For this post and a couple more in this series, I’m going to talk about a subject near and dear to my heart – composite pressure vessels. This week, I’m going to describe two different types - all composite and composite overwrapped pressure vessels (COPVs) - and some of the sizes and shapes of these things and why they are the size they are. And just so that you know, composite pressure vessels are really just composite wrapped pipelines with the ends capped off. For those of you that are curious, look at US Patent Number 6,146,482, Patton et. al.
But first, why make a composite pressure vessel? Steel and aluminum pressure vessels are everywhere you look. Scuba divers typically use either a steel or an aluminum air tank on their back. But these things are pretty heavy – especially when you have double steel tanks – believe me, I had those. Pretty unwieldy out of the water.
Nowadays the higher end scuba tanks are aluminum lined carbon/epoxy overwrapped – COPVs. These tanks are about a quarter the weight of a steel tank that holds the same pressure and volume of air. And they are less than half the weight of an aluminum tank. This is the main motivator for use of a composite wrapped pressure vessel – weight. And with the price of carbon fiber coming down significantly in the past decade or so, these things are pretty much everywhere.
There are small ones that are purpose built to hold specialty gases and usually have a thermoplastic liner – like the two pics to the right.
There are medium size ones like the pic to the left. These are mostly for holding air at high pressure, commonly at 4000 psi or above. These cylinders also typically have more air in them than do their metal counterparts. These are called “Type 3 or Type III” pressure vessels, and they are used for more than just scuba diving. They are the air source for the fireman’s breathing apparatus. They are also used for medical oxygen in hospitals because they are much easier to transport than their metallic cousins.
And there are really large composite pressure vessels – primarily to store and transport things like liquid hydrogen and liquid oxygen. The pic to the rightis of an Omni tank that mounts on a trailer just like the tanker trucks you see rolling down the road. This one in particular is intended to transport liquid hydrogen or oxygen and was built by an Australian manufacturer, Omni, with the help of Lockheed Martin and the Australian Advanced Manufacturing Growth Centre.
This one comes in two different types, one that is lined with a fluoropolymer (called a Type IV) and one that is not lined at all, just carbon/epoxy (called a Type V). Both the polymer liner and the carbon/epoxy are excellent at very high pressures and cryogenic temperatures, unlike most metallic pressure vessels.
And of course, where would the space program have gone if it weren’t for composite pressure vessels.
Almost all liquid fueled rockets use composite pressure vessels to store either the liquid oxygen and hydrogen or hydrazine fuel. These pressure vessels are extremely tough and can withstand not only launch, but also re-entry into the atmosphere.
The pic to the left is a recovered hydrogen tank from a Falcon 9 booster. And the pic above is of a group of composite cryogenic pressure vessels made by military vehicle company Mega Engineering, headquartered in Huntington Beach. The vessels in this picture are intended to be installed in either aircraft or spacecraft – and it appears that the large one is about the same size as the liquid propellant tank from the second stage of the Falcon 9 re-entry that is in the upper right pic. Amazingly enough, that SpaceX pressure vessel is from the second stage of a Falcon 9 launch in late March of 2021 that came apart over Oregon and Washington. What you see strapped to that pallet was found in a farmer’s field in southwest Grant County in Washington State.
Like I said, these things are tough and made to last a very long time with extremely rough service. That thing survived re-entry into the atmosphere and also the impact when it hit the ground.
In the next post in this series I’m going to talk about how these things get made, what sorts of machines you need to make one, and something about the business of filament winding of composite pressure vessels, lined or unlined. The ones without a liner are a bit more difficult to build, but they are extremely light and strong.
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