Editing Nuclear thermal rocket (section)

=== Gas core ===
[[File:Gas Core light bulb.png|thumb|upright=1.0|right|Nuclear gas core closed cycle rocket engine diagram, nuclear "light bulb"]]
[[File:Gas Core open cycle.png|thumb|upright=1.0|right|Nuclear gas core open cycle rocket engine diagram]]

The final fission classification is the [[Gas core reactor rocket|gas-core engine]]. This is a modification to the liquid-core design which uses rapid circulation of the fluid to create a [[toroid (geometry)|toroidal]] pocket of gaseous uranium fuel in the middle of the reactor, surrounded by hydrogen. In this case, the fuel does not touch the reactor wall at all, so temperatures could reach several tens of thousands of degrees, which would allow specific impulses of 3000 to 5000 seconds (30 to 50&nbsp;kN·s/kg). In this basic design, the "open cycle", the losses of nuclear fuel would be difficult to control, which has led to studies of the "closed cycle" or [[nuclear lightbulb]] engine, where the gaseous nuclear fuel is contained in a super-high-temperature [[quartz]] container, over which the hydrogen flows. The closed-cycle engine has much more in common with the solid-core design, but this time is limited by the critical temperature of quartz instead of the fuel and cladding. Although less efficient than the open-cycle design, the closed-cycle design is expected to deliver a specific impulse of about 1500 to 2000 seconds (15 to 20&nbsp;kN·s/kg).{{citation_needed|date=June 2019}}