The problem with fuelling rockets

The last few years have gotten us in touch with new, radical green groups that seem to seek mankind’s return to a pre-industrial state. Sometimes one would be forgiven to assume that some of those radicals want to expel humans from Earth. 

If only humans disappeared, Earth could heal. As if nature was a big intelligent sentient being that actively keeps things in balance. A bit like Ewa from the Avatar movie. Everything is connected and kept in balance. Another metaphor for a perfect hive-mind-like dictatorship. History is full of those that wanted to get rid of the unwanted. Continents like the Americas and Australia have been colonized and built on the backs of those unwanted in older countries.

All through history, the unwanted sought new frontiers establishing new societies. And the ultimate frontier is space. 

Hence, I do agree with a part of the doomsayer’s message. Humanity must leave this planet and thrive in space.

Far fetched and foolish? Remember, the greatest tales of humans going where no one had gone before all started with simple ambition. Christopher Colombus appeared no less foolish to his contemporaries than some of the space visionaries may appear to us today. Finding a new, faster way to India by going west. Yeah right. 

Last Saturday, humanity made a big step to the stars, and judging by what we have seen from Elon Musk and Jeff Bezos, there is way more in the pipeline. Discovery has become thrilling again.

A SpaceX rocket topped with a manned crew module and a two-person cargo launched into space to RDV with the International Space Station. Now you will say that this is nothing special as humans have gone to space for more than 50 years by now. This only matters to Americans as their capability to conduct human spaceflight without any non-US assistance is restored.

Still, something larger is at play here. And it goes way beyond the circumstance that the rocket and the spaceship are built, owned and operated by a private company. This is an important stepping stone on the way to the Moon and then to the planets. And thats what matters. A step that we expected to happen decades ago. It just seems we were not ready then.

But now, space has become a business. And a business needs reliability, repeatability, and also profitability. Throwing vast sums of money at a project for some show-off moment and/or virtue signaling is simply not getting us where we need to get. 

And the first thing that springs to mind is the incredible waste spacefaring was before the advent of SpaceX. Every time a rocket was launched, the entire thing was waste after the event. Zero reusability. The Space Shuttle tried to remedy this cost problem but only made it worse. Every single refurbishment if the Shuttle costs more than an entire conventional rocket built from the scratch. Things got more expensive rather than more affordable. 

Thats why SpaceX but also their competitor Blue Origin have gone all the way to reuse rockets. Is there anyone who has not seen one of those Falcon first stages land vertically on the drone ship? I remember comics from the 70s and 80s. They had plenty of comic rockets that lifted off and landed vertically. But even my childish brain understood at the time that this was bogus. Rockets lift off vertically, and then they are gone.

Or so we thought until the Falcon showed us otherwise. Jawdropping …

Yet, it’s not like those stages can be reused immediately after refueling, stocking fresh meals, and passing the broom. They are not destroyed anymore as this used to be the case with rockets so far. Still, every single use case requires a pretty overhaul of the entire booster. 

And the biggest work item when doing so is the rocket engine. Just take a look at them when the boosters land. The lower halves of the booster are black from the soot the engines create. And the engines have hard carbon sediments that need maintenance. 

Reusing those rockets is not a simple turnaround like you do it with an airplane. Refurbishing a Falcon is still a major piece of work. With the requisite delay and cost attached to that. 

When rockets were throw-away-items it did not matter much that one single launch literally cluttered the critical parts up. If you want to reuse the rocket, things are different. Whatever the fuel does to the engine during launch affects the number of times you can reuse the engines. It also affects how long turnover takes and what it costs to get them boosters back into the race. 

To understand the conundrum better, we need to take a little excursion into rocket propulsion and fuels. I will focus on liquid fuels here as we need variable thrust and that’s hard to pull off with solid fuel thrusters.

A rocket is basically nothing but a tube filled with stuff that is thrown out at one end to push the rocket itself away in the opposing direction. The principle is well demonstrated with a waterjet surfer. A person stands on a platform that spews out water under high pressure to lift a platform and with it the surfer. 

A spacefaring rocket produces a little more lift than needed for one single person and also has farther to go than just a few meters. And there is no tether long enough to deliver enough water.

The solution is a buildup of pressure by rapid combustion of something highly flammable. Fuel is fed into a combustion chamber where it combusts and then throws out the exhaust gases at the nozzle creating the push needed.

As rockets are not air-breathing engines, they have to bring the oxidizer for the reaction with them. That will be liquid oxygen (LOX) in most cases.

Both – fuel and LOX – have to be pumped at very high pressure into the reaction chamber where they combust. This requires very powerful pumps. And to get those very high pressures reliably, a part of the fuel is used in the pre-burner that makes the pumps turn.

This is a very high-performance machine and as such it does not have a lot of tolerance for crap messing with the process. The crap I talk about is residue from the combustion process building up and clogging equipment. 

Many different fuels have been used and even more tried out but today, 2 fuels form the backbone of liquid-fuel rocketry today. RP-1 is an especially refined form of Jet Fuel. Which is itself an especially refined form of Kerosene. This makes a middle distillate occupy one corner. And liquid hydrogen called LH-2 on the other side.

Each of them has distinct advantages and drawbacks. RP-1 is commonly used for lower stages as it provides great energy density which allows for slim but powerful first stages. LH-2 engines burn clean and are very efficient. Generally, you want to use RP-1 when you are close to the surface and slowly push a lot of air out of the way working your way up. And you want to use LH-2 when there is not much or no air and speed is very high.

RP-1 is rather easy to produce and store and also comparatively cheap. But it also is a middle distillate and those fuels are cloggy and do not perform well in space. And its also a very dirty fuel leaving behind a trail of soot and oxides. LH-2 is very hard to produce and store and hence very expensive but it performs well in space and does not have the soot and clogging problem.

And thats part of why going to space is so expensive. Lots of different equipment for different phases of the journey and expensive, dangerous fuels to feed on. And it all needs a lot of maintenance work after every launch. If one single-engine using one single fuel would burn cleanly it would make things a lot easier – and cheaper. 

But more on that next week.

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