Could beta(-) emitters be used to build a compact light-space-drive?

[consuli]

One of the main issues to send orbiters to (light years) faraway locations is the propulsion problem. Conventional chemical fuels cannot provide enough energy by weight to produce that much thrust.

Nuclear fission provides a lot of energy by weight, but usually radiation energy doe not produce any thrust, as the radiotion goes omnidirectional anywhere.

In the case of beta-negative radiators the emitted particle from the isotope is an electron. And for acclerated electrons however, we already know a lot of energy conversions to transform them into light (which easily can be redirected in a certain direction using mirrors).

The following acclerated electron to light transformation are known:

1. Cherenkov radiation in transparent mediums like water
2. Neon and other nobel gases in flourescent tubes
3. Phosphor (e.g. in a CRT screen of an old school TV)
4. Certain crystals that are used in beta-radiation-scintillators

So, if a short- or medium lifed beta-negative radiating isotope would be put in the middle of a miniature rocket, which further space would be filled with such a transformation medium, light would be generated. Ideally all outer walls of such miniature rocket would be coated with sinousid mirrors, that redirect of the generated light, which can only escape at the rocket back. Thus, the light escaping at the back of the rocket will cause forward thrust.

My questions:

1. Has anyone propsed such nuclear fueled light-space-drive already? (If so, where can I find it?)
2. How much thrust would be generated from e.g. one kilogram 137-cesium, if the energy efficiency would be 50%?

 

[Vanadium 50]

Why not show us your calculations?

 

[gleem]

@Vanadium 50 I was in the processes of determining the feasibility of this process when you posted so here goes. Assuming my calculations are correct 1 kg of Cs137 generates 461 watts of power. How did I get that number. 1Kg of Cs137 is 7.3 moles with 4.4x10²⁴ nuclei. The rate of energy generation is equal to the number of disintegration per unit time multiplied by the average energy produces per disintegration . The number of disintegration per second is equal to the number of undecayed nuclei times the decay constant λ which is equal to ln2/half-life. The half life of Cs137 is 30.2 yrs or 9.52x10⁸ seconds. So λ =7.28x10 ^-10/second. Each decay generates about 0.9 MeV of energy. One MeV of energy is 1.6x10^-13 joules.

Thus the rate of energy generations is 0.9x1.6x10^-13x7.28x10^-10x4.4x10²⁴ = 461 j/second or 461 watts.

Now most of the beta particles will not get out of the block of Cesium and you need to encapsulate it since it is very chemically reactive so you loose about 1/4 of the energy and thus power. You are down to 346 watts. But wait I forgot its low melting point 83 deg F. So it would be a liquid at ambient temps. Most Cs source are in the form of the chloride but this would still get hot (how much I do not know) This amount of Cs137 (86500 Curies) unshielded would produce radiation exposure levels at 10 meter of about 270 R/hr ( lethal to 50% of humans with 2hr exposure). To reduce it to safe levels for shipping 5mR/hr at 1 meter would require about 0.8 m of lead (over 2m³ or over 22000 Kgm. The gammas are 662 keV fairly penetrating so you need a very thick phosphor absorber that is transparent to the light generated. I think the efficiency of converting the energy to propulsive energy will be significantly less than 50% ,

Compare this to a perfect absorbing one square meter solar sail which at the position of the Earth receives over 1300 watts and generates about 5 μNt of thrust. I would not invest any more time in thinking about the idea of using an isotope as a means of propulsion.

 

[consuli]

Thanks for your calculations gleem! As far I could reproduce your calculations, they seem to be correct. Only wikipedia states 0.51 MeV for Cesium-137 beta-decay. (Other common medium lilfe isotopes like Strontium-90 or Cobalt-60 are in the same range, thus would not produce significantly more energy).

However, your 1300 Watt for the solar panel of a long range orbiter is unrealistic. The most recent long range orbiter was New Horizons. It has a total launch weight of 478kg and its solar panels produce only 245 Watt.

If it would be possible to design an orbiter that contains 200kg Cesium-137, it would have gross power generation of 14'600 Watt (following your calculations). This only by normal decay. If it would be possible to create a chain reaction in some kind of beta-isotope-reactor, it could be multiples of that.

Ideally the radiation shielding and the transforming medium would be same.

Of course, there are a lot of unsolved questions. E.g.
- ideal transformation medium
- cost of Cesium-137
- environmental damage in case of 200kg Cesium-137 exploding in the sky
- ...

 

[consuli]

If I am allowed to repeat my first question, that has not been answered yet:

What is the more common label for "my" more or less obvious light-space-drive design? Nuclear powered direct photon drive? However, I do find anything in Google and it is not listed in https://en.wikipedia.org/wiki/Nuclear_propulsion.

 

[gleem]

However, your 1300 Watt for the solar panel of a long range orbiter is unrealistic. The most recent long range orbiter was New Horizons. It has a total launch weight of 478kg and its solar panels produce only 245 Watt.

I think you misinterpreted my statement. The 1300 watts is the amount of light energy that is passing through a square meter of space at a point 93 million miles from the Sun. It has nothing to do with a photo voltaic solar panel. Efficiencies of these panels are typically around 20%. The New Horizon spacecraft is powered by a radioisotope thermoelectric generator not solar panels. Pluto is about 40 farther from the Earth than the Sun making the Sun's light intensity 1600 times smaller and solar panels useless.

If it would be possible to create a chain reaction in some kind of beta-isotope-reactor, it could be multiples of that.

I know of no chain reaction involving Beta particle. A 200 Kg Cs source would probably vaporize due to the heat generated within.

Ideally the radiation shielding and the transforming medium would be same.

No it would require 42 cm more of Lead to achieve the same exposure rate that I calculated.

Of course, there are a lot of unsolved questions. E.g.
- ideal transformation medium
- cost of Cesium-137
- environmental damage in case of 200kg Cesium-137 exploding in the sky

If released in an explosion it would be 8 times more than that released by the Chernobyl disaster.

 

[mfb]

So, if a short- or medium lifed beta-negative radiating isotope would be put in the middle of a miniature rocket, which further space would be filled with such a transformation medium, light would be generated. Ideally all outer walls of such miniature rocket would be coated with sinousid mirrors, that redirect of the generated light, which can only escape at the rocket back. Thus, the light escaping at the back of the rocket will cause forward thrust.

That is incredibly wasteful. As in: Orders of magnitude of thrust lost in the conversion processes.
A hot surface emits infrared radiation. A simple hot surface at the back of the rocket will give net thrust already, mirrors can increase that more. You can use the whole energy that way, and you can use a nuclear reactor. The thrust will still be tiny, but at least measurable this way.

 

[Rive]

Nuclear fission provides a lot of energy by weight, but usually radiation energy doe not produce any thrust, as the radiotion goes omnidirectional anywhere.

Your original plans are just too weak and wasteful. What you have to consider is that:
- we do know a lot of 'energy conversions' for fission too
- what fission produces the most is actually not just radiation but radiation and mechanical (!) energy (as heat).
- absorption of radiation means it is converted to heat//mechanical energy too.
- since heat is mechanical energy it can be easily converted to thrust directly or indirectly.

 

[consuli]

mfb and rive, you have mentioned very good points. Also infrared radiation could be brought into distinct direction by mirroring and thus generate thrust.

However this approach might be more tricky than the beta-negative isotope design, because beta radiation passes through a thin encasing that holds the isotope in place (e.g. out of beryllium-oxide), but most of the infrared radiation will get lost at it, I guess.

Maybe a reactor could be created more easily with nuclear fuel, that decays to a beta-negative isotope with a very short life (seconds) and then the beta radiation will be converted to thrust (as supposed above) ?

 

[mfb]

However this approach might be more tricky than the beta-negative isotope design, because beta radiation passes through a thin encasing that holds the isotope in place (e.g. out of beryllium-oxide), but most of the infrared radiation will get lost at it, I guess.

What do you expect to happen to the radiation? The heat produced will go somewhere. As long as you keep the "back" of your rocket warmer than the front (which is trivial with the heat source at the back) you get thrust.

Maybe a reactor could be created more easily with nuclear fuel, that decays to a beta-negative isotope with a very short life (seconds) and then the beta radiation will be converted to thrust (as supposed above) ?

That's again a needless complication. You just need a source of heat, that is all.

 

[etudiant]

Ideally, one would want the radiated particles and photons to be all collimated and ejected using some sort of 'nuclear nozzle'. That would bypass all the complications of heating working fluids currently required.
To achieve that requires a nuclear decay that is all charged particles and a reflector surface to redirect the particles ejected in the wrong direction. Both aspects seem technically very challenging.

 

[mfb]

Fission-fragment rockets have been considered: Use fission products as direct exhaust.

Just using the heat would be a nuclear photonic rocket.

More concepts here. None of these ideas are new.

 

[consuli]

Indeed, the list of nuclear propulsion designs in https://en.wikipedia.org/wiki/Nuclear_propulsion#Spacecraft is very interesting. However, a lot of these concepts are already pretty old (from right after WW2).

It would be interesting to have some kind of pre-screening scheme to evaluate them. E.g. in dimensions of
- maximum power/ thrust
- maximum range per kg nuclear fuel
- ease of technical realisation

 

[anorlunda]

Indeed, the list of nuclear propulsion designs in https://en.wikipedia.org/wiki/Nuclear_propulsion#Spacecraft is very interesting. However, a lot of these concepts are already pretty old (from right after WW2).

It would be interesting to have some kind of pre-screening scheme to evaluate them. E.g. in dimensions of
- maximum power/ thrust
- maximum range per kg nuclear fuel
- ease of technical realisation

Engineers often find that limiting cases are very useful. If the limiting case is not good enough then all real world cases are not good enough either by definition. That saves an enormous amount of effort evaluating the many real world cases.

For interstellar rockets, the limiting case I believe would be matter-antimatter annihilation. Never mind where to obtain and store the antimatter. If that is not enough to get where you want to go fast enough, no other rocket will either.

 

[mfb]

An efficient antimatter/matter annihilation rocket with enough fuel has no trouble with interstellar travel within a human lifetime. If you can direct all the released energy backwards as massless (or nearly massless) particles you get a specific impulse of 30,000,000 s. You can accelerate at 1 g for one year with a mass ratio of e¹. You can constantly accelerate at 1 g for 2.5 years and then decelerate for the same time with a mass ratio of 148 (2 stages, probably) and reach a star at a distance of 11 light years. Every additional year of coast time in between adds 6.7 light years.
If you take fuel for 6 years of total acceleration (might need a third stage) you reach 20 light years without coast phase and 11 light years extra per year of coasting.
If you are fine with a mass fraction of 22,000 (probably looking at 4 stages now) you have 10 years of acceleration (5+5). The spacecraft now reaches a distance of 170 light years in 10 years for the spacecraft , plus 89 light years for each year of coasting. With a journey of 30 years you reach a distance of about 2000 light years.
If you can accelerate at 2 g you can also travel 90 light years in 5 years, again plus 89 light years for each year of coasting. 30 years now get you 2300 light years away.

 

[anorlunda]

An efficient antimatter/matter annihilation rocket with enough fuel has no trouble with interstellar travel within a human lifetime.

Thanks MFB. Could we coin a new unit used for online discussions of space drives. Let's say 1 "mad" is defined as the capability of a matter-antimatter-drive. As your post shows, the missions that can be accomplished with 1 mad (for the 1G case) can be defined. It is also clear that no space drive could exceed 1 mad, making the mad both useful and understandable.

A proposed drive scheme could be rated at perhaps 0.01 mad based on the most optimistic energy considerations.

I'm hoping to simplify future threads by considering only energy rather than getting into the messy mechanisms that make it work. But that hope is probably completely mad.

 

[mfb]

With a bit more realistic parameters: Project Valkyrie is a proposed antimatter-based spacecraft concept. It would use antimatter to initiate fusion until it reaches ~10-20% the speed of light and then switch to direct matter/antimatter annihilation directly. With a mass ratio of 22 (at the time of transition, presumably) it can reach 0.92 times the speed of light and slow down again.
The energy requirements even for a single trip are way beyond anything we can plan for today.