Engine cooling – why rocket engines don’t melt

The fuel rail in a modern diesel engine is operating at 25-30,000 psi all the time.

People don't grasp what an unbelievably complex engineering problem that is. It's at least an addition of difficulty at the same magnitude as building a steady-state 63000 hp turbine pump in the first place.

Mechanically implementing it in the inherently steady-state design rocket of most rocket cycles. Having variable controls able to work at those pressures. Testing structural dynamics for a range of harmonic conditions instead of one. And do all of that with materials that need to tolerate temperatures going from cryogenic to white hot, without allowing thermal expansion to affect the mechanical tolerances of parts running at thousands of RPMs. And now you have varying flow rates and negative pressures in the lines coming from the external tanks, so have to design such that cryogenic liquids (that normally would require immense positive pressure to keep liquid) don't spontaneously boil or cavitate or cause a shock-like wave (think water hammer turning off your bath faucet) under changing negative pressures.

It's really difficult even for seasoned engineers to grasp the scale of difficulty involved.

A .22 LR rifle bullet might acquire 200 J in 2 ms, which means the firing gun is producing 100 kW mechanical, plus probably another 300 kW thermal. So another way of thinking of this is that an engine dissipating 10 GW is equivalent to something like 25000 handguns firing at once, without ever stopping.

That's not the power output of the rocket (which someone quoted below). That's the power required to move the fuel and oxidizer into the rocket engines. Pumping the same amount of liquid can move a 100,000 ton floating city across the ocean at 40mph.

edit: if you watch gas generator tests on youtube the gas coming out is dark because it's very fuel rich which keeps it cool (in a relative sense)

A jet engine can't have such power density because it uses gaseous air which is about 1000 times less dense than liquid oxygen and only contains 20% oxygen.

Everything follows from that.

The pumps are not challenging temperature wise since they pump cold liquids.

The turbine is challenging, but the temperature can be limited by varying the ratio of propellants in the preburner (very lean or very rich means lower temperature). If you use lean, then it's a very oxidizing environment. If you go very rich, there's soot (if you use fuels with carbon).

And the chamber is not so challenging because there is so much cool liquid available for cooling.

You can boil water with a candle and a paper cup.

A high performance jet engine is a harder problem than a medium performance rocket engine.

Considering all the manufacturing, precision manufacturing involved the V2 effort, it's astounding they could do it at all.

The most similar technology is a boiler feed pump (used to feed the boilers in a steam turbine system, e.g. in a ship or power station) - these work in a similar way, high pressure steam is bled off from the system to drive a small turbine, which drives a high pressure pump to feed the boiler, which feeds the 'main' steam turbine. Similar pressure (>100 bar), but the turbine on the rocket engine needs to handle much higher temperatures (hot gas rather than steam).

Not quite literally, in e.g. Soviet engine designs some gas generators are oxygen-rich, they produce oxidized exhaust of high temperature, and the engine still doesn't burn (usually).

> So, the turbopump has to outperform the combustion chamber in terms of pressure and flow rate.

Yes, the injector pressure drop can't be too small.

He still loves SpaceX, and to be fair, who doesn't. You may not like Elon Musk, his fanboys, the outrageous claims, and the way the company is run, but most of the exciting news in rocketry for the last decade are about SpaceX. But for the last few years, it is clear that he makes some efforts to be impartial and focus on the technical aspects.

And you should watch the videos he made with Elon Musk when he visited SpaceX. I expected little more than an ad for SpaceX, and it turned out surprisingly technical and hype-free.

I watched the video version of the article linked here this morning, and don't recall any part that was fawning over anyone or anything in particular.

What is it about his presentation that puts you off?

I would be really interested to know why "caring too much" is a disqualification.

Reminds me of the launches I have seen where I really wanted to soak in the thumping sound of the rocket engines, but the people nearby are just screaming at the top of their lungs… "America!!!" as if that's helping anything at all.

He's also friends with a previously Tesla-focused YouTuber (name redacted, but rhymes with "Ken Mullins") who imho sold out to affiliate and sponsorship deals to the point where I lost all respect for him (the friend) and can no longer trust a word he says. So I have a suspicious eye on Tim's fire hose of cash harvesting mechanisms, but he seems to be staying legit so far. And he does have fantastic content. Also I'm not saying he doesn't deserve the cash he's earning; he puts it to good use and has a growing staff, as I understand it.

For Pre-Launch Previews they are written first by a team member.

The goal is to bring the subject down to a level where everyday people can still follow. It's not meant to be a college course, so of course there will be some dumbing down.

OTOH at the point that you're EDM-drilling thousands of micron-scale holes in your combustion chamber for film cooling, you may start to need practical experience with different things.

I while ago I read--but barely understood--a book that went into a lot of this: "Ignition! An Informal History of Liquid Rocket Propellants" by John D. Clark.

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IIRC there were some cases where a fuel was not militarily-acceptable because you would need to warm/thaw the mobile missile prior to firing in a Russian winter, or other cases where a permanent missile-silo meant it was cost-effective to run heating/refrigeration all the time, etc.

> [I]n applications which do not require a low freezing point, hydrazine itself is fused, either straight or mixed with one of its derivatives. The fuel of the Titan II ICBM doesn't have to have a low freezing point, since Titan II lives in a steam-heated hole in the ground, but it does need the highest possible performance, and hydrazine was the first candidate for the job.

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Another fuel-choice issue involves how badly it might self-destruct if anything unusual happened:

> [I]n the summer of 1960, we tried to fire a 10,000-pound thrust Cavea B motor. [...] Well, through a combination of this and that, the motor blew on startup. We never discovered whether or not the [detonation] traps worked—we couldn't find enough fragments to find out.

> The fragments from the injector just short-circuited the traps, smashed into the tank, and set off the 200 pounds of propellant in that. (Each pound of propellant had more available energy than two pounds of TNT.) I never saw such a mess. The walls of the test cell—two feet of concrete—went out, and the roof came in. The motor itself—a heavy, workhorse job of solid copper— went about 600 feet down range. And a six-foot square of armor plate sailed into the woods, cutting off a few trees at the root, smashing a granite boulder, bouncing into the air and slicing off a few treetops, and finally coming to rest some 1400 feet from where it started. The woods looked as though a stampeding herd of wild elephants had been through.

> As may be imagined, this incident tended to give monopropellants something of a bad name. Even if you could fire them safely—and we soon saw what had gone wrong with the ignition process—how could you use them in the field?

> Here you have a rocket set up on the launching stand, under battlefield conditions; and what happens if it gets hit by a piece of shrapnel? LRPL came up with the answer to that. You keep your monoprop in the missile in two compartments: one full of fuel-rich propellant made up to A. = 2.2 or 2.4, and the other containing enough acid to dilute it to X = 1.2. Just before you fire, a can-opener arrangement inside the missile slits open the barrier separating the two liquids, you allow a few seconds for them to mix, and then push the button.

Looking back at history, maybe what goes on top actually do keep the peace in the world, so maybe we could safely go back to building solids at scale for peaceful space transportation. But not likely without an American nod, I would imagine.

The V2 had other foundational inventions - the ballistic missile shape, and the gyroscopic guidance system.