Low-Pressure NTR for Dissociated Hydrogen Speed

Nuclear Thermal Rockets (NTR’s) once seemed like a fantastic idea to the Sci-Fi greats who were involved in the early days of the post-War Space Age.

Prometheus with Alpha and Beta ready to fly

In the UK Arthur C Clarke wrote of a two-stage hybrid nuclear rocket “Prometheus” for Moon missions in his “Prelude to Space” (written in 1947, but published in 1951). To get the required performance Clarke fudged a little for the air-breathing nuclear Ramjet Beta component, hoping a high temperature system could be created to provide the required thrust. To get the required performance for the Moon return-mission, the Alpha component depended on methane heated to a high enough temperature to mostly dissociate into carbon and hydrogen and increase the exhaust velocity with a high atomic hydrogen fraction.

In the USA Robert Heinlein wrote many tales with Nuclear Thermal Rockets – his 1947 “Space Cadet” featured monatomic hydrogen sub-orbital rockets. By 1950 the first hard performance data of high temperature reactors was available and the first nuclear thermal rocket designs were appearing in the journals. His most detailed discussion was in “The Rolling Stones” (1952) about a family on the Moon buying a second hand NTR spaceship to take to Mars, then the Asteroids and Saturn. The characters mention tanking up on monatomic hydrogen for the long range destinations. Back in the infancy of cryogenics, monatomic hydrogen seemed potentially stable over long periods of time. Using just H, rather than H2, increases the exhaust velocity of an NTR by 50% or so.

Low Pressure Nuclear Thermal Rocket schematic

Atomic hydrogen doesn’t like being by itself and quickly recombines into molecular hydrogen all by itself. However once molecular hydrogen gas is hot enough, it starts breaking apart and this fact can improve exhaust velocity. That’s the basic physics idea behind the Low-Pressure Nuclear Thermal Rocket (LPNTR) which feeds the gas into a really hot reactor, but at a low enough pressure to minimise the recombination of the hot atomic hydrogen. The improvement is significant – the Specific Impulse jumps from NERVA’s 850-925 seconds to 1210-1350 seconds.

Titan Base from "The Invisible Enemy"
Titan Base in the year 5050 CE

However making hydrogen is a non-trivial exercise – about 120 MJ/kg when making it from steam. Chemically free hydrogen is rare anywhere but the Sun and the Gas Giants. And one other place. Titan, which has an atmosphere that’s 0.1% hydrogen. While that doesn’t sound like much, it’s readily extracted and liquefied for a lot less energy than cracking it out of water ice. The total mass of H2 is about 675 billion tonnes. The delta-vee to launch to the Earth-Moon system from the surface of Titan is about 4.8 km/s – round it to 5 km/s to account for gravity losses. In energy terms roughly 12.5 MJ/kg. Using the high-thrust mode of the LPNTR, the mass-ratio is ~1.54, meaning the rocket can be mostly payload. For the long cruise back home, the reactor’s fission-product decay heat coupled by heat exchanger to a thermophotovoltaic system can run a cryo-cooler to keep the hydrogen chilled.

In terms of energy expenditure, sourcing hydrogen from Titan makes more sense than anywhere else in the Solar System. Hohmann Transfer Orbit launch windows open more frequently for Saturn-Earth, every 12.4 months, versus 26 months for Mars-Earth, thus more frequent delivery opportunities. The Transfer time is 6 years, but this can be scheduled for. Faster elliptical (~3 years), parabolic (~2 years) and hyperbolic orbits are possible for higher hydrogen expense, but for a well established automated delivery schedule the Hohmann transfer is sufficient.

4 Replies to “Low-Pressure NTR for Dissociated Hydrogen Speed”

    1. I was born in 1970 – the year the Dream of Space Age 1.0 died – and grew up wondering why the dreams of the 1960’s didn’t happen. Wasn’t until I learned more about US politics that it began to make sense. That and what the first Space Probes actually found – a cold, barren Mars and an oven-hot, crushing lava-plain Venus – which killed the initial impetus inspired by Heinlein and Clarke.

  1. I wonder whether or not cyclers would be practical for regular Titan transfers. Perhaps it is time for a more ‘grown up’ approach to treaties governing the use of nuclear technology in space, or perhaps now is not the time to consider easing restrictions… Specific impulse of 1210-1350 seconds though!

    1. There’s no treaty against reactors for power or propulsion. Just against independent nuclear explosives. The NTR – and the PFNTR – suffers badly from lack of propellant for really high performance. It’s a waste to ditch the oxygen released by cracking the hydrogen out of water. That can be improved by feeding oxygen back into the reactor exhaust stream to create an “after-burner” – the Isp goes down, but the LOX used goes up, increasing the value-for-megajoules return from the reactor. Really useful for high speed missions as a ferry to the Moon or similar high-thrust missions.

      Of course, on Titan, there’s two propellants that can be readily pumped from the atmosphere – nitrogen or methane. For a Space-Tug that launches from Titan to an Earth-Return trajectory (delta-vee 5 km/s) a nitrogen-propellant NTR only needs to run at about 2400 K. The mass-ratio is the “golden ratio” of 5 which maxes the energy efficiency. Methane can be used, but it breaks down and introduces carbon into the exhaust stream, which can choke the flow by condensing. Of course that process can be used to separate hydrogen from methane for less energy input than making it from ice, but it does require doing something about the carbon. A hot oxygen flush can burn it off, but that limits the reactor components to non-oxidisable materials. A smart chemical engineer might figure out a good way to do it for much less energy than pyrolysing or electrolysing water.
      One additional inefficiency I didn’t add to my discussion was the conversion of reactor heat into electrical power. The surface of Titan represents a huge heat sink, so a reactor could run *very* efficiently. But you’re already on Titan – why make hydrogen??

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