Torpedo Engine Technology for a Venus Space Probe?

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Georg Wilhelm Friedrich Hegel, philosopher. I know people who have gone their entire lives without finding a passion. I am fortunate that I found mine early on.

Figure 1: Current US Navy Torpedoes. (Source)

I just finished reading an interesting article on a NASA proposal for a Venus space probe that uses power generation technology developed for a US Navy torpedo program back in the 1980s. Like many spacecraft, torpedoes need power generation systems that are small, generate massive power for a short period, and must be storable for years with the ability to turn on almost instantly with high reliability.

Back in the 1980s, the US Navy faced the threat of deep diving, high-speed Alfa-class submarines operated by the Soviet Navy. Standard torpedo power systems at that time either used seawater batteries (eg. Royal Navy Sting Ray) or Otto-fuel driven internal combustion engines (e.g Torpedo MK 48). Both of these technologies have their advantages and disadvantages.

• Seawater Battery
• Advantages: electrical propulsion is quiet, very reliable, no exhaust to remove, storable long-term
• Disadvantages: low energy density
• Otto-Fuel
• Advantages: high energy density, storable long-term
• Disadvantages: noisy, exhaust must be removed

The need to remove engine exhaust is a problem for a deep-diving torpedo. As the torpedo goes deeper, the engine must work against the back pressure of the ocean in order to remove the exhaust. This means that an Otto-fueled torpedo goes slower as it goes deeper. This creates an intrinsic limit on the operating depth of these torpedoes. The Alfa-class submarine could dive so deep that it was possible for it to go below the operating depth of many torpedoes.

The US Navy's solution was developed by the University of Pennsylvania in the form of the Stored Chemical Energy Propulsion System (SCEPS). The SCEPS engine uses the chemical reaction between lithium (fuel) and sulfur hexafluoride (oxidizer) to generate heat that can drive a boiler and turbine (Equation 1).

 Eq. 1 $\displaystyle 8\ \text{Li}\ \text{+}\ \text{S}{{\text{F}}_{6}}\to \text{6}\ \text{LiF}\ \text{+}\ \text{L}{{\text{i}}_{\text{2}}}\text{S}\ \text{+}\ \text{heat}$

A key advantage of this chemical reaction is that its reaction products occupy less volume than the input fuel and oxidizer.

Historically, NASA would have used plutonium-238 thermelectric batteries for its space applications. However, the US has lost its plutonium-238 production capacity (article, article) and is now going through multi-year process to rebuild that capacity. While this rebuilding is going on, the SCEPS engine may provide a useful alternative power source.

This is not the first time torpedo technology has been used in spacecraft. For example, a friend of mine managed a thermal battery design group that built the batteries for the Galileo Probe that descended into Jupiter's atmosphere. Very similar batteries have been used in torpedoes for decades because they can be stored for years, generate tremendous power for a short period of time, and are very reliable.

I should mention that the SCEPS engine ended up not being deployed on any active US Navy torpedo because of issues that are not relevant to a space deployment.

For those of you interested in the history of US torpedo development, Figure 2 is excellent (large photo, ~6 MB).

Figure 2: Pictorial History of US Torpedo Development.

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