How Can We Have NTR SSTO?

[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.)