I’ve spent the last few days reading about tanks. Not the armoured kind. The kind that hold liquid at -250C something degrees without leaking, cracking, or slowly boiling away into nothing. I did not expect to find this interesting. So, almost every rocket that has ever left the ground runs on fuel that is, at the moment it is loaded, colder than anywhere on the surface of the Earth has ever naturally been. We are not talking about a home freezer. We are talking about liquid hydrogen at -253C. Below roughly -150C, you are in a different category of physics altogether. It has a name. Cryogenics.

What is actually going on at that temperature

At that kind of cold, gas stops behaving like gas and turns into liquid. That is the entire point. A gas takes up an enormous amount of space. Squeeze it cold enough and it collapses into a liquid that takes up a fraction of the volume. That is why rockets carry liquid oxygen instead of gaseous oxygen. That is why hospitals store liquid helium instead of gaseous helium for MRI magnets. Compact storage is not a nice to have here, it is the whole business case.

But cold enough to liquefy a gas is also cold enough to make ordinary engineering fall apart. Three problems show up immediately. First, storage. You need a container that can hold something at -200C without leaking or cracking, often while also holding serious pressure. Second, insulation. Heat wants to leak in constantly, and the moment it does, your liquid starts turning back into gas. This is called boil off, and it is quietly the most expensive line item in this entire industry. Third, handling. Moving cryogenic liquid through valves and pipes without either the liquid warming up or the equipment failing is its own discipline.

None of this is new. Rockets have run on this since the sixties. What is new is who is trying to make the container itself lighter.

The container problem

Traditional cryogenic tanks are metal, steel or aluminium, because metal does not care about extreme cold the way other materials do. It holds its shape, it holds pressure, it does the job. The problem is weight. Metal tanks are 5-10x heavier than a composite tank built to hold the same volume. On a rocket, or a drone, or anything that has to leave the ground, every Kg matters.

So the obvious fix is to build the tank out of carbon fibre composite instead. Same strength, 6-10x lighter. This is not a new idea either. It has existed since the eighties for exactly one purpose, holding pressurised gas at room temperature. CNG cylinders, hydrogen fuel tanks in cars, breathing apparatus for firefighters, all built this way, all extremely mature, all shipping at real industrial volume today.

Here is the part that took me longer to understand than it should have. A composite tank built to hold pressurised gas at room temperature, and a composite tank built to hold liquefied gas at -253C sound like the same product. They are not. They are solving almost entirely different problems, and one of them is dramatically harder than the other.

The room temperature version only has to fight pressure. Wind the carbon fibre right, pick a liner, and you are containing force in one direction. Decades of precedent, mature manufacturing, shipping today at industrial scale.

The cryogenic version has to fight pressure and temperature at the same time, and temperature is the one that breaks things. Cool a composite down that far and the resin and the carbon fibre inside it shrink at different rates. Do that once and you might get away with it. Do it on every fill and drain cycle, which is exactly what a working tank has to survive, and the resin starts developing microscopic cracks. Those cracks are wide enough for hydrogen to slowly leak through the tank wall. A tank can pass a pressure test on day one and still fail after a handful of cryogenic cycles. This is not a solved problem anywhere in the world. NASA and Boeing have been working on large scale composite cryogenic tanks since 1987, with real funding behind it, and it is still not commercially deployed at scale.

Classification

Anyone who has seen a pitch deck in this space has run into the phrase Type 3, Type 4, Type 5. It is a universal industry classification for how a pressure vessel is built, and everyone in this space uses the same five categories.

Type 1 is all metal, steel or aluminium, the oldest and cheapest, and still by far the largest category by volume today.

Type 2 is metal with a partial composite wrap.

Type 3 is a metal liner fully wrapped in carbon fibre, the metal holds the shape and the composite carries the structural load, mature and well understood.

Type 4 swaps the metal liner for a polymer one, lighter again, and this is the one doing most of the growing right now, mostly for compressed hydrogen in vehicles.

Type 5 has no liner at all, the composite is the entire structure and the seal, the hardest version to engineer, and still not commercialised at any real scale anywhere outside of space.

Adoption of anything past Type 1 is purely a weight versus cost trade. There is no regulation pushing anyone off metal. It only happens where the weight actually matters, vehicles, aircraft, rockets. Everywhere else, metal wins forever, quietly, without anyone noticing.

Who is actually building this

This is an interesting case, because once you separate the two problems, the compressed gas tank and the cryogenic liquid tank, the list of people who can actually do the second one gets very short very fast.

In India, Time Technoplast is the volume player. They claim to be the world’s second largest composite cylinder maker, and they have PESO approval across LPG, CNG, and now compressed hydrogen. But everything they build is the room temperature version. Gas kept as gas, under pressure. No insulation, no boil off management, because none of that is needed for what they make.

INOX India is the other Indian name, and this one is actually cryogenic. They built cryogenic systems for ISRO’s own launch pad, they supply ITER, and they are picking up serial aerospace orders from private space companies abroad. But their tanks are mostly metal, not composite. They have solved the cold problem. They have not solved the weight problem.

Which is what makes the space genuinely open. Nobody has cleanly combined both, a composite wall light enough to matter, holding a true cryogenic liquid, at any real commercial scale, anywhere in the world. The closest anyone has come is a German company called Cryomotive, which has built a cryo compressed hydrogen tank.

And then there is a small Hyderabad team, Onnes Cryogenics trying to build this.

The problem is real, the physics is unforgiving, and almost nobody on the planet has fully cracked it. Combination of a real unsolved problem with a short list of people attempting it, kind of where the interesting money either gets made or gets lost. I have not decided which one this is yet.