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The role of helium in the Starliner spacecraft

We will try to briefly describe the role of helium in the Starliner's propulsion system, where are the risks when there is such a leak and why, in my opinion, Boeing's engineers approved the flight even though they knew even before the launch that the helium line was leaking

The Starliner spacecraft separates from the Centaur launcher. Illustration: Boeing
The Starliner spacecraft separates from the Centaur launcher. Illustration: Boeing

On June 7 we reported here that the crew of Boeing's (cursed) Starliner spacecraft managed to dock the spacecraft in the International Space Station, while overcoming a helium leak in its control engines.

Here the thought crossed my mind - why do the engines need helium? 

Usually my articles are not topical and you can read them at any time, but this time I deviated a bit from my usual. We will try to briefly describe the role of helium in the Starliner's propulsion system, where the risks are when there is such a leak and why, in my opinion, Boeing's engineers approved the flight even though they knew even before the launch that the helium line was leaking [1]. 

The Starliner spacecraft includes 64 engines that play different roles, 28 of which are control engines, RCS (Reaction Control System). These are the engines that drive the spacecraft and control its angular motion [2]. Some of these engines can be seen in the figure below

How does an engine containing helium work. Illustration: Alex Shapira
How does an engine containing helium work. Illustration: Alex Shapira

The RCS engines of the Starliner are made by Aerojet Rocketdyne, model MR-104 [4] [5]. Produced with monopropellant thruster technology, that is, the fuel is flowed over a catalytic substrate that causes the fuel to ignite and thus produce the required thrust. This technology is very old and was first tried even before World War II. The big advantage - there is no need for an oxidizer, this of course reduces the complexity of the system and increases its reliability. Although much more efficient space propulsion technologies have been developed over the years, this type of thruster is still used in a variety of systems, including the Starliner.

Well, Helium gas Used in the control engines of the spacecraft to compress the fuel. The figure below shows a simplistic diagram of a fuel system of a typical 3-engine single push engine (arbitrarily I drew three engines, their number of course varies according to need). I couldn't find the diagram of the Starliner propulsion system online, but I estimate it is quite similar.

Fuel is compressed in the storage tank by gas and separated from it with the help of a rubber diaphragm. When a command is given to open the valve, the fuel comes out of the tank to a specific motor that produces the required angle for the spacecraft.

Let's remember, according to the engineers' decision, the spacecraft can have several tanks, this is to create redundancy so that if a malfunction occurs in one fuel/helium tank, it will not affect the entire system.

Ok, so after digging a little deeper, what can we conclude? First of all, indeed a helium leak could be a serious problem. If the helium leaks out, the fuel will not be compressed and will not be supplied to the engine. Secondly, the malfunction apparently happened in the helium filling tap.

Now the question arises, why was the spacecraft launched after all and in the end the mission was successfully completed? Here I am already entering the world of conjecture and a bit of speculation, but we will still try to give an educated guess. From all the searches I did, I found no data about the size of the leak, but there is talk of a "small" leak [6], so it could be that despite the leak, there is still enough helium pressure left to push the fuel towards the engine. Another issue is of course the redundancy. As said, the Starliner has 28 engines, but they don't have to be fed from a single tank. Apparently there are several parallel "branches" in the spacecraft, each of which consists of a group of RCS engines connected to a single tank. Such planning creates redundancy - if there is a fault in one or even more branches, the task can still be continued. This hypothesis receives further confirmation, because there is talk of shutting down three to five RCS engines.

Like all my articles, this article is also dedicated to our daughter, Michal, deceased.

Sources

[1] NASA and Boeing monitoring 3 helium leaks on Starliner ahead of ISS docking

[2]  CST-100 STARLINER

[3] Starliner Wikipedia

[4] Aerojet Rocketdyne Ships Starliner Re-entry Thrusters

[5] MR-104J 440N

[6] Boeing Starliner's helium leak explained by engineer – 'Crew is in no danger'

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