fission fragment propulsion

[farmerben]

Quote

Rotating fuel reactor

A design by the Idaho National Engineering Laboratory and Lawrence Livermore National Laboratory^[1] uses fuel placed on the surface of a number of very thin carbon fibres, arranged radially in wheels. The wheels are normally sub-critical. Several such wheels were stacked on a common shaft to produce a single large cylinder. The entire cylinder was rotated so that some fibres were always in a reactor core where surrounding moderator made fibres go critical. The fission fragments at the surface of the fibres would break free and be channeled for thrust. The fibre then rotates out of the reaction zone to cool, avoiding melting.

The efficiency of the system is surprising; specific impulses of greater than 100,000 s are possible using existing materials. This is high performance, although the weight of the reactor core and other elements would make the overall performance of the fission-fragment system lower. Nonetheless, the system provides the sort of performance levels that would make an interstellar precursor mission possible.

 

 

 

https://en.wikipedia.org/wiki/Fission-fragment_rocket

[HebaruSan]

Neat. What's the approximate TRL?

[farmerben]

I dunno what TRL means.  I think the TWR is rather low.  But ion thrusters are in the 5000s ISP range, so they are not quite up for interstellar voyaging.

[HebaruSan]

4 minutes ago, farmerben said:

I dunno what TRL means.

Oh, sorry: 

https://www.nasa.gov/directorates/somd/space-communications-navigation-program/technology-readiness-levels/

[RKunze]

47 minutes ago, HebaruSan said:

Neat. What's the approximate TRL?

Should be around 2. Principles are well understood, engineering does not need any obvious unobtainium, but nobody built one yet.

Could have something to do with the fact that its exhaust is the same kind of highly radioactive fission products that are carefully not let out of containment in terrestrial fission reactors 

[HebaruSan]

26 minutes ago, farmerben said:

I think the TWR is rather low.

That was my guess as well, but your link gives an estimate of 4.5 kN in exchange for somewhat lower specific impulse:

Quote

By further injecting the fission fragment exhaust with a neutral gas akin to an afterburner setup, the resulting heating and interaction can result in a higher, tunable thrust and specific impulse. For realistic designs, some calculations estimate thrusts on the range of 4.5 kN at around 32,000 seconds Isp, or even 40 kN at 5,000 seconds Isp.

... whereas https://en.wikipedia.org/wiki/Ion_thruster says:

Quote

Ion thrusters in operation typically [...] possess thrusts of 25–250 mN

So "45000 times stronger thrust than ion engines" would be a defensible summary.

[farmerben]

Part C must be a vacuum filled Beryllium chamber.  The other part can be an actively cooled helium sterling cycle engine.  The electricity derived could support quite a lot of systems.

It's unclear whether the discs will be coated with U235 or some more exotic unobtanium.  Americium or Curium might be better.  This is because of high energy alpha particles interacting with the beryllium will excite neutrons driving it above criticality.  And you want to a fissile material that has a large cross section for both fast and slow neutrons.  

Edited August 20 by farmerben

[Nuke]

its a very dirty drive. not sure if dirtier than orion or not. dont fly near habitable worlds. 

[darthgently]

Something recent showing some interest is still there

https://www.nasa.gov/general/aerogel-core-fission-fragment-rocket-engine/

Can the fragment makeup be selected for low-ish half-life (like thorium's 500 years)?

[AckSed]

8 minutes ago, darthgently said:

Something recent showing some interest is still there

https://www.nasa.gov/general/aerogel-core-fission-fragment-rocket-engine/

Can the fragment makeup be selected for low-ish half-life (like thorium's 500 years)?

Looks interesting. For the record, SGL is "Sun gravity lens", and this is the rocket proposed to send a spacecraft 550AU to hang out there and use the warping effect of the Sun's gravity to image exoplanets through their Einstein ring.

There are a few wrinkles, though:

For one, sending one spacecraft with a single imaging platform will limit it to viewing one part of the ring - best bring a swarm. Two, any planet will only be observable for 40 minutes before moving out of focus.

But the key takeaway (and to bring it back to FFREs) is that this is an excellent precursor to interstellar travel. If you can make it here, you can make it anywhere! (Sorry not sorry.)

[darthgently]

1 hour ago, AckSed said:

Looks interesting. For the record, SGL is "Sun gravity lens", and this is the rocket proposed to send a spacecraft 550AU to hang out there and use the warping effect of the Sun's gravity to image exoplanets through their Einstein ring.

There are a few wrinkles, though:

For one, sending one spacecraft with a single imaging platform will limit it to viewing one part of the ring - best bring a swarm. Two, any planet will only be observable for 40 minutes before moving out of focus.

But the key takeaway (and to bring it back to FFREs) is that this is an excellent precursor to interstellar travel. If you can make it here, you can make it anywhere! (Sorry not sorry.)

With the huge Isp of FFRE, the SGL craft could move to different focal points many times.  If anything a constellation could be a lot smaller if one is willing to endure the reorientation time

Edited August 20 by darthgently

[Nuke]

23 hours ago, AckSed said:

If you can make it here, you can make it anywhere! (Sorry not sorry.)

*eventually.

[darthgently]

On 8/20/2024 at 12:37 PM, AckSed said:

If you can make it here, you can make it anywhere! (Sorry not sorry.)

... I'm thinking about a song