The Liquid Droplet Radiator

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One of the biggest challenges when designing the Gen1 Enterprise will be getting rid of all the waste heat. After all, there will be gigawatts of waste heat that must be radiated out into space somehow since here is no air in space to help carry away waste heat like back on earth. Thus radiating away this thermal energy mainly as infrared radiation is the only option.

Shown above is a concept for using a “liquid droplet radiator”. The idea is to emit a thin sheet of liquid metal droplets from one of the aux engine hulls and collect it at the other aux engine hull. As the droplets pass through space they very efficiently dissipate the thermal energy that they are storing. Liquid droplet radiators have had a lot of research done on them, though one has never been implemented in a real spaceship. But the concept is sound, so there is no reason to think that it can’t be done.

The alternative approaches to a liquid droplet radiator have problems. First, gigantic arms with gigantic fins could be added to the Gen1 Enterprise. This follows the reaction from John Elliott, flight system lead on NASA’s Outer Planet Flagship Mission, when asked about the Gen1 Enterprise. He said:

“Reactors might be scalable to those high power levels, but the radiation shielding and heat rejection system would be a huge design challenge,” said Elliott, who qualified his comments as a personal perspective rather than official NASA or Caltech views. “I’d hesitate to say it couldn’t be done with sufficient resources, but it probably wouldn’t end up looking like the original design.”

In last week’s article at Space.com about the Gen1 Enterprise, a similar concern was raised:

“Adam Crowl, an engineer with Icarus Interstellar Inc., a nonprofit foundation dedicated to interstellar exploration, pointed out that a spaceship built with a sufficiently powerful nuclear reactor would need large thermal radiators, ruining the classic Enterprise look.”

If the goal is to build the Enterprise, the ship has to look sleek like the Enterprise is supposed to look. Otherwise that defeats the basic goal of latching on to the form of the Enterprise to generate broad-based excitement about building the first large interplanetary spaceship. Attaching giant arms with giant fins to the Enterprise would look very wrong and thus is not a suitable solution to the waste heat problem.

The liquid droplet radiator shown above also alters the look of the Enterprise, but it’s hoped that this addition will look fine and perhaps even add a visually interesting new feature to the ship. The sheet of moving liquid droplets will likely be above 400 degC, so the droplets will glow a faint red. The sheet will also be partly transparent; it will stand out less than as shown in the image above. So the sheet of moving droplets will appear as a shimmering red thin plane of light that is connecting the two aux engine hulls. I bet some graphic artists could have some fun trying to see how this might look.

It should be noted that this is yet another case where the Enterprise’s form is functional. The two long aux engine hulls work nicely as places to emit and collect the sheet of liquid droplets. And the heat that is radiated away from the sheet is far from the saucer hull, and roughly co-planar with it, thus making the saucer hull less prone to re-absorbing the radiator’s thermal radiation.

As I mentioned here, another method to try to get rid of the waste heat is to use the entire outer aluminum hull of the Enterprise as a radiator. At first glance this seems to work since there is so much hull area to work with (the Gen1 Enterprise is a big ship). The hull will get very hot, over 300 degC, but that is still well below the melting point of aluminum. However, at such high temperatures the mechanical properties of the aluminum hull will change and it should not be assumed that the hull remains structurally sound. As a reference, the space shuttle’s  aluminum hull was not allowed to get above 175 degC because structural failure could occur.

The liquid droplet radiator is not a certain fix to the problem of high waste heat with the Gen1 Enterprise. The liquid droplet radiator allows the radiator temperature to get much higher, and this gets around the problem of trying to use  the Enterprise hulls as the radiators and these hulls then getting too hot. But if the liquid droplet radiator runs very hot, while it can still work okay as a radiator, the nuclear electric power generation system efficiency will crater and now a new problem is created.

I will write more about the liquid droplet radiator and give temperature calculations in a future post.

(Reminder: As a last resort solution to solving the waste heat problem, the Enterprise engine powers can be reduced by a factor of three. This will cause the Enterprise to take around 8.5 months to get to Mars. I’m hopeful that it doesn’t come to that.)

I should also point out that Michel Lamontagne has started a Thermal wiki page at BTEWiki.org. This is a work in progress.

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