Thread by @GBruhaug

Ok last time I posted a thread about making antimatter, now let's talk about using it in a rocket! Antimatter rocket concepts cover the full gamut in performance from launchers off Earth to interstellar speed machines and everything in between! 1/22

We should first talk about why antimatter? The interest is in the extreme energy density, which then could allow for rockets with very, very high specific impulse. This is like the gas mileage for a rocket and means that we don't need much antimatter to go very fast! 2/22

Antimatter should also allow for reasonably high thrust to weight ratios (TWR), which means the rocket can accelerate up to speed on a reasonable time scale. Once again it comes back to that awesome energy density of ~9E10 MJ/kg, about 1000X fission and 300X fusion! 3/22

Now there is a little hiccup here (ignoring making and storing antimatter). The reaction doesn't turn entirely into *useful* energy. It makes ~2 GeV per reaction, but that is split between multiple types of pions which then all quickly decay. 4/22

The charged pions can be redirected and used with magnetic fields, but the neutral ones don't respond to fields and decay into really high energy gamma rays very quickly! We lose about half the energy no matter what, which is a challenge for safety and efficiency. 5/22

Now the simplest engine we can make is a solid core antimatter thermal engine. This is like an NTR but puts antimatter into tungsten. It gets similar performance (~1000 sec ISP), but has no minimum size unlike an NTR. Materials limits prevent higher performance. 6/22

The next obvious step up would be to try and use magnets and make a gas core antimatter rocket! This actually doesn't typically win anything in ISP, but might win in mass of the engine since you don't need a big block of tungsten and instead make the charged pions circle. 7/22

A combo gas/solid core concept has been explored as well. This absorbs some gamma energy in a tungsten shell while also using magnets to circulate the charged pions. ISP's >1700 sec seem doable, but it's all a trade on gamma absorption. 8/22

This has even been considered for an SSTO rocket and spaceplane! There were some safety issues to consider though... Don't be near the launch and remember the engine stays hot after shut down just like a nuclear rocket! 9/22

Moving past this we can go for a plasma core rocket. There hasn't been much study here, but the antimatter energy efficiency is typically quoted as ~2%.. The ISP should be many thousands to maybe 10's of thousands though! That is going to get you going fast... 10/22

Pulsed antimatter-metal explosions have also been considered (kind of like Orion), and look much more efficient! The use of heavier atoms is a good way to get more charged particles (and more efficient use of antimatter) and could give us >10^5 sec ISP at the upper end! 11/22

We can also consider mixing in uranium and making an antimatter-fission rocket! This lets us have a gas/plasma core nuclear rocket (3-10K ISP) but without the concerns about maintaining a critical mass. Thus the leak rate of U can be kept very low without any tricks. 12/22

If fission is on the table, then I would be remiss to not mention ICAN-II! The idea is to compress fission and fusion fuel with large particle beams and then slam it with antimatter to get a sort of tiny nuclear bomb. It's kind of like ICF and gives ~10K ISP. 13/22

There are a couple versions of this concept of trying to use antimatter to make nuclear pulse propulsion easier. It's important to remember that we still need to form and compress fuel though! Antimatter doesn't just magically make nuclear chain reactions occur. 14/22

Another take on this is Antimatter Initiated Microfusion (AIM). This traps antimatter and fusion fuel in Penning traps to try and get a significant amount of fusion for thrust. Some U may also be added to get more charged particles from the antimatter. >67K ISP is claimed. 15/22

Personally I have my doubts about both of those previous concepts being as "easy" as claimed. ICF with antimatter and fission mixed in sounds... tricky and the AIM concept seems to rely on some questionable fusion burn physics. It would all be fun to test though! 16/22

The final concept worth mentioning here is the antimatter-fission sail. Here we spray antimatter at a solid sail of U or U trapped in an electrostatic trap. This then fissions and the fission fragments send us on our way! This gives us 10^6 sec ISP, which is cooking! 17/22

The advantage to all of these antimatter+nuclear fuel concepts is that they don't rely solely on antimatter for the energy and increased ISP. We are looking at milligrams to grams of antimatter total for missions to even nearby stars! These would all be "slow" missions. 18/22

To get going *really* fast we need to directly use the pions for propulsion! This monster will be consuming 100's of ug/sec of antimatter and can achieve speeds >40% of light speed! They have to dump insane amounts of heat though, notice the 10K km radiator! 19/22

The above is the classic Frisbee antimatter rocket, while other concepts like the Valkyrie have made it into movies and even ideas like antimatter ramjets using the interstellar medium have been considered! This is one option for interstellar missions on a human lifetime. 20/22

Where the ultimate limits lie on antimatter rockets, I am not quite sure, but I can say they are a long ways off! For now, I think the antimatter-fission sail, the gas-solid core mix and the plasma core are more interesting to consider for now. 21/22

References! 22/22





arc.aiaa.org/doi/abs/10.251…
arc.aiaa.org/doi/10.2514/6.…
ntrs.nasa.gov/api/citations/…
arc.aiaa.org/doi/abs/10.251…
sciencedirect.com/science/articl…
nets2021.ornl.gov/wp-content/upl…