Dear Max!

"Is my proposal feasible?" Probably not.

You can lead the question to a similar problem at conventional drive systems: Would it be possible to point the thermal movement of all particles in a given direction?

But every question is it worth to be asked. I don't understand the downvote above. I remind of many academic questions with solely theoretical content.

Regards!

Hans

Actually, it is totally doable (sort of). First instead of solid Pu, make it a gas (plasma really), then contain it in a (leaky) magnetic mirror (https://en.wikipedia.org/wiki/Magnetic_mirror), the alpha particles would shoot out the leaky side. Now, the problem is complex, but we will simplify and make many assumptions to get an simple answer. Pu (~AMU=239 or 240, or 241) is heavy, in fact it is almost 60 times heavier than the alpha particle (~AMU=4), the alpha provides a thrust kinetic energy of 5 MeV ( the typical alpha energy, and assumes all the kinetic energy can be translated into thrust energy, it cannot), your "fuel" then has a kinetic energy thrust to weight ratio (cheating to make the math simple => 1 AMU = 931 MeV using E=mc^2) of 0.005. A chemical rocket engine like the Russia RD-180 has a thrust to weight ratio of about 75! Basically this is saying that you are taking a lot of useless mass with you that you have to accelerate, and you cannot get off the ground on Earth. Now weak thrust is not really the issue, it only counts when you are trying to leave a gravity well, once in space what is really important is Isp (Specific impulse), for alphas this is actually quite high, about 1.1E7 s (assuming thrust velocity is 1.1E8 m/s), compare this to a typical chemical rocket, the RD-180 is about 3000 m/s. To get any meaningful thrust (since we have all the extra mass, we need a significant thrust ), let's say a Newton, we would need a mass flow of .0000001 kg/s. This seems small, but .0000001 kg of alphas gives1E-7/6.6421608e-27 = 1.5E19 alphas/sec, this also means you need 1.5E19 Bq of radioactive decay...Pu 241 has 110 Ci/g or 4E12 Bq/gram, thus we need 3.6E6 grams of Pu per N of thrust, or 3.6 tonnes of Pu. This means the space craft will be very heavy, the Space Shuttle dry weight was about 68 tonnes, and lifts off with about 51 tonnes of fuel, so just the fuel weighs 90%, but all this fuels leaves! Your Pu engine is 5% fuel and 95% extra weight. This is also approximately 1% of all the Pu that has every been produced by man (all isotopes). So, 3.6 tonnes accelerated by 1 N means you will gain 2.8E-7 m/s/s, or 28 microns/s/s. This means in 10 years your fuel will be traveling at 88 m/s or 196 mph. Not very fast on interstellar scales. Oh and the fuel has a half-life of 14 years, so every 14 years your thrust decays by half (but not the weight, that stays about the same)  and you need the most radioactive one you can find to make thrust, so in 100 years you will have basically zero thrust. technically 0.5^7=0.7% of the original thrust or .007 N. These are pretty rough calculations (with several short-cuts), but you get the idea. Lots of mass, not moving very fast. 

Sorry forgot to answer your question....1kg of Pu-241 gives 4E15 Bq (alphas/s, we will assume all radioactive decay is into alphas), this is a mass flow of 2.7E-11 kg/s, so the thrust is (Force=exhaust velocity[m/s] * mass flow[kg/s]) =.003 N  So 1 kg accelerated for 10 years at 0.003 N is 9.46E5 m/s or 2 million mph (not accounting for the loss of thrust due to decay). This is pretty fast, and this is why small satellites can use micro newton ion thrusters for long periods of time! But now add some mass just to control the alphas (say a modest 50 lbs or 25 kg) now your 0.003 N in 10 years is only 37,000 m/s or 82,000 mph, compared to our fastest spacecraft to leave earth (New Horizons) at 36,373 mph, which was acquired from an Atlas V rocket in less than an hour. Voyager 1 is the fastest spacecraft ever (thanks to gravity assists ) at 38,090 mph. New Horizons weighed 487kg at launch. So thrust from 1 kg of Pu now pushing 487 kg for 10 year would only be 4100 mph. Again, not very fast. As the fuel increases to gain more thrust to reduce time accelerating you have a game of diminishing returns. As from above you see 3.6 tonnes of Pu after 10 years is moving at  only196 mph.

Dear Krik!

Thanks for your precise calculation!

Yor first statement relate to the Ion Drive. It is always realised. But Max don't mean this engine.

Furthermore, the decay of 93-pu-238 lead finally to the stabel state of 82-pb-206. (I'm not realy sure.) This means that we have about eight alpha decays and much more "fuel".

In my opinion this is a thought experiment. Huygens-Cassini had 38kg Pu on board and it was quite dangerous...

Regards!

Hans

Hans, 

True, I did not take the full decay chain into account, but you can't get all those alphas out anyway. In a mirror or other magnetic nozzle, the alphas are actually spiraling around the field lines not along them, so the KE is all wrapped up in synchrotron oscillations. The real thrust of a mirror machine would come from the plasma temperature, which basically would kill the whole idea...

UNLESS you use a fusion or fission reaction to heat the plasma as a gas-dynamic engine. It ends up being like a mile long, but might work.

Cheers,

Kirk

Kirk,

so it's possible that with this technology we can propel a 1kg cube sat to 1000Kms-1 in a period of about ten years.

Sure just not with Pu, that has too many issues. But other ways are feasible.

These guys certainly think so...(even faster, but probably closer to < 10 grams!)

...they want to do an Alpha Centauri fly-by!

http://www.rdmag.com/news/2016/04/hawking-milner-launch-starshot-space-exploration-project?et_cid=5228565&et_rid=384787723&location=top&et_cid=5228565&et_rid=384787723&linkid=http%3a%2f%2fwww.rdmag.com%2fnews%2f2016%2f04%2fhawking-milner-launch-starshot-space-exploration-project%3fet_cid%3d5228565%26et_rid%3d%%subscriberid%%%26location%3dtop

Kirk,

what about the 946Kms-1 you calculated, where is it coming from?

Can't something close to that be realized using plutonium-238?

Kirk, an Alpha Centauri fly-by would be interesting like the Voager-Missions.

Max, your proposal is actual a thought experiment. Thinking about the dangers of using Pu as fuel... Furthermore you have to neutralize the alphas and solve some other technological problems. But it's interesting nevertheless.

I would calculate the speed as following:

The mass of all alphas is about 32/238≈0,13 of the Pu-mass. If the average speed of all alphas would be ≈15000km/s, the resultant speed of the remaining Pu (probably Pb-nuclei) should be ≈2000km/s. But you have to calculate the effective load, technoligical equipment etc. What would be the actual speed?

For the decay of 94-pu-238 see: http://atom.kaeri.re.kr:8080/ton/index.html

I believe this is possible since Alpha particles can easily be shielded by a sheet of paper, they can only affect you when they get into your body.

Alpha particles are positively charged so we can easily deflect them with an electromagnetic field to provide directional thrust.

Max, what I mean to say is that the 946 km/s is a theoretical maximum for accelerating the 1 kg of Pu (no payload), but you will never be able to extract all the KE from the alphas. The are emitted isotropically and randomly in time, thus your Pu fuel would need to be atomized to be able to extract the alphas, and not very dense (as you pointed out alphas don't go very far), so the machine would by necessity be very large and very hot to keep the Pu a gas, and structure to magnetically confine it. Thus you would need much more energy to keep it that way...etc. etc, making it basically impractical to get the alphas out of the Pu and in one direction to provide thrust, without providing much more energy. Basically entropy prevents you from extracting all this KE into thrust.  Also, not all of the radioactive decay will go into alphas (depending on the isotope and isotopic purity), gammas will be produced from internal decay processes, and also from alphas colliding with electrons or nuclei, producing more gammas (technically high energy x-rays), so shielding/distance of a payload would be needed as well, making it more massive (shielding/support structure). This is why you can't just use a chunk of Pu as an engine.  Chunks of Pu are good for generating heat though, if you need a couple 100 watts for 10's of years.