Nuclear pulsed propulsion

[lamontagne]

I've contacted Troy Howe, one of the Howes behind the Pulsed Plasma rocket that just got a Phase II NIAC grant, and he was gracious enough to give a few hints about where they are with this project.  Although the engine design is advanced, the phase II grant will cover some of the development of a vehicle that would use the technology for the usual Mars access and some outer planets work, plus of course more simulations and research.

I think it would be interesting to do a vehicle design on the basis of this engine, and see how it compares to other solutions and what the researchers eventually work out.  This would also include some of the comparaisons that Interestedengineer has done showing the requirements for Nuclear to make it competitive with Chemical propulsion, and probably include other proposals such as the 3F idea the LMT champions and some of prof Winterberg's ideas.
The end result should be a series of images, some kind of spreadsheet, and if a competent writer shows up with adequate credentials, perhaps a paper?

BTW the PPR engine, a pulsed fission at 1 hz, is close to what can be used for fusion designs such as Vista or similar ones operating at the same frequency so the work might be applicable to other designs and propulsion systems.

[InterestedEngineer]

Post with some interesting details

https://forum.nasaspaceflight.com/index.php?topic=60353.msg2589246#msg2589246

[InterestedEngineer]

Video presentation to the interstellar group

[InterestedEngineer]

  This would also include some of the comparaisons that Interestedengineer has done showing the requirements for Nuclear to make it competitive with Chemical propulsion,

Howe's pulsed fission is simulating at an Isp of 5,000 (50km/sec) and a thrust of 100kN

Before I'd calculated that the threshold to overcome the benefits of a refuelabe Starship with EDL to be at about an Isp of 2,000 and a thrust exceeding 10kN.  So this one definitely fits the bill.  However unlike say NTR there's little hope of refueling at Mars or some other destination.

He's estimating about 50km/sec total deltaV for the ship, or a mass ratio of 2.72.  I didn't get any sense of dry weight, but he indicated a cargo of a rather measly 18t.   Still, it's in the 10s to 100s of tons of fuel, so mass production of HALEU and the ice pellets will be a problem to solve (and a separate thread)

He's talking 2 months flight to Mars, for a one way trip 25km/sec on the way out and 25km/sec braking, not sure if that includes a return flight which would be 12km/sec out, 12km/sec brake, 12km/sec return 12km/sec brake.  The dividing by 4 with no refuel and no EDL at Mars makes the break-even something over 4,000 Isp, which means it's barely cutting it.

To be really interesting as a tug that exceeds Starship in a useful manner for either quick cargo or quick human trips to Mars.  It needs a cargo of 100t or so to be really interesting, and the logistics of cargo transfer from a Starship to the transfer vehicle (and back to a Starship on Mars) add a bunch of cost to and hassle to the trip planning.   Presumably standardized cargo containers for Starship will help with cargo.  Not sure what to do about humans to Mars

I haven't read OP'S pdfs yet...

[InterestedEngineer]

A list of problems off the top of my head after watching the video and reading the main paper:

https://doi.org/10.1016/j.actaastro.2022.03.020

1.  How does the fuel storage work when you've got the moderator and the HALEU all together in a fuel storage mechanism?  Won't it be shooting thermal neutrons around causing issues?  By issues, I mean a runaway nuclear reactor kind of issues...

2.  How do you efficiently store a bunch of ice+HALEU+ iron pellets and feed them into the accelerator?  By a bunch I mean on the order of 100t.  At a few kg per pellet that's say 50,000 pellets.

3. How do you protect the crew/cargo from the large bursts of neutrons coming from this drive?

4.  How does a fragile sapphire exhaust bell work with very large 100kN+  bursts vibrating the heck out of everything?  There was no shock absorber in the system...

[lamontagne]

A list of problems off the top of my head after watching the video and reading the main paper:

https://doi.org/10.1016/j.actaastro.2022.03.020

1.  How does the fuel storage work when you've got the moderator and the HALEU all together in a fuel storage mechanism?  Won't it be shooting thermal neutrons around causing issues?  By issues, I mean a runaway nuclear reactor kind of issues...

2.  How do you efficiently store a bunch of ice+HALEU+ iron pellets and feed them into the accelerator?  By a bunch I mean on the order of 100t.  At a few kg per pellet that's say 50,000 pellets.

3. How do you protect the crew/cargo from the large bursts of neutrons coming from this drive?

4.  How does a fragile sapphire exhaust bell work with very large 100kN+  bursts vibrating the heck out of everything?  There was no shock absorber in the system...

1-2. I think the best references for fuel storage will be found in the Orion project.  They apparently worked with Coke vending machine fabricators on ways to handle multiple cans at a high rate with no risk of bloccage.  Scott Lower's work on Orion might be helpful here.
3.  I think that is the purpose of the rotating drums.  Or at least one of the purposes.
4. That's a big issue.  Again Orion may come in handy, or possible the work on nuclear light bulbs, although that was a constant thrsut device, so no impact.

I think the thrust comes from the expansion into the nozzle, and that it's the nozzle structure that handles the impact.  So we would need to have shock absorbers for the nozzle, and isolate this from the barrel exhaust. Saphire will be pretty tough, and it's not a huge engine.  My guess is that the thrust only exists for a small fraction of a second, and is way over ten tonnes during that time.  So I expect the inertia of the nozzle and shock absorber reaction time will be important.

It may make sense to have more than one engine.  Looking at the model of the nozzle, it's just 1-2m accross or so.  This may just be a small prototype, of course.  They must have many itterations at many sizes.

[lamontagne]

I think that a sensible application for this would be a thrust stage for a Starship.  My guess is that the 18 tonnes is an heritage from trying to fit this into a SLS upper stage faring, and that this is not the best way to go today.
As a NIAC proposal, I get that they need to respect 'official' parameters and work with SLS.  But I don't think we need to limit ourselves to this.  Starship gives you a lander, a life support system and enough resources to at least get new water from Mars for the bullets, or 'pulse units' in Orion vocabulary.  Because I don't expect this to be the first ship to Mars, but perhaps an eventual competitor to Starship, if the economics work out.

[Twark_Main]

"Bullets"? 

We are still talking about nuclear bombs, correct? Or was there some other component part of Orion (or some other "Orion" proposal entirely) that my mind is momentarily blanking on, one that uses bullets?

We all know Orion includes a bomb ejection system, but "bullet" of course has a more specific meaning than just "any projectile."

[seggybop]

1-2. I think the best references for fuel storage will be found in the Orion project.  They apparently worked with Coke vending machine fabricators on ways to handle multiple cans at a high rate with no risk of bloccage.  Scott Lower's work on Orion might be helpful here.

With this system the fuel "bullets" are concentric cylinders of uranium with a water moderator core. The system overall is vaguely like a partially reusable gun-type bomb. If the projectiles are all stored together, they could potentially act as a nuclear reactor and heat up, with consequences ranging from melting the ice cores to melting everything.  Orion would use stable self-contained bomb units that wouldn't have any possibility of reacting with eachother, so they only needed to worry about the mechanics.

[lamontagne]

"Bullets"? 

We are still talking about nuclear bombs, correct? Or was there some other component part of Orion (or some other "Orion" proposal entirely) that my mind is momentarily blanking on, one that uses bullets?

We all know Orion includes a bomb ejection system, but "bullet" of course has a more specific meaning than just "any projectile."

Not quite.  The bullet turns into a plasma, but it's not really a nuclear weapon or a weapon of mass destruction in the sense of the Outer Space Treaty. The engine has a barrel, and the object that goes down the barrel is a bullet.  I do like the euphemism of ''pulse unit'' that Orion used.
I do wonder about the burn up fraction.  Seems like a significant waste of nuclear fuel if the burn up fraction is very low.  Of course if we do turn away from fission as a power source for Earth in the near future, the uranium would be a pretty low value product.  Anyone notice in the papers how much uranium there is in each bullet?

[lamontagne]

1-2. I think the best references for fuel storage will be found in the Orion project.  They apparently worked with Coke vending machine fabricators on ways to handle multiple cans at a high rate with no risk of bloccage.  Scott Lower's work on Orion might be helpful here.

With this system the fuel "bullets" are concentric cylinders of uranium with a water moderator core. The system overall is vaguely like a partially reusable gun-type bomb. If the projectiles are all stored together, they could potentially act as a nuclear reactor and heat up, with consequences ranging from melting the ice cores to melting everything.  Orion would use stable self-contained bomb units that wouldn't have any possibility of reacting with eachother, so they only needed to worry about the mechanics.

Why do these units have more probability of reacting than an Orion pulse unit?  I would have thought that by definition an Orion pulse unit holding materials in a non critical arrangment would always be more dangerous that somethnig storing the material in entirely separate parts.  Won't they, be definition, be non critical?

[seggybop]

Sticking a large amount of somewhat enriched uranium in one volume, particularly with some moderating water, may be enough by itself to produce a reaction. Nuclear reactors in a basic form are not really difficult to create (see the natural ones at the Oklo mine, or the original Chicago Pile). I think this is the type of issue InterestedEngineer was referring to.

The Orion units, assuming they're basically the same as bombs, have all of their fissile material buried deep inside a casing that contains any extraneous radiation, so it wouldn't be a concern with them. That's not to say the Orion devices couldn't be more dangerous for other reasons, like the chance of setting an individual one off at the wrong time and evaporating the whole ship.

[john smith 19]

"Bullets"? 

We are still talking about nuclear bombs, correct? Or was there some other component part of Orion (or some other "Orion" proposal entirely) that my mind is momentarily blanking on, one that uses bullets?

We all know Orion includes a bomb ejection system, but "bullet" of course has a more specific meaning than just "any projectile."

That's debatable.

I've never seen any details on the Orion packages but they appeared to be self-contained modules AKA low yield H-bombs.

This system OTOH only goes supercritical when the package, the control drums and the HEU at the barrel end align. IOW it's a very poor nuclear weapons design.

Which (in principle) make it a much more socially acceptable design provided  you can get past the issues about the use of nuclear energy in space.   

But what would you call something that's accelerated down a cylindrical support structure (let's call it a "barrel") at roughly M4.7?

[Asteroza]

But what would you call something that's accelerated down a cylindrical support structure (let's call it a "barrel") at roughly M4.7?

A burrito? (see Alameda-Weehawken Burrito Tunnel https://idlewords.com/2007/04/the_alameda_weehawken_burrito_tunnel.htm)

But seriously, naming conventions like pellet or BB or slug might work. Or pearls. Technically the uranium shell is surrounding the water so a sabot shell. Maybe Pez dispenser.

Or the yutes on TokTak might call it the Demon Core Pooper...

[john smith 19]

Video presentation to the interstellar group

Let's just take a moment to consider the person who worked this out.
So

Control drum --> spinning continuously have sine effect on neutron output -->Different rpm gives more complex output --> use Fourier analysis to create high intensity neutron pulse

Control drums are pretty much SOP for NTR and some space nuclear power designs. But I've never seen the notion of spinning them, or at different rates to create a complex modulation of the neutron output .

I could be wrong, but I think this is could be a genuinely original development in rocket engineering.

 

There are a couple of possible tweaks to the design.

My instinct is to try and keep the packages as simple as possible and put as much of the complexity on the vehicle.

So watching this video from will tell you that  the drums would need to spin at odd multiples of a baseline spin rate and have falling levels of moderation. So if the baseline is 1rpm and full moderation the 6th drum is 11rpm with 1/11 th the moderation. BTW Fourier analysis says nothing about what pattern those drums should be in. Might be an issue. Might not. 

The conceptually simple solution is to spin the drums up with a set of electric motors and a battery and they align at peak output just as it comes up to the muzzle.

My instinct is to spin them up before launch and time it so they spin down to the level that gives maximum pulse. Since this happens outside the barrel they are not being moderated at this point.

The second point is the use of HEU. The work on KiloPower showed what a monumental PITA this is IRL. In fact even HALEU is difficult unless you can get access to government uranium that's been blended down from weapons stock.

Alternatives would be to use PWR grade (about 4-5% U235) and back it with a reflector. Reflector properties were considered in SDI papers in the 80's and SNP power reactor projects in the 90's. Be and BeO were the front runners but MgO was IIRC the winner. Much cheaper and way less toxic. MgO actually has a long history in nuclear work as it's the insulator for Mineral Insulated Cable used in lots of NPP's and fire alarm systems. In powder form it's cheap, has a very high Mp and could be compressed if you wanted a high density solution.

I've never seen any details of the Orion pulse modules but AFAIK the weaponeers who worked on it had a background in implosion based bombs, which would suggest an implosion design with efficient use of uranium. If Howe's design can replace a complex explosive implosion by a design keeping most of the U on board (in the barrel and muzzle), which is in effect reused on every pulse this could be the Holy Grail of in-system space flight. Relatively cheap, fast travel within the solar system using known physics (used in a creative way).

Lastly in materials science "Sapphire" implies single crystal Alumina. If you're talking polycrystalline Alumina that's much easier to make as a coating. Multiple routes from temperatures ranging around 1000c (essentially burning AlCl3) dating from GE's work on the Nuclear Aircraft programme to lower temp methods developed for semiconductor mfg. Otherwise you're talking machining panels to fit a nozzle contour, which will be challenging.

If people can get over their aversion to fission in space this has a lot of (potentially, given no hardware has been built) very attractive features.

Nice work.

[LMT]

I've never seen any details of the Orion pulse modules but AFAIK the weaponeers who worked on it had a background in implosion based bombs, which would suggest an implosion design with efficient use of uranium. If Howe's design can replace a complex explosive implosion by a design keeping most of the U on board (in the barrel and muzzle), which is in effect reused on every pulse this could be the Holy Grail of in-system space flight. Relatively cheap, fast travel within the solar system using known physics (used in a creative way).

3F / MOX would be a better candidate for that title, due foremost to its higher pulse energy or specific energy, which makes fast turnaround possible -- vital for fleet tug economics.  And it, too, uses known physics in a creative way, etc., but without the open issues of PPR.

The cost of a PPR bullet is an interesting question.  What's a plausible price for their "1200 g HALEU"?

In comparison, a 3F device has more complex fueling and structure, but breeding would cut cost, and mass-production in plastic seems feasible.  There are certainly trade-offs, and favorable 3F points, to consider.

[lamontagne]

This project aims to calculate design elements for the PPR propulsion system, and compare them to other similar systems:
- The proposed mission is a one way mission to Mars.
- The reference baseline is a 4 month (120 days) transfer by the SpaceX Starship.
- The target gain is improved transit times compared to Starship, in order to reduce radiation exposure.
- The Orion nuclear pulse rocket is also used as a design reference, as will be other alternatives as per interest by participants.
- The target is delivering a payload of  100 tons to Mars, plus the payload vehicle and some landing propellant. 
- Use of InSitu resources should be included.
- Aerobraking may be used if applicable, possibly only on the payload.
- Engine cooling systems and secondary power systems are included in the elements to be analyzed.

I'll try to post a preliminary spreadsheet today.

[lamontagne]

Reference systems:

Starship   Chemical propulsion
Orion   Pulsed nuclear rocket
PPR           Pulsed  plasma rocket
Vasimir   Nuclear electric, Chang  Diaz 39 day Vasimir
3F           Improvement of the Winterberg mini nuke design
Vista           Nuclear fusion pulsed propulsion

The design effort is mostly on the PPR secondary systems. 

[LMT]

I'll try to post a preliminary spreadsheet today.

Include propellant cost in this spreadsheet.  One bullet's HALEU, at 20% enrichment, would cost ~ $30,000 near-term.

[lamontagne]

This is the initial spreadsheet.  Most of the tabs hold unrelated stuff from former projects, only the first two tabs have been adjusted for this PPR one.

The closed cycle page will include a Brayton closed cycle system for the PPR, eventually.