind of thrust is useful for visiting Jupiter, Saturn, Neptune, Uranus, or Pluto. Mercury to Mars would be much easier to reach using much higher thrust and lower Isp. Mars is 140 million miles from Earth, on average. Jupiter is 340 million miles from Mars, on average.

There is nothing "game changing" about Newtons of thrust per GW of input power, because it's unusable, and even they noted that. Thrust is not simply about velocity, although high velocity helps mightily since KE = 0.5*m*v^2. Thrust is also about mass flow rate. If mass flow rate is infinitesimally small, then even when velocity is near the speed of light, total propulsive force remains quite small. Fission fragments are moving at a healthy percentage of the speed of light, but the force generated is quite small because the mass flow rate is quite. An insanely high Isp typically equates to insanely low thrust. The nuclear salt water rocket is a notable exception, but it's equivalent to a continuous nuclear explosion going off inside your engine's rocket nozzle. You'd best have an incredibly stout nozzle design.

Therefore, the best usage for fission fragments is to bombard particulates or gases to impart a portion of that stupendous velocity into something with enough mass that a lot more total force is being generated through BOTH velocity and mass. Our fission fragments generate 1,000,000s of Isp, but only 5N of thrust. Well, great. Let's slow those speed demons down by smacking them into a lot more physical mass, thus accelerating that mass through heating, to something that still represents incredible speed- far higher than a chemical rocket engine, but far more force at the same time. Adding 200X more mass to accelerate, we arrive at 5,000s of Isp and 1,000N of thrust.

1,000N is a far more usable amount of thrust than 5N or 10N, when the end goal is to accelerate to escape velocity from low orbit, and quickly, to avoid barbecuing those aboard the spacecraft with the radiation maelstrom of Earth's Van Allen Belts. Spending a few hours to a couple of days in the Van Allen Belts is not going to hurt anyone. Absent very heavy radiation shielding to absorb the ionizing radiation, spending weeks to months in the Van Allen Belts is enough to impart a fatal radiation dose, or severe illness if you survive. The intensity of the radiation source, the kind of radiation being absorbed, where in your body it's absorbed, and the total length of the exposure will all significantly change the outcome. The Apollo astronauts transited through the Van Allen Belts in a matter of hours, and walked away with a radiation exposure equivalent to a few chest X-rays. It wasn't ideal, but nobody was hurt by that, nor would we expect them to be hurt, because both the kind of radiation and the total dose was quite small. Neil Armstrong (82) and Michael Collins (90) died of old age. Buzz Aldrin is still alive and fighting, just 94 years young, a mere spring chicken compared to some of our last remaining WWII veterans.

Where does that leave us?

Thermalization of fission fragment energy into a propellant is the ultimate goal for producing high thrust at a reasonably high Isp. Another AFFRE concept skirts around the low fission cross-section problem of ordinary Uranium and Plutonium by mixing the nuclear fuel and "afterburner" propellant into an aerogel infused with Uranium and Carbon or Silica "dust". The fission fragments produced by fission are immediately thermalized into the propellant, imparting tremendous velocity as the super-heated gas rockets out of the engine. The Isp of that engine was still insanely high (5,000s to 8,000s) while producing multiple kiloNewtons vs tens of Newtons of thrust.

Regardless of whether you mostly produce high velocity fission fragments or sensible heat lower velocity thermal photons, the only way to get kiloNewtons of thrust is to dump all of that energy into the propellant. If you only get your thrust from fission fragments, then you get very little thrust, far too little to justify the expense of your nuclear reactor, unless the vehicle is headed towards the outer planets and thus has many months of time available in which to accelerate and then decelerate near arrival.

Some additional details:

1. I would like to point out that this particular purpose-built propulsion device (not simply a nuclear reactor powering a propulsion device- fission itself is the source of the thrust) operates on both electromagnetic and electrostatic principles at the same time. It is direct nuclear electric propulsion. If the electricity used to power the electromagnet was provided by fission fragments siphoned off to a MHD device, then it would be a fully integrated nuclear-electromagnetic propulsion system.

2. The "special oil" used as the moderator for this reactor design is in fact a specialty hydrocarbon made from Carbon and Deuterium. Whereas "normal oil" is made from plain old Hydrogen, this oil substitutes Deuterium for Hydrogen, to provide the neutron moderation properties required to sustain fission. It's a very interesting reactor design, as most "normal" reactors use D2O (heavy water) or BeO (Beryllium Oxide) to reflect neutrons back into the fissile material. Although the proposal does talk of using the fissile Americium isotope known as "Am242m", further design refinement does indicate the use of plain old Uranium or Plutonium fuel, both of which are far more abundant than Am242m.

3. My proposed uses Carbon dust mixed with fuel or CO2 gas as the propellant for the afterburner. We could repurpose captured CO2 from Earth as the thrust enhancement source. I don't think anybody will complain if we take a little into space, and Venus would be an ideal refueling depot if someone does. Isp would be further reduced from 32,000s, but a much lower Isp, near 5,000s, would be far more useful for swiftly transiting through the high radiation fields of Earth's Van Allen Belts.

The thrust of the H2-fueled proposal in the NASA NIAC document I linked to was, 50N per 1,000MW using fission fragments alone. The injection of H2 into the "afterburner" portion of the device absorbed thermal energy from the highly energetic fission fragments to increase thrust to low kiloNewtons.

4. Making better use of thermalization within the core of the reactor would drastically reduce the size and mass of the radiator array. Since that's what the fission fragments are ultimately used for in ALL afterburner-equipped fission fragment engines, why not start there the way the further refinements to the AFFRE engine concept did? Unless we intend to go well beyond Mars, we should give up on the wild Isp figures. Incidentally, this was also how the "nuclear lightbulb" functions, and its Isp was 1,500s to 3,000s. AFFRE is best used and described as a more efficient open cycle gas core nuclear thermal rocket engine that operates over much saner temperature ranges that don't risk vaporizing the engine if something goes slightly wrong with the cooling subsystem.

5. I think a further refinement of the core's geometry to a four-nozzle horseshoe could yield the ability to perform thrust vectoring without the use of moving parts.

Calliban · 2024-11-06 14:55:15

If fission products are thermalising, then this is essentially a gas core fission rocket. There is nothing wrong with that for the purposes of interplanetary propulsion. It can produces exhaust velocity capable of taking us anywhere in the solar system without staging and with a modest propellant mass fraction in most cases. It won't be very impressive from a thrust point of view, because waste heat removal will constrain engine power output.

Thermalisation is inevitable unless fuel particles are very small (allowing efficient fission product escape) and have a low areal density, reducing the probability that an ejected fission product collides with a neighbouring fuel particlebefore escaping into the exhaust. That raises two obvious problem. (1) It will be challenging to sustain criticality in a diffuse dust unless volume is extremely large, because the mean free path of neutrons is a function of fuel atom density; (2) Power density (and thrust) will be low, because the fuel density is low and the power output from any individual fuel particle is limited by its ability to radiate excess heat beneath its melting point. That makes acceleration comparable to an ionic propulsion.

One potential solution: have the fuel in the form of long, thin axial fibres, whose axis points to the nozzle exit. Wrap a superconducting magnetic coil around the engine, producing a static magnetic field with field lines running parallel to the direction of the fibres. When positively charged fission products escape from the fibres, they will spin along the magnetic field lines. This forces them to exit the nozzle along magnetic flux lines, reducing their potential to impact neighbouring fuel fibres.

Something like this:

Last edited by Calliban (2024-11-06 15:20:08)

New Mars Forums

Announcement

#1 2024-11-05 12:21:24

fission fragment propulsion from dusty plasmas

#2 2024-11-05 12:22:29

Re: fission fragment propulsion from dusty plasmas

#3 2024-11-05 12:23:56

Re: fission fragment propulsion from dusty plasmas

#4 2024-11-05 12:28:07

Re: fission fragment propulsion from dusty plasmas

#5 2024-11-05 21:58:45

Re: fission fragment propulsion from dusty plasmas

#6 2024-11-06 14:55:15

Re: fission fragment propulsion from dusty plasmas

Board footer