How to build a rocket engine

How to build a rocket engine

Martin Jud
Zurich, on 06.05.2021
Translation: Patrik Stainbrook

They aren’t part of NASA or ESA, and yet they’re about to fly high at supersonic speeds. In nine seconds, the Swiss Academic Spaceflight Initiative rocket will climb to 30,000 feet. For this to succeed, it needs a powerful engine.

If you’re looking to soar, you need ambition. Thankfully, the Academic Space Initiative Switzerland (ARIS) has it in spades. At ARIS, more than 150 students from ETH Zurich, HSLU, ZHAW, OST and the University of Zurich research, develop and build space-related hardware every year. Each year, a new rocket is built to compete against models from other colleges and universities around the world.

At last year’s Spaceport America Cup 2019, the world’s largest engineering competition for rocket construction, the Swiss HEIDI rocket had already found huge success: she finished second in her category.

HEIDI’s journey at the Spaceport America Cup 2019 earned her a top 2 finish.

The event was unfortunately cancelled in 2020 due to Corona, while 2021 will only feature virtual flights. At least there was a new competition in Portugal, celebrating its debut last year – the European Rocketry Challenge.

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Winning competitions is a goal at ARIS, though not the primary one: an object is expected to be placed in orbit by the end of the decade. This could be either a proprietary rocket, or a satellite launched by a larger space company. But competitions are important, as talented young builders don’t just get to show what their tech is capable of, but gain experience as well. This is only possible to a limited extent in Switzerland.

Anyone who wants to launch a rocket in this country isn’t allowed to fly high due to the (usually) heavily congested airspace and current legal situation. Therefore, flights completed in Switzerland in the past were only small hops followed by a test of the recovery system – the parachute.

The 2019 HEIDI test flight in the canton of Jura.

Here’s another shot of last year’s EULER rocket during a drop test in our beautiful country. Unfortunately, the launch at the autumn 2020 Portuguese Challenge had to be aborted due to complications with the flight computer. Nevertheless, the ARIS team won a Technical Excellence Award.

Drop test for the EULER 2020 rocket in the canton of Lucerne.

The ARIS team has been aware of the fact that it needed to build its own engine since the explosion of TELL back in June 2018. That one managed to stay intact for about one and a half seconds.

Slow-motion video of 2018’s TELL rocket exploding.

Not that homemade engines are immune to exploding, but success or failure would be entirely dependent on oneself.

For the 2019 HEIDI flight in the New Mexico desert, as well as for EULER 2020, engines were bought in, as none had been available before. In the meantime, a lot has happened; in 2019, part of the test infrastructure and a first small prototype engine RHEA were developed and then tested in December 2019. In 2020, a first flight-scale Hybrid Rocket Engine was developed, built and successfully tested. A second version is currently being developed, which is to compete in a rocket for the first time in Portugal in autumn.

But how is a rocket engine built in the first place?

A hybrid sorbitol engine, candle wax and nitrous oxide

To build your own Rocket Engine, you first need the right minds to implement it.

Developing the first flight-scale ARIS rocket engine with IRIDE in 2020.
Developing the first flight-scale ARIS rocket engine with IRIDE in 2020.

Eight people are responsible for the first version of the IRIDE engine. It will be revised this year as part of Project DAEDALUS by six others and integrated into a rocket for the first time.

They take the first version of the engine, improve it and integrate it into the rocket.
They take the first version of the engine, improve it and integrate it into the rocket.

Here you can see Team IRIDE in full.
Here’s Team DAEDALUS.

However, the six new team members aren’t entirely on their own. To ensure a perfect knowledge transfer of the previous year’s projects in all details, coaches and ARIS alumni are on hand. One of them is Shady Elshater. He can be seen in the top left of the first picture and has already worked on the RHEA and IRIDE projects as Project Manager and System Engineer. Furthermore, he’s responsible for the explanations that will now follow. Seems like press work suits him.

In addition to quick thinkers, ambitious goals are needed:

  • A hybrid rocket engine, which functions reliably and is modular in design, is being developed.
  • The thrust is said to have a peak power of 5000 newtons and should be able to take rockets to altitudes of up to 10,000 metres.
  • The burning time should be ten seconds. The ignition phase before that adds up to four seconds.
  • Furthermore, a lightweight design is to be used and thrust throttling integrated.

Five kilonewtons of thrust is roughly equivalent to the weight force of half a ton – that’s how much thrust is needed to keep it in the air. Or to accelerate a rocket weighing 81 kilograms to over 8.5 g. It’s intended to deliver four kilogram payloads with a standard size of three cubesats up to 10,000 metres. More precisely, this year a 6.34-meter-long, 81 kilogram rocket called PICCARD with a diameter of 17.9 centimetres should reach its target at an altitude of 30,000 feet or 9144 metres with a maximum speed of 1.05 Mach in nine seconds.

PICCARD measures more than six metres and is expected to fly at supersonic speeds.
PICCARD measures more than six metres and is expected to fly at supersonic speeds.
Image: ARIS

Mach 1 corresponds to the simple local speed of sound. That’s 343 metres per second or 1235 kilometres per hour at a temperature of 20 degrees Celsius, dry humidity and an ambient pressure of 1 atm (physical atmosphere at 0 metres above sea level). The targeted top speed of 1.05 Mach corresponds to 1296.54 km/h. However, flying faster than sound is already possible at a lower speed, since the speed of sound is relatively strongly dependent on temperature. For example, if a flying object is to reach the sound barrier at an altitude of 10 kilometres, it must travel only 300 metres per second at the prevailing ambient temperature of -50 °C.

Hybrid rocket engines use solid propellant combined with a liquid oxidiser.
Hybrid rocket engines use solid propellant combined with a liquid oxidiser.
Image: ARIS

This will all be powered by a hybrid rocket engine, meaning that solid propellant will be combined with a liquid oxidiser. ARIS uses nitrous oxide, better known as laughing gas, as the oxidant. It’s stored in a pressurised tank as shown in the illustration above. Opening a pneumatic valve directs the gas into an injection nozzle, which places it into the engine casing in an atomised form similar to a shower head. Within the housing, the gas is ignited and forms a fire vortex, which allows the solid propellant component to burn evenly and, together with it, provides the required thrust through the nozzle.

ARIS uses a combination of candle wax (kerosene) and sorbitol as a solid propellant. Sorbitol is used by bakers for sweetening or by doctors as a laxative for enemas. Things which can also be bought or imported in Portugal or the USA.

A water-cooled rocket nozzle in a cargo container

Moving from theory to practice requires good plans that build on existing concepts. Furthermore, sponsors are essential. For example, to manufacture individual parts (based on CAD drawings) or to provide a safe test site. Meaning that construction, which takes place at ETH Zurich, all has to be done on a mobile platform. Which is why the team decided on a test bench in a freight container.

This shot of Team IRIDE in an open container could just as easily have come from a science fiction movie.
This shot of Team IRIDE in an open container could just as easily have come from a science fiction movie.
Image: ARIS
The three-part container holds the test bench, gas tanks and electronics.
The three-part container holds the test bench, gas tanks and electronics.
Image: ARIS

The cargo container has three separate rooms. In the first, a test bench is set up onto which the engine is mounted so that the nozzle can fire from the container.

The first room contains the test bench. It also provides the nozzle with water cooling and access to the oxidiser, whose tank is located in the second room.
The first room contains the test bench. It also provides the nozzle with water cooling and access to the oxidiser, whose tank is located in the second room.
Image: ARIS

When the engine is eventually installed in a rocket, it will receive a handmade nozzle made of carbon fibre with a graphite insert. However, this piece won’t be used for multiple flights due to excessive temperatures. To keep nozzle wear low during testing, a copper nozzle with water cooling is used on the test bench instead.

The test bench in detail.
The test bench in detail.
Image: ARIS
The engine in detail.
The engine in detail.
Image: ARIS

The engine is mounted on the test bench on the left and fed with the oxidiser from the right. When the main valve opens, a sensor monitors the mass flow of the oxidiser. Furthermore, a thrust measurement system is installed to ensure that correct data is obtained after the engine has been fired. Just this first room already contains more than 15 sensors that measure every single parameter of the engine and the flowing oxidiser.

The second room provides space for gas tanks.
The second room provides space for gas tanks.
Image: ARIS

The second, middle room provides space for the oxidiser tank, which can be seen on the far left. Next to it on the outer wall are three nitrogen tanks. The first among them assists the pressurisation system. The second leads to the pneumatic system, which is responsible for opening and closing the valves. The third nitrogen tank can be used to flush the engine.

To the right of the three nitrogen tanks, an additional safety wall is installed during operation, behind which there are two more tanks containing nitrous oxide. These are used to fill the oxidiser tank, which is directly connected to the engine.

The third container room houses measurement modules, safety relays as well as the power supply system, surveillance camera system and many cables – all the electronic hardware is located in this area. This is where the entire setup is controlled. As well as all test data is collected.

Image: ARIS
Image: ARIS

Over two kilometres of cable are installed throughout the structure.

Image: ARIS
Image: ARIS

And a special gadget that, unfortunately, only one person at a time is allowed to operate later outside the container at a safe distance.

5 kilonewtons, sure: Blast off!

The launch button as seen above had been ready for a while back in late summer 2020 when after a year of planning, manufacturing and assembly, everything was finally ready for the first tests. Components that couldn’t be manufactured by the company itself were realised with the help of sponsors.

Cooling water flows through the grooves of this special nozzle after it has been set up on the test bench.
Cooling water flows through the grooves of this special nozzle after it has been set up on the test bench.
Image: ARIS

The special metal nozzle attached to the water cooling system was produced externally. In contrast, the original carbon nozzles were handmade on site.

Fuel production is just as homemade: kerosene and sorbitol, with a little aluminium powder for extra kick, are carefully heated in the right proportions and poured into a tube of phenolic resin. Before all being spun around like a drum.

The engine is first assembled under a bright sun. A good omen.
The engine is first assembled under a bright sun. A good omen.
Image: ARIS
The modular design has many advantages. If something breaks, it can be replaced without complications.
The modular design has many advantages. If something breaks, it can be replaced without complications.
Image: ARIS

First assembly of the engine occurred on a sunny July day at Zurich Hönggerberg, cheering up Team IRIDE as well as the sponsors and professors. Everything seemed to fit. All systems were ready for ignition, requiring the container to be moved.

Somewhere in the middle of nowhere: a soon-to-be launchpad.
Somewhere in the middle of nowhere: a soon-to-be launchpad.
Image: ARIS

By truck, the entire test setup was shipped across 60 kilometres. All heading for Ochsenboden, in the canton of Schwyz. Rheinmetall has a test centre there. Any rocket engine ignition can only be performed in very remote locations.

The engine is ready to be tested.
The engine is ready to be tested.
Image: ARIS

Preparation usually takes around a few hours per ignition. Everybody has a fixed task – just like during an actual launch. Thanks to over a year of collaboration and knowledge regarding every last system detail, everything ran smoothly.

It just works.
It just works.
Image: ARIS
Every team member is absolutely focused.
Every team member is absolutely focused.
Image: ARIS

The engine is filled with solid fuel for the first time, fixed on the test bench and connected to the tank. The impressive water cooling is a real eye-catcher.

Image: ARIS
Image: ARIS

After one final tank check, the entire team disappeared into a bunker. During ignition, a safe distance from the container must be maintained. The bunker also contains an emergency stop switch and five monitors: they display sensor data, the control panel for opening and closing all valves and surveillance videos.

Image: ARIS
Image: ARIS

One last look through the bunker windows, and there they are: the firing flag and the red LED bar in the container. The system was armed.

Image: ARIS
Image: ARIS

Butterflies. In but a moment, a powerful spark would be lit. Hopefully. The anticipation was breathtaking, and the tension in those few moments before ignition was almost unbearable. In a moment, everything that had been accumulated for more than a year would be released at once. Not just in the engine, but in each of the eight team members.

Would IRIDE provide enough thrust? Would the rocket engine achieve the desired 5 kilonewtons with a burn time of 10 seconds?

Find out in this bombastic 4K video. Goosebumps, guaranteed:

What a brilliant jet of flame. And what emotions.

As the evaluations assembled after 19 ignitions with a total burning time of 89 seconds showed: the target wasn’t just achieved, but exceeded by quite a bit. A total of twelve successful tests were performed, the longest of which lasted 16 seconds. The 5 kilonewtons set during the firings top out the engine with a peak force of up to 7994 newtons. 60 per cent more power than anticipated.

In the meantime, the new DAEDALUS project has also tested the enhanced engine a dozen times with great success. This should put PICCARD in a solid position for October’s event in Portugal. I wonder if it’ll manage to break the sound barrier.

Will Switzerland become an independent space nation by the end of the decade thanks to ARIS?

Stay tuned!

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I find my muse in everything. When I don’t, I draw inspiration from daydreaming. After all, if you dream, you don’t sleep through life.


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