Why D–D
Abundant fuel and a self-breeding cycle.
Deuterium is found in ordinary water and can be extracted on board or at a refuelling point. The D–D cascade needs no tritium stockpile: tritium and helium-3 form inside the plasma and then burn together with the deuterium.
| Reaction | Products | Role in the cascade |
|---|---|---|
| D + D | T + p | produces tritium |
| D + D | ³He + n | produces helium-3 |
| D + T | ⁴He + n | burns the tritium |
| D + ³He | ⁴He + p | burns the helium-3; all products are charged |
If the tritium and helium-3 burn completely, charged products carry a noticeably larger share of the energy than in D–T. Assumption: complete burn-up. In a real plasma some intermediate nuclei escape unburned, so the gain is smaller.
Challenges
Barriers to ignition.
- D–D reactions are much slower than D–T, so higher temperature and denser compression are needed.
- The intermediate tritium and helium-3 must burn within the duration of a pulse.
- The cascade needs a high pulse rate. Whether a single engine can reach it has not been shown.
- There are fewer neutrons than in D–T, but they remain, so shielding is still needed.
Implications for mission performance
Transit time depends on exhaust power, spacecraft mass and how completely the fuel burns. We will publish transit estimates only after the cascade model has been checked: a premature number here is easily misleading.