How Can We Have NTR SSTO?

[sanman]

For those who'd say it's absolutely not practical/possible, please at least see this as a thought exercise to make the impossible possible.

If we really, really wanted/needed to do NTR SSTO, what would be the best way to do it?

What could be done to mitigate risks to make flights as safe & reliable possible, and to minimize wider hazards of nuclear contamination in the atmosphere?

Regarding the mass penalty from radiation shielding, could reductions in shield mass be possible for unmanned vehicles?

How could propellants be used for radiation shielding?

How could a nuclear reactor/engine be modularized or ruggedized to help it survive intact, in the event of loss of control over the vehicle or breakup of the vehicle?

Would it be possible to use a highly-enriched nuclear fuel to achieve higher burnup-fraction, for great fuel efficiency?

--------


So I'll confess that my renewed interest in this topic stems from the recent Russian test of its new 9M730 Burevestnik missile, which is purported to use nuclear ramjet propulsion. According to statements from the Russian president, the missile was apparently test-flown for a duration of 14-15 hrs, and apparently monitored by a NATO observation vessel as well as by ground monitoring stations in nearby countries (eg. Norway), with no anomalous radiation levels being reported. The statements also mentioned the reactor using liquid-metal coolant, as per submarine reactors, but being far smaller in size. The flight regime was sub-sonic, but it was claimed that faster versions could one day be possible (hypersonic?) The nuclear fuel was said to be highly enriched to some high fraction below weapons-grade.


Fine, fine, everyone is free to take the statements with a grain of salt, or reject them outright, etc.
But for the sake of discussion, supposing the propulsion system is able to perform as advertised, with its relatively small form-factor.

(Disclaimer: not seeking weapons discussion, just referenced what looks to be recent real world hardware, to serve discussion on how to do SSTO via NTR)


But this then offers the possibility of having a nuclear-powered scramjet-style SSTO that could use the atmosphere for propulsion by staying in the upper atmosphere for as long as possible while ramping up toward orbital velocity. This could allow significant savings in propellant mass and gross takeoff weight. The much greater energy density available from nuclear could help to overcome the tyranny of the rocket equation.

I was thinking that scramjet-style HOTOL would be better than VTVL nuclear rocket, because the flight profile is lower-G and less stressful aerodynamically, which helps safety.
Scramjet-style trajectory however means more heat buildup and thermal issues from frictional heating, due to staying in the atmosphere for longer.
Was also thinking that densified LCH4/LOX (less coking) could be our onboard propellant for getting up to ramjet speed, and later also providing the final boost to orbital velocity.

---


What are the key considerations that would make/break the NTR SSTO, from the technical and overall practical standpoints?

[Starship Trooper]

Back in the early 90s, when SSTO (Delta Clipper, Venture Star) was the aspiration, reusability was part of the package.  Now 30 years later, we have achieved booster reuse with Falcon 9 and Super Heavy, (with a landed New Glenn booster awaiting reuse).  Cost effective upper stage reusability is still an unfulfilled aspiration, but booster reusability has already changed the game.

NTR SSTOs would only make sense if they were fully reusable, and there is the rub.  No one is going to be happy about frequent reentry of NTR SSTOs until safe and reliable reentry of conventional upper stages is a proven and mature technology.  In other words, NTR SSTOs can't even be considered until we have at least a couple decades of routine reliable upper stage reuse experience behind us.  (The Shuttle experience is not encouraging)

[sanman]

Back in the early 90s, when SSTO (Delta Clipper, Venture Star) was the aspiration, reusability was part of the package.  Now 30 years later, we have achieved booster reuse with Falcon 9 and Super Heavy, (with a landed New Glenn booster awaiting reuse).  Cost effective upper stage reusability is still an unfulfilled aspiration, but booster reusability has already changed the game.

Alright, but DC-X & X-33 were chemical propulsion, which imposes excruciatingly tight margins for doing SSTO.
It's not that SSTO is inherently less reliable than multi-stage to orbit, but rather that it's too difficult to achieve with chemicals.
If we were purely comparing nuclear SSTO vs nuclear multi-stage-to-orbit, then you & everyone would probably agree that SSTO helps KISS.  (aka. "best stage is no stage")

NTR SSTOs would only make sense if they were fully reusable, and there is the rub.  No one is going to be happy about frequent reentry of NTR SSTOs until safe and reliable reentry of conventional upper stages is a proven and mature technology.  In other words, NTR SSTOs can't even be considered until we have at least a couple decades of routine reliable upper stage reuse experience behind us.  (The Shuttle experience is not encouraging)

But let's say we're planning for that day when there's been a couple of decades routine reliable chemical upper stage re-use, etc. Things seem to be on track for eventually achieving that.

We want nuclear SSTO not because it's cool/sexy, but because we want to free ourselves from Tyranny of Rocket Equation.
We want to be able to scale to launch vehicles with larger payload mass & mass fraction, to enable better economies of scale and more mission flexibility.
We want to be able to send ships to Mars without having to send up 4 other orbital re-fueling flights just to support the main flight.
We want power/energy to spare, so that we won't have to fret over every gram and kilogram being flown.

We don't know how to do fusion yet, so we want to go with fission, accepting its known issues/non-idealities.

[redneck]

The problem being that you're not free of the rocket equation. And you're stuck with hydrogen for any practical NTR. Even with a feasible NTR, the takeoff mass will be over 60% hydrogen* with attendant tank mass and thermal issues.

*Unless you can somehow run a lot more heat than anyone expects since "normal" NTRs are seriously limited in allowable temperatures.

[Paul451]

Alright, but DC-X & X-33 were chemical propulsion, which imposes excruciatingly tight margins for doing SSTO.

Not as tight as NTRs.

Chemical rockets have thrust/weight ratios upwards of 70:1 (Merlin is closer to 200:1). NTRs are in the theoretical range of 7:1, and more likely 4 or 5:1. Add shielding mass (even for uncrewed rockets) and it's worse. Gravity losses kills NTRs as a first stage. Super-kills it for an SSTO.

NTR would be struggling to be useful as a second stage. AIUI, proposals were to use them for a third stage; ie, once they've achieved minimal orbital velocity and you can switch to high efficiency, low thrust engines for long BLEO cruise phase.

It might, perhaps, be possible to use NTR as a launch vehicle from the moon. But I suspect even Mars would be too much.

Jet engines (and hence air-breathing NTRs serving the same role in a doomsday missile) can have much lower thrust/weight, because wings do the work of fighting gravity, the engines mostly only need to fight drag. [For example, the stupidly powerful F119 from the F-22 has a thrust/weight of around 8:1, giving the whole jet a thrust/weight slightly above 1:1. Modern turbofan engines are around 5 or 6:1.]

[sanman]

Thanks for nice responses

The problem being that you're not free of the rocket equation. And you're stuck with hydrogen for any practical NTR. Even with a feasible NTR, the takeoff mass will be over 60% hydrogen* with attendant tank mass and thermal issues.

*Unless you can somehow run a lot more heat than anyone expects since "normal" NTRs are seriously limited in allowable temperatures.

But hydrogen only seems to be favored for Isp reasons. For SSTO we'd want to balance that against thrust, which methane is better on.
If you're referring to neutron radiation, it doesn't care about what chemical form that hydrogen is in, as long as it has the necessary density. LCH4 has plenty of it, and is easier to densify than LH2.

The thing is that we'd be trying to achieve most of our deltaV via ramjet propulsion, thru nuclear heating of airstream.
The onboard propellant is just for the initial and final phases of our ascent, while everything in between would be nuclear-ramjet.

A main challenge however is how to achieve nuclear-ramjet heating at high mach numbers.
Maybe that's where we'd have to start gradually using onboard propellant mixed into airstream at higher mach flows.

Alright, but DC-X & X-33 were chemical propulsion, which imposes excruciatingly tight margins for doing SSTO.

Not as tight as NTRs.

Chemical rockets have thrust/weight ratios upwards of 70:1 (Merlin is closer to 200:1). NTRs are in the theoretical range of 7:1, and more likely 4 or 5:1. Add shielding mass (even for uncrewed rockets) and it's worse. Gravity losses kills NTRs as a first stage. Super-kills it for an SSTO.

NTR would be struggling to be useful as a second stage. AIUI, proposals were to use them for a third stage; ie, once they've achieved minimal orbital velocity and you can switch to high efficiency, low thrust engines for long BLEO cruise phase.

It might, perhaps, be possible to use NTR as a launch vehicle from the moon. But I suspect even Mars would be too much.

Jet engines (and hence air-breathing NTRs serving the same role in a doomsday missile) can have much lower thrust/weight, because wings do the work of fighting gravity, the engines mostly only need to fight drag. [For example, the stupidly powerful F119 from the F-22 has a thrust/weight of around 8:1, giving the whole jet a thrust/weight slightly above 1:1. Modern turbofan engines are around 5 or 6:1.]

Thanks for this. So thrust is the limitation, due to the problem of working fluid and power transfer to the exhaust stream.

But can't we get away with less thrust if it's an air-breathing ascent trajectory making use of lift from wings?
The fact that Burevestnik could fly for 14-15 hours could allow for a more gradual ascent, while exploiting lift to offset gravity losses. Re-entry could be the reverse of that trajectory. But then there's the issue of heat buildup.

Alternatively, could we elevate thrust using nuclear-chemical hybrid propulsion?
Perhaps something like a "nuclear pre-burner" that heats up reactant flowstreams before they go to main combustion chamber?

It's mind-bobbling that we're TRAPPED in the world of chemicals and their energy limitations. There's got to be some better way.

[Starship Trooper]

It's mind-bobbling that we're TRAPPED in the world of chemicals and their energy limitations. There's got to be some better way.

We are not trapped, there are other potential technologies limited only by imagination and capital investment.

For one, giving your vehicle/payload a quick start with a mass driver is doable, a Chinese company is working on that now.

For another, beamed power from the ground or orbit is another option, allowing use of a monopropellant.

While slightly dated now, the 1991 book by Mallove & Matloff, The Starflight Handbook was a good review of all sorts of propulsion concepts

[Vultur]

Thanks for nice responses

The problem being that you're not free of the rocket equation. And you're stuck with hydrogen for any practical NTR. Even with a feasible NTR, the takeoff mass will be over 60% hydrogen* with attendant tank mass and thermal issues.

*Unless you can somehow run a lot more heat than anyone expects since "normal" NTRs are seriously limited in allowable temperatures.

But hydrogen only seems to be favored for Isp reasons. For SSTO we'd want to balance that against thrust, which methane is better on.

If you use anything other than hydrogen, your Isp is not good enough to *remotely* make up for the bad mass ratio introduced by the heavy reactor+ shielding.

If I wanted to make an SSTO, I'd focus on really low mass fractions not Isp. An expendable SSTO is IMO well within current technology (I believe a tank stretched F9 first stage would manage it, especially with landing legs deleted; F9 first stage is already supposed to be 95.5% propellant fraction or about mass ratio 22, and a tank stretch would improve that). I think a reusable chemical SSTO is probably possible today if you use tricks like Starship's tower catch... Just not economically practical. It'd be huge for its payload size.

It's mind-bobbling that we're TRAPPED in the world of chemicals and their energy limitations. There's got to be some better way.

There are other options, solid-core NTR is just a particularly bad one. (This is sad, because NERVA was super cool and I used to really hope it'd be revived. But I think the practicalities just don't work.)

Most of the other ground to orbit options require a lot of very expensive ground infrastructure, like laser launch, SpinLaunch, advanced gun launch, etc.

Also, "energy limitations" for chemical rockets aren't really the issue. The fuel cost to orbit is quite tiny. It's bespoke hardware and huge staff costs that make launch expensive. Thus, going to nuclear (more expensive hardware, more regulatory costs & thus more staff needed) doesn't help.

But, in the spirit of the thread, if someone wanted to build an NTR SSTO regardless of practicality (and had total exemption from any environmental or safety requirements) the only real way to do it would be to get away from the solid core NTR and remove the requirement to keep the reactor solid, so you can use really high temperatures. A gas-core NTR could probably do SSTO quite easily.

[Robotbeat]

Nuclear thermal in general is worse than chemical for SSTO. Maybe an aggressive gas core would change that, but the efficiency would be lower than a chemical rocket. Kirk Sorensen has blogged it on selenianboondocks.com

Chemical rockets are more efficient than people think, and high performance nuclear thermal requires HEU which is much more expensive than natural gas per joule plus you need hydrogen propellant as well for high performance NTR, and you don’t even get the benefit of the 142MJ/kg embodied energy. Dumb.

Chemical rockets are literally better than NTR for RLVs.

[spacenut]

What about a fusion reactor engine that ejects plasma for thrust?  Only problem is the mass of electromagnets to contain the plasma in the combustion chamber and the nozzle.  I may have answered my own question, but I think it would use less hydrogen than a fission engine, thus maybe less mass for the spacecraft itself. 

Two stage to orbit seems to be the best solution right now, reusing the chemical first stage and developing a returnable second stage.  I do like Stoke Space's upper stage solution if it works well. 

[Robotbeat]

Note that fusion has too high of an Isp to be energy optimal.

People underestimate how good chemical rockets are. It takes 32MJ/kg to launch up into orbit assuming no losses of any kind, just kinetic and potential energy. A good chemical rocket like Falcon 9 already gets 320MJ/kg or so, and that’s including everything. If you had a truck carrying gravel up a hill, your chemical to potential energy conversion for that gravel is likely to be worse than 10%. Plus natural gas is like the cheapest energy source per joule that there is, other than pure sunlight and maybe LEU. Starship has the potential to get cost to orbit as low as transpacific air freight. It’s just not that much of a constraint.

If you want to get better, use rotating tethers. Then you can even beat the efficiency limit because you can reuse energy from reentry of another payload to launch assist a payload.

[Vultur]

Nuclear thermal in general is worse than chemical for SSTO. Maybe an aggressive gas core would change that, but the efficiency would be lower than a chemical rocket. Kirk Sorensen has blogged it on selenianboondocks.com

Chemical rockets are more efficient than people think, and high performance nuclear thermal requires HEU which is much more expensive than natural gas per joule plus you need hydrogen propellant as well for high performance NTR, and you don’t even get the benefit of the 142MJ/kg embodied energy. Dumb.

Chemical rockets are literally better than NTR for RLVs.

Absolutely, I don't think it makes any economic (or regulatory) sense, but in terms of the thread question of what it would take to make it possible - I think a gas core of some type is probably the best answer.

Even if you insist on SSTO, though, I think chemical would actually be easier.

[Starship Trooper]

What about a fusion reactor engine that ejects plasma for thrust?  Only problem is the mass of electromagnets to contain the plasma in the combustion chamber and the nozzle.

There are a raft of issues here, which the 1970s "Project Daedalus" design tried to address.  Note, this was a concept for a fusion powered interstellar probe.  The low thrust/mass ratio of such a fusion drive is not suitable for planetary ascents.  Wiki is good enough for an introductory synopsis: https://en.wikipedia.org/wiki/Project_Daedalus

Anyway, your magnets cannot contain the energetic neutrons produced by deuterium/tritium fusion.  So little propulsion would ensue and the neutrons would slowly disintegrate your spacecraft.  Therefore such concepts are based on deuterium/helium-3 fusion, which ejects energetic protons, which can be channeled by electromagnets.  Unfortunately, the fusion is exponentially harder to do:

Deuterium/Helium-3 (D-3He)

Difficulty of implementation. The D-3He reaction requires high temperatures and a limited temperature range compared to the D-T reaction. This is because the D-3He synthesis cross-section is lower than that of D-T, and the D-3He reaction requires more energy.

Problems with conditions. For example:
The need for high plasma density - The D-3He reaction requires a density product over a retention time that is 50 times higher than that of D-T.

No reliable source of helium-3 — helium-3 is a rare and expensive isotope that is not produced on an industrial scale. (Daedalus required 50,000 tonnes of fuel, of which ~ 30,000 tonnes would be helium-3.  The concept was to mine the atmosphere of a gas giant to get the 3He.  These guys thought big!)

Control complexity - Plasma retention and control of synthesis product accumulation are important for the D-3He reaction.

So basically, good luck with all of that.  The British boffins in the 1970s BIS thought implementation of such ideas might be half a century away.  Now they are looking to be over a century away, if ever ...

[sanman]

I'd once read that NASA has funded research into "Direct Fusion Drive" technology.

I recall we'd also had a thread on here about a Polywell fusion-based propulsion concept proposed by Bussard

Also, could something similar to Orion be done using laser-triggered Inertial Confinement Fusion of fuel pellets?

Note that fusion has too high of an Isp to be energy optimal.

But we should be able to use that high Isp to entrain additional propellant mass for increased thrust.
A ramjet design could have the high Isp fusion exhaust accelerating the incoming airflow.

[sanman]

What about a fusion reactor engine that ejects plasma for thrust?  Only problem is the mass of electromagnets to contain the plasma in the combustion chamber and the nozzle.

There are a raft of issues here, which the 1970s "Project Daedalus" design tried to address.  Note, this was a concept for a fusion powered interstellar probe.  The low thrust/mass ratio of such a fusion drive is not suitable for planetary ascents.  Wiki is good enough for an introductory synopsis: https://en.wikipedia.org/wiki/Project_Daedalus

Anyway, your magnets cannot contain the energetic neutrons produced by deuterium/tritium fusion.  So little propulsion would ensue and the neutrons would slowly disintegrate your spacecraft.  Therefore such concepts are based on deuterium/helium-3 fusion, which ejects energetic protons, which can be channeled by electromagnets.  Unfortunately, the fusion is exponentially harder to do:

Deuterium/Helium-3 (D-3He)

Difficulty of implementation. The D-3He reaction requires high temperatures and a limited temperature range compared to the D-T reaction. This is because the D-3He synthesis cross-section is lower than that of D-T, and the D-3He reaction requires more energy.

Problems with conditions. For example:
The need for high plasma density - The D-3He reaction requires a density product over a retention time that is 50 times higher than that of D-T.

No reliable source of helium-3 — helium-3 is a rare and expensive isotope that is not produced on an industrial scale. (Daedalus required 50,000 tonnes of fuel, of which ~ 30,000 tonnes would be helium-3.  The concept was to mine the atmosphere of a gas giant to get the 3He.  These guys thought big!)

Control complexity - Plasma retention and control of synthesis product accumulation are important for the D-3He reaction.

So basically, good luck with all of that.  The British boffins in the 1970s BIS thought implementation of such ideas might be half a century away.  Now they are looking to be over a century away, if ever ...

But how much 3He do you really need just for SSTO? We're not talking about supplying world energy needs. Let's suppose that if net energy-positive fusion were possible with 3He, then it gets prioritized for space applications, since it's supposed to be safer and less hazardous.

And what's so terrible about using Tritium, btw? It has a half-life of just 12 years, and you'd only need a small amount of it.

[Paul451]

Just to put it to bed, air-breathing launch is also "sounds good but doesn't make sense in practice". The trajectory for launch necessitates getting above the atmosphere as quickly as possible. You only gain a small amount of delta-v before you run out of air, during which you have to deal with high drag and thermal stress (plus the air-scoop is always heavy. Screwing your thrust/weight anyway.)

Nuclear plus air-breathing is piling garbage on garbage and wondering why it doesn't improve the smell.

I'd once read that NASA has funded research into "Direct Fusion Drive" technology.

Meh. NASA put money into the EM-drive.

Also, could something similar to Orion be done using laser-triggered Inertial Confinement Fusion of fuel pellets?

Have you seen the size of the laser-ICF reactors, versus the energy out? They make NTR reactors look dainty.

Note that fusion has too high of an Isp to be energy optimal.

But we should be able to use that high Isp to entrain additional propellant mass for increased thrust.

Sure. And after all that faffing around, you end up with something that has roughly the same thrust and Isp as a chemical rocket. Except the chemical rocket is 10% as heavy, costs 0.1% as much, is more reliable, and isn't spewing radiation.

Chemical fuels are optimal for Earth launch.

I know it's hard to get your head around, but it's true. We found the ideal case first. (The trick has been to get the construction and operating costs down to where the price of propellant is dominating the cost of launch.)

There are other applications where high-Isp, low-thrust rockets make sense. In which case, solar-electric is optimal out to the asteroid belt. After that, Jupiter and beyond, NEP becomes optimal. (Future tech, like fusion, might also compete with NEP, in that specific application.)

[Starship Trooper]

But how much 3He do you really need just for SSTO?

Mate, I love your interest in space, but I'm thinking your background is not STEM?  Anyway Daedalus was an optimistic minimum model of what a fusion reactor for spaceflight could be.  Dry mass of the reactor and craft, minus fuel and scientific payload, was 3500 tonnes.  The thrust was given as 7,540,000 newtons, which converts to ~ 769 tonnes of force.  So the thrust is less than a quarter of what would be needed to get the reactor off the ground, as I alluded to in my earlier post.  Fusion, if ever achieved, would be like ion drives for use in space only.  The low thrust is only redeemed by high ISP and the ability to thrust for years.

Tritium will not help you in any fashion.  You won't have thrust, just fast neutrons that are uncontrollable and so destructive they will limit the operational life of ground based D-T power reactors, if any are ever built.

[Starship Trooper]

Dear Paul,

You misinterpreted my off the cuff response to the OP's other question

"And what's so terrible about using Tritium, btw? It has a half-life of just 12 years, and you'd only need a small amount of it."

PS: Honors graduate of Caltech here, yes that was a very long time ago, but I'm not senile.  One of my proudest moments 'back in the day' was when "our" Voyagers were launched. And with 48 years of TLC, both are still working on reduced power and approaching a light-day out from Sol.  Back in the 1980s, there was a proposal for sending an ion drive probe out to the focus of the solar gravitational lens, over 3 light-days out.  Maybe people can start dreaming big again.

[sanman]

But how much 3He do you really need just for SSTO?

Mate, I love your interest in space, but I'm thinking your background is not STEM?  Anyway Daedalus was an optimistic minimum model of what a fusion reactor for spaceflight could be.  Dry mass of the reactor and craft, minus fuel and scientific payload, was 3500 tonnes.  The thrust was given as 7,540,000 newtons, which converts to ~ 769 tonnes of force.  So the thrust is less than a quarter of what would be needed to get the reactor off the ground, as I alluded to in my earlier post.  Fusion, if ever achieved, would be like ion drives for use in space only.  The low thrust is only redeemed by high ISP and the ability to thrust for years.

Yes, I understand that most of the devices used in mainstream fusion research are really big and heavy.
I wasn't talking about the mass of the machine, I was talking about how much of the rarified fuel you need to scrounge up for aneutronic purposes, since that's a concern with Helium-3.
Helium-3 is also produced as a by-product of nuclear reactors and weapons stockpiles, since it's a decay product. There's also the helium produced from the natural gas extraction industry, of which some small fraction is separated off by cryo-distillation as 3He.

Tritium will not help you in any fashion.  You won't have thrust, just fast neutrons that are uncontrollable and so destructive they will limit the operational life of ground based D-T power reactors, if any are ever built.

I understand that D-T fusion produces the extra neutron radiation, but that's a price you pay for the extra neutronic shielding effect to overcome Coulomb barrier.
Apologies, I wasn't trying to suggest using Tritium for D-T fusion, I was only mentioning Tritium because it too is a source of the rarified Helium-3, which is produced from Tritium decay. That's all I meant - sorry for the confusion.

Quick Google search says total worldwide annual production of 3He is 15-20kg.

So if nuclear fusion were to somehow be viable for space launch purposes, aren't our existing 3He sources enough to get us off the ground, without having to start strip-mining lunar surface or diving into Jupiter's upper atmosphere?

[Vultur]

Chemical fuels are optimal for Earth launch.

I know it's hard to get your head around, but it's true. We found the ideal case first. (The trick has been to get the construction and operating costs down to where the price of propellant is dominating the cost of launch.)

Yes, at least for internally powered vehicles. There's a possibility that some kind of high infrastructure approach like laser launch or Spinlaunch would make sense at a large enough volume of the right line of payloads, depending on how cheap & reliable the TPS maintenance on a reusable vehicle can be made.

[Robotbeat]

Helium-3 can be extracted from bulk helium, and so is fairly common in the US we just don’t extract it much. It can also be bred by making tritium and letting tritium decay.

Or mined from the clouds of Saturn or Uranus.

If we had a good use for it.

[edzieba]

NERVA-style solid-core NTR: not viable. TWR is far too low.

TORY-style solid-core nuclear ramjets for initial acceleration: not viable, maximum velocity too low (SLAM's mach-3 sprint was limited by inlet heating)

Gas-core 'nuclear lightbulbs': potentially not physically impossible, but first you'd have to invent an optically transparent gas core reactor. Good luck!

That leaves you with pulsed fission-fusion, i.e. Project Orion. Physically possible, stupendous thrust and ISP, no problems at all with SSTO (and single stage to Earth departure velocity, for that matter). But very unpopular with the neighbours.

[Starship Trooper]

That leaves you with pulsed fission-fusion, i.e. Project Orion. Physically possible, stupendous thrust and ISP, no problems at all with SSTO (and single stage to Earth departure velocity, for that matter). But very unpopular with the neighbours.

Of note, the last atmospheric nuclear test was done on October 16, 1980 by China.  Since then, no one, not even Pakistan or North Korea, has thought it would be a good idea to nuke the air we and our food sources breathe.

[sanman]

Maybe fission is out of the question then -- maybe we'll have to find a way to make fusion work.
At least fusion doesn't mostly result in unstable products that continue to be radioactive - aneutronic being the safest

And while most of the main fusion experiments rely on very heavy equipment, not all concepts are heavy.
There's Polywell  (I remember there used to be a forum site dedicated to it, where I'd sometimes post)

There's the Dense Plasma Focus approach by Lawrenceville Plasma Physics, led by Eric Lerner (I used to occasionally donate $20 to them once in awhile. I was always impressed by the earnest transparency of their efforts, which they'd chronicle on their blog)
Their idea is to achieve fusion through this plasmoid that gets pinched, and the kinks pile up to form a knot, which squeezes the plasma to achieve fusion. Apparently Lerner got this idea from his grad student research on the natural astrophysical version of this phenomenon in quasars.

They're applying that method for achieving aneutronic fusion by the harder p+11B route, which poses a higher upfront energy barrier, but allows the possibility of better energy recovery through charged products.

The apparatus does not seem to be super large though, so if it works then I wonder if it could perhaps be further optimized and adapted for use on a launch vehicle.
But this is a pulsed fusion device, not a continuous one.

https://www.lppfusion.com/

[InterestedEngineer]

Think about the fundamental physics of reaction drives without even bothering to worry about whether it's fission, fusion, antimatter, or whatever, and you will find the energy density is enough to create x-rays, which will annoy your neighbors and eat a hole in whatever ground is behind the rocket.

Let's suppose we want an exhaust velocity of 20,000m/sec, and we have a 100t SSTO rocket.

if our TWR needs to be 1.5, then we need 1.5MN of thrust.

Now in a rocket the mass flow is force = mdot * exhaust velocity, and thus fixing force at 1.5MN and velocity at 20e3 m/sec we get an mdot of 75kg/sec.

now calculate the energy rate of that 75kg/sec.   It's 75/2 * (20e3)² = 15GJ/sec or 15GW.

Now, when that exhaust hits the atmosphere or the ground, you are going to turn much of that 15GW into heat energy, and when you heat 75kg with 15GW you get a temperature (for let's say water) of 100,000K.  Notice I'm only counting kinetic energy.  And it's very difficult to emit that kind of exhaust without imparting some thermal energy on the exhaust (typically even ion engines have only a 66% efficiency, leaving 33% thermal energy).

Now by Wien's Law your peak spectral radiance is 29nm.

You are now emitting deep UV and soft x-rays just from the exhaust hitting the atmosphere, generating a ton of plasma. 

You'll eat a hole instantly in any ground structure.  Your neighbors won't like the x rays.

Now contrast this to Starship's booster, which  is doing so with about 25,000kg/sec of exhaust at 3300m/sec.   The force is 82MN, the (kinetic) energy rate is 135GW.  Little or no plasma is formed, and it's easily absorbable by a lot of water (the deluge)

TL;DR - if you use a high exhaust velocity for launching from earth you concentrate all that energy into a few kilograms of matter per second and your exhaust is plasma which will emit anything from deep UV out to gamma rays, depending on how much energy is in that plasma.   That stream of plasma will etch deep holes in the ground and have a pretty long range (km probably) for doing so.

I haven't even considered the atmospheric effects such as the supersonic shock of a plasma going through the atmosphere.  Think of lightning, but with far higher energy rate and a continuous one at that, not one that lasts a few milliseconds.

[sanman]

Alright - back to fission again. I know you guys will feel I'm flogging the dead horse, but here's another idea to consider:



So madame is pointing to irregular shaped nuclei (this one happens to be dumbbell shaped)

If we have something like a dumbbell-shaped nuclei, then it offers the possibility of achieving a cleaner break in the weaker middle bridge-point, instead of breaking it apart like a pinata and spraying stuff (neutrons) all over the place.

We get fission fragments that are heavier but also likely more stable.
Beryllium-10 is obviously quite light, and so there isn't a whole lot more decay chain its products can undergo. The resulting helium nuclei (aka. alpha particles) are obviously going to be maximally stable.
These alpha fragments are also charged, which makes it easier to harvest their resulting energy, just as the aneutronic fusion fans like to do.
Fine, you're getting a couple of neutrons released along with those 2 larger alpha fragments, but that can be worth it given how much energy you're getting out of those alphas.

Here we can afford to have our nuclear fission products as our direct exhaust stream, albeit with overly high Isp.
So we no longer have the problem of 'working fluid' and heat transfer limitation which limits thrust.
Our overly-fast high-Isp fission products can perhaps be used to augment the Isp of some heavier chemical propellant exhaust.

Even if Be-10 doesn't work out, there may be other irregular-shaped nuclear isomers out there which might have useful fission characteristics that we could usefully exploit, perhaps even for propulsive purposes.

Even if it costs us energy to make exotic irregular-shaped nuclei like this, we can still make it here on the ground for subsequent use during flight, just like we do with so many other rocket fuels.


Maybe proton-spallation could work to break/fission our Be-10 dumbbells, by snatching a neutron or two from the dumbbell bridge which holds the nucleus together. You'd probably want to tune your proton spallation energy to be just enough to break the bridge, to avoid unnecessary neutron release from pinata-breakage.
There are newer medical cyclotrons and laser-driven proton accelerators which accelerate protons, while not being ridiculously large.

[edzieba]

Alright - back to fission again. I know you guys will feel I'm flogging the dead horse, but here's another idea to consider:



So madame is pointing to irregular shaped nuclei (this one happens to be dumbbell shaped)

If we have something like a dumbbell-shaped nuclei, then it offers the possibility of achieving a cleaner break in the weaker middle bridge-point, instead of breaking it apart like a pinata and spraying stuff (neutrons) all over the place.

We get fission fragments that are heavier but also likely more stable.
Beryllium-10 is obviously quite light, and so there isn't a whole lot more decay chain its products can undergo. The resulting helium nuclei (aka. alpha particles) are obviously going to be maximally stable.
These alpha fragments are also charged, which makes it easier to harvest their resulting energy, just as the aneutronic fusion fans like to do.
Fine, you're getting a couple of neutrons released along with those 2 larger alpha fragments, but that can be worth it given how much energy you're getting out of those alphas.

Here we can afford to have our nuclear fission products as our direct exhaust stream, albeit with overly high Isp.
So we no longer have the problem of 'working fluid' and heat transfer limitation which limits thrust.
Our overly-fast high-Isp fission products can perhaps be used to augment the Isp of some heavier chemical propellant exhaust.

Even if Be-10 doesn't work out, there may be other irregular-shaped nuclear isomers out there which might have useful fission characteristics that we could usefully exploit, perhaps even for propulsive purposes.

Even if it costs us energy to make exotic irregular-shaped nuclei like this, we can still make it here on the ground for subsequent use during flight, just like we do with so many other rocket fuels.


Maybe proton-spallation could work to break/fission our Be-10 dumbbells, by snatching a neutron or two from the dumbbell bridge which holds the nucleus together. You'd probably want to tune your proton spallation energy to be just enough to break the bridge, to avoid unnecessary neutron release from pinata-breakage.
There are newer medical cyclotrons and laser-driven proton accelerators which accelerate protons, while not being ridiculously large.

AKA the Fission Fragment engine.

[tuomasn81]

Maybe fission is out of the question then -- maybe we'll have to find a way to make fusion work.
At least fusion doesn't mostly result in unstable products that continue to be radioactive - aneutronic being the safest

And while most of the main fusion experiments rely on very heavy equipment, not all concepts are heavy.
There's Polywell  (I remember there used to be a forum site dedicated to it, where I'd sometimes post)

There's the Dense Plasma Focus approach by Lawrenceville Plasma Physics, led by Eric Lerner (I used to occasionally donate $20 to them once in awhile. I was always impressed by the earnest transparency of their efforts, which they'd chronicle on their blog)
Their idea is to achieve fusion through this plasmoid that gets pinched, and the kinks pile up to form a knot, which squeezes the plasma to achieve fusion. Apparently Lerner got this idea from his grad student research on the natural astrophysical version of this phenomenon in quasars.

They're applying that method for achieving aneutronic fusion by the harder p+11B route, which poses a higher upfront energy barrier, but allows the possibility of better energy recovery through charged products.

The apparatus does not seem to be super large though, so if it works then I wonder if it could perhaps be further optimized and adapted for use on a launch vehicle.
But this is a pulsed fusion device, not a continuous one.

https://www.lppfusion.com/

For the record here are some papers on DPF and IEC ("Polywell") fusion propulsion:

C.L. Leakeas: Parametric Studies of Dense Plasma Focus for Fusion Space Propulsion with D - He3 (1991) https://apps.dtic.mil/sti/tr/pdf/ADA234578.pdf

Sean D. Knecht, Robert E. Thomas, Franklin B. Mead, George H. Miley, H. David Froning: Propulsion and Power Generation Capabilities of a Dense Plasma Focus (DPF) Fusion System for Future Military Aerospace Vehicles https://apps.dtic.mil/sti/pdfs/ADA446973.pdf

H. D. Froning: Fusion Propulsion and Power for Future Flight (1995 & 1992) https://ntrs.nasa.gov/citations/19960021082

H.D. Froning & Robert W. Bussard: Fusion-Electric Propulsion for Hypersonic Flight (1993) (attached)

Robert W. Bussard & Lorin W. Jameson: From SSTO to Saturn’s Moons: Superperformance Fusion Propulsion for Practical Spaceflight (1994) (attached)

[Vultur]

Alright - back to fission again. I know you guys will feel I'm flogging the dead horse, but here's another idea to consider:



So madame is pointing to irregular shaped nuclei (this one happens to be dumbbell shaped)

If we have something like a dumbbell-shaped nuclei, then it offers the possibility of achieving a cleaner break in the weaker middle bridge-point, instead of breaking it apart like a pinata and spraying stuff (neutrons) all over the place.

We get fission fragments that are heavier but also likely more stable.
Beryllium-10 is obviously quite light, and so there isn't a whole lot more decay chain its products can undergo. The resulting helium nuclei (aka. alpha particles) are obviously going to be maximally stable.
These alpha fragments are also charged, which makes it easier to harvest their resulting energy, just as the aneutronic fusion fans like to do.
Fine, you're getting a couple of neutrons released along with those 2 larger alpha fragments, but that can be worth it given how much energy you're getting out of those alphas.

Here we can afford to have our nuclear fission products as our direct exhaust stream, albeit with overly high Isp.
So we no longer have the problem of 'working fluid' and heat transfer limitation which limits thrust.
Our overly-fast high-Isp fission products can perhaps be used to augment the Isp of some heavier chemical propellant exhaust.

Even if Be-10 doesn't work out, there may be other irregular-shaped nuclear isomers out there which might have useful fission characteristics that we could usefully exploit, perhaps even for propulsive purposes.

Even if it costs us energy to make exotic irregular-shaped nuclei like this, we can still make it here on the ground for subsequent use during flight, just like we do with so many other rocket fuels.


Maybe proton-spallation could work to break/fission our Be-10 dumbbells, by snatching a neutron or two from the dumbbell bridge which holds the nucleus together. You'd probably want to tune your proton spallation energy to be just enough to break the bridge, to avoid unnecessary neutron release from pinata-breakage.
There are newer medical cyclotrons and laser-driven proton accelerators which accelerate protons, while not being ridiculously large.

AKA the Fission Fragment engine.

Which would have extremely good Isp but very low thrust - definitely not usable for any kind of launch vehicle.

[leovinus]

Think about the fundamental physics of reaction drives without even bothering to worry about whether it's fission, fusion, antimatter, or whatever, and you will find the energy density is enough to create x-rays, which will annoy your neighbors and eat a hole in whatever ground is behind the rocket.

Let's suppose we want an exhaust velocity of 20,000m/sec, and we have a 100t SSTO rocket.

Some nice points and calculations here. I wish the thread title would included TSTO because Sänger-style with a nuclear second stage was perceived as more manageable. At least that is what some 60s documents indicate. I need to parse the thrust and Isp figures in more detail. Environmental considerations are a whole other kettle of fish which is why it was nice to see calculations.

[sanman]

AKA the Fission Fragment engine.

Wow - that was an amazing resource - thanks for that. Plenty of reading to be had there.

But for what you've referenced, those fission fragments are still the classical heavier radioactive ones prone to decay.

What I'd outlined would mean light-weight fission fragments (alpha particles), which sounds like Unobtainium, but that Beryllium-10 isomer is real and might enable this, even if it's something exotic that we'd have to manufacture.

Also, we wouldn't be relying on a controlled chain reaction from natural fission events in the Beryllium-10, which is relatively stable, but rather we'd be carrying out proton spallation to actively break the Beryllium-10 nuclei, hopefully in a clean way without too many messy unstable fission products and their neutron emissions.

That dumbbell structure of the Beryllium-10 nucleus is an oddity that deserves to be examined further, and investigated for possible exploitation.

Which would have extremely good Isp but very low thrust - definitely not usable for any kind of launch vehicle.

Could we perhaps use the high Isp to augment chemical propulsion? Or does that just add too much complexity?

[Vultur]

Which would have extremely good Isp but very low thrust - definitely not usable for any kind of launch vehicle.

Could we perhaps use the high Isp to augment chemical propulsion? Or does that just add too much complexity?

I've seen something called a LOX-augmented NTR mentioned, though not with a fission fragment engine - it was suggested for a "standard" (NERVA style solid core) NTR, add LOX to the hot hydrogen stream to add thrust at the expense of specific impulse.

You're still adding the dry mass of a nuclear reactor though. And the Isp isn't as good as a pure NTR. So probably not viable for Earth launch.

--

I don't think the key difficulties with SSTO are even really chemical fuel issues, though. They're at least as much reentry/reuse issues. I think an *expendable* chemical SSTO is actually well within the current state of the art, it just doesn't make economic sense to build.

[InterestedEngineer]

SSTO is about 9.2km/sec when you consider gravity and aero losses.

For an exhaust velocity of 3300 (Raptor-SL), that gets you a mass ratio (MR) of 16.2.  Just for an expendable dry mass of 100t (including payload) that's a fuel tank of 1500t, and that tank will mass about 3% for methalox or 50t, or HALF your allowed dry mass.  You are simply approach the limits of the rocket equation.  mass ratios above 10 (including cargo) or so are VERY difficult to pull off at the same time as a TWR of 1.5.

So let's try this for an exhaust velocity of 7000m/sec (NTR in atmosphere). MR is 3.72, even with LH2 this is reasonable, the real problem is TWR.  The engine just masses far too much, it's "just by itself" TWR is 5:1.    Given the fuel requirement (no fuel tank, just magic force field) you are down to a TWR of 5:3.72 or 1.34.

Now add in a real tank (H2 tank mass is 10% of fuel mass), we're talking TWR of less than 1.  It'll never get off the ground.

We haven't counted shielding yet.  HOpe your cargo doesn't care.

Good luck with EDL.  The tail weight is off the charts, so COM and COD won't match.  Then there's testing re-entry of a spent nuclear engine with all sorts of short half life highly radioactive nuclides...


the mismatch of wishful thinking to reality is really quite stark.

[Vultur]

SSTO is about 9.2km/sec when you consider gravity and aero losses.

For an exhaust velocity of 3300 (Raptor-SL), that gets you a mass ratio (MR) of 16.2. 

Sure, but F9 first stage is supposed to be propellant fraction 95.5% or MR 21+.

I think the original (balloon tank) Atlas was even higher, it just had really low TWR early engines.

I am pretty sure a tank stretched F9 first stage could achieve SSTO, there's just no *point*. The payload would be way way smaller than current F9 expendable.

This is another problem with solid core NTRs; the reactor is so heavy that it's not clear that the higher Isp gives you anything relative to a lower Isp but vastly lighter chemical stage.

Then there's testing re-entry of a spent nuclear engine with all sorts of short half life highly radioactive nuclides...

This is another key problem with any kind of nuclear SSTO concept, yeah. Without not just reuse, but quick easy turnaround, there's really no great advantage to SSTO; the point is supposed to be skipping the stacking/integration for airplane like "gas and go" reuse.

(Well, and no staging events = more reliable, but that doesn't seem like as large an advantage as maybe it once did. I don't think any of F9's recent problems were staging related.)