Translated by Daen de Leon and Jørgen Skyt.
(Blog 1 of 2, how to start a bi-propellant rocket engine, by Jørgen Skyt)
I believe every healthy child, at one time or another, has been fascinated by fire.
I certainly have.
I will never forget my excitement when my older brother poured gasoline in a maze built into an earth bank in the backyard and set it on fire, the flames running up and down and in and around the maze. It was so cool!
Our mother did not share our enthusiasm.
But I think the straw that broke the camel’s back was when I discovered how exciting zinc-sulfur mixtures could be. They made the most amazing mushroom clouds! I had no idea that it was a commonly used rocket fuel. One day I discovered what happens when this reaction is enclosed in a matchbox. It was only made of cardboard, so how could that be a problem!
While the doctor phoned round the city’s shops, my mother informed my school about the incident. I will never forget my physics teacher’s sarcastic: “Sooo, Jørgen … have you learned something?” when, one week later, I turned up for lessons with scabs on my face, one eyelid still shut – and with slightly shorter hair than the flowing style I used to have. The city’s businesses had in the meantime, as if by magic, completely sold out of sulfur and zinc powder!
It’s really a bit sad when young people are not allowed to play with fire under controlled conditions. I myself have done my best to teach my own children about fire and how to handle black powder and the likes. Apart from a few cases of spontaneous anxiety outbreaks, there has to date only been material damage.
Also in an educational context, I have with some success used fire as the catalyzing factor that can get students to listen. For example, when I allowed young remedial students to juggle burning snowballs, soaked in butane, and launching fireballs with a trebuchet one dark night.
Is it dangerous? Well, of course! Otherwise it’s not interesting for young men who want to conquer the forces of the magic of our world! If you don’t try it with “grown-up permission”, under controlled conditions with adequate supervision, how wrong will it go when you do it covert, without any guidance and with no clue of what you are doing?
Is it there any lesson in it? Well, of course! You can’t make snowballs out of powdery snow. Case closed! But if you spray a small bucket of powdery snow with fluid lighter gas, it turns into a somewhat heavier snow-porridge, mixed with butane (which boils at -0.5 degrees Celsius) and propane (which boils at -43 degrees Celsius). Propane evaporates instantly into the atmosphere (“please do not smoke!”), while butane turns the snow into a sort of porridge, which behaves like melting snow, at a constant temperature of -0.5 degrees Celcius, until all the butane is gone. This snow can burn, but will never get unbearably hot to the touch. So you can go out on the frosty lawn and juggle burning snowballs from hand to hand and be surprised that you don’t burn yourself. Isn’t this lesson?
If we look at the entertaining launches with burning trebuchet ammunition, I think there’s real evidence of learning. In the video below, the kids find it hard to understand why I put out the fire with a fire blanket, they’ve been trying so hard to light up in the first place. In Danish, you can hear that those who have figured out the reason for putting out the fire (to heat up the oil so the fire can easily be reignited and survive the powerful launch) are passing this on to the others. They have completely got the point! They’ve got a hint of an insight that may be important in their future lives. Now, that is ”learning”! And this was an evening where we were just supposed to enjoy ourselves.
There is something mythical about being able to control an inferno of heat and destructive forces. Fire can be not only a mortal enemy, but a trustworthy friend as well. Being able to control fire is the main difference between humans and animals.
Occasionally, the fire can send us on a magic journey into our soul. Fire is associated with dreams, with great victories – and with love!
Danish composer Otto Brandenburg wrote a touching song called “Two Candles on a Table”. And who can forget Johnny Cash’s “Ring of Fire”?
We are connected to fire – that’s what makes us human! Animals are usually terrified of it. But what about women? Yes, of course! If fire is presented in the right context, tears will break the boundary between fear and romance! 🙂
However, I had a bit of bad luck a few years back, lighting a fire on some patio tiles for some young guests from Greenland, who were visiting a female friend of mine. The boys were seriously bored with adult company, so I taught them how to make fire. Right there on my friend’s patio tiles. They were excited and haven’t forgotten it since, so I’ve heard – but my friend found it a bit unfortunate that I had chosen her patio tiles for the intertainment. Like the boys, she’s never forgot me for this incident!
There obviously also is something called “due diligence” regarding the use of fire on the premises of a female friend. This is how I learned that ”fire has to be presented with due sensitivity”.
Fire doesn’t happen on its own. It can demand hard work, great insight into materials, and can be a technological breakthrough. Or also just pure dumb luck, such as in the early Pleistocene, when man first learned to handle fire. In the beginning fire was probably caused by random acts of nature. For example, when lightning stroke a tree.
I thoroughly recommend that readers watch – or watch again – the entertaining and wordless “Quest for Fire” that can be found on several legal streaming services. A movie that revolves around the art of making fire in the Pleistocene.
The fire triangle describes what’s needed for fire to happen. There must be something combustible, something hot and something oxidizing present for fire to evolve and persist.
Actually, there’s also a fourth factor that is often forgotten, of which most scouts knows all about:
The heat produced by the fire must exceed the heat loss through radiation and the heat lost to the supply of fuel and oxidizer!
A good example is how easy it is to put out a small fire by blowing on it: the cooling effect of the large volume of air exceeds the heat produced through combustion – and so the fire goes out. Who hasn’t tried to turn on a gas burner with a lighter, only to watch the lighter flame get blown out?
Do Not Do This At Home!
I’ve personally taken part in an experiment where we put out a fire by adding large amounts of gasoline, aerosolized with pressurized air. There’s both fuel and oxidizer in a good mix, but in such large quantities, and introduced so quickly to the fire, that the heat of evaporation simply killed the fire stone dead.
So you can’t just turn on a rocket engine with a single spark or a burning match shoved up the rocket engine: when you open up the taps and several kilograms of atomized liquid oxygen and alcohol blowns out through the injector, the temperature drops dramatically.
In the summer of 2015, we had many test runs of BPM-5, including one where an igniter failed to light. There we had the opportunity to see this happening in reality.
It’s seriously chilly in the chamber when LOX is vaporized and boils at -183 degrees. The result is a dense cloud of ethanol snow in a soup of liquid oxygen, which ends up splashing out through the nozzle, or in short: “Welcome to picnic on Enceladus”!
You really need to pump some Joules into that cloud before combustion is self-sustained!
Ignition of a BPM-5 rocket motor happens in several closely coordinated steps:
- We start with firing up the igniter (a pyrotechnic device that makes a fountain of magnesium sparks which is spread out in the combustion chamber, just below the injector).
- Once we’ve confirmed that the igniter is working as it should, we go to the “pre-stage” when a modest fraction of the full flow is fed in. But the “pre-stage” is itself in two parts, starting with “priming” where the cooling jacket is filled with alcohol, just before we open up the LOX valve. Then both valves is opened until we have a steady but modest flow (about 10%) on both alcohol and LOX.
- When the chamber pressure measurement tells us that the fire is self-sustained during “pre-stage” we go to “main-stage” where both valves are opened to deliver full flow.
If the igniter doesn’t provide enough heat, the flow should be reduced so much during “pre-stage” that there’s virtually no cooling of the chamber wall. At such a low flow, we also run the risk that the injector design we have chosen won’t efficiently atomize the LOX and fuel mix as required, as the injector is designed for “main-stage”.
This can provide poor and unstable combustion, which can cause a burn-through of the inner chamber wall if the mix is disproportionately high in oxygen in one area, so we get a “hot spot” and burn-through in fractions of a second. So we have to be ABSOLUTELY sure that we have both initial flow and “oomph” enough before we open the floodgates, with its roughly 10-fold greater flow during “main-stage”.
It would be a real shame if this transition phase caused the fire to get blown out into the expansion nozzle from where the flame front sooner or later would propagate back into the chamber, resulting in a massive and extreme compression and destructive “hard start”. That is what firemen know as “backdraft” – but this is on steroids!
A lesson learned …
On December 21, 1939, the night was cold and clear at Kummersdorf West, when von Braun, Riedel, Dornberger and another technician, Heinrich Grunow went to an outdoor test stand. Three concrete slabs, five meters long and three meters high, formed an open shelter that could be sealed off by means of a pair of metal doors. The roof, made of wooden boards with roofing felt, was removed prior to testing. Inside the test chamber was situated the first rocket engine that von Braun had developed under Dornberger’s management, a roughly 50cm long pear-shaped motor, made of duralumin and designed to provide a thrust force at around 3kN. Pipes and wires led from the test stand to the control room on the other side of a wall where technicians Riedel and Grunow sat in safety during the testing while they sent fuel and oxidizer into the combustion chamber. Meanwhile Dornberger sought safety behind a tree.
As the youngest member of the group, it fell to von Braun to ignite the rocket engine with a can of flaming petrol at the end of a 4 meter long stick, which he introduced to the cloud of liquid oxygen and alcohol that blew out of the nozzle. Immediately, there was a huge BANG, the test stand was engulfed in flames and the explosion ripped the steel doors off their hinges and red-hot pieces of shrapnel flew everywhere. While the flames died down, all that was left was a twisted and chaotic mass of metal and charred cables. Riedel and Grunow emerged from the control room to find both Dornberger and von Braun miraculously unharmed. The reason was the delayed ignition in the expansion nozzle from which the flame front propagated up the fully-filled combustion chamber where all the fuel detonated in a split second, resulting in the devastating explosion. (From “Breaking The Chains Of Gravity” by Amy Shira Teitel)
These “hard starts”, where the engine gets pushed significantly harder than it is designed for during a split second, are not unusual. We have experienced them, since they’re a naturally occurring phenomenon while commissioning a new engine, at least until all the operational parameters are nailed down.
But “hard starts” can be anything from “a bit to loud pop” through “deformation hardening of the chamber” to “devastating explosion”. Regardless of the degree of damage, we try to avoid them altogether, as the “mild” version alone can damage load cells and piping, and weaken couplings and joints.
The really essential thing is to avoid the kind of “hard starts” where the engine is destroyed, or there is a risk that someone may be injured. We try to avoid this by carefully thinking through “due diligence”.
We hope you can understand why we spend a lot of time and effort on the safety aspects of every test. There can always be small errors, overseen details or misunderstandings. That’s why we at Copenhagen Suborbitals constantly develop and maintain a tight organization around all tests, with dedicated tasks and procedures for everything from safe distances and radio communication and risk evaluation to the protecting of pensioners who are taking the poodle for a walk. We do what we can to take all undesired outcomes into account and deal constantly and immediately with any issues that might arise.
Of course, we want the igniter to stay in place until mission control decides that it has played its part. This has been solved so far by mounting the igniter on a long stick that was blown out of the chamber when we reached “main-stage”, where some plastic strips melted through.
It’s a funny little low tech solution that has proven its worth over many years, including on the Russian Soyuz rocket’s 32 engines where there’s an almost identical igniter-stick in each one. As late as March 12, 2016 there was a launch abort because one of the igniters could not be confirmed lit:http://www.russianspaceweb.com/resurs-p3.html
Murphy’s Law: “If something can happen, it sooner or later will!”
We spend a lot of time worrying about the above – and also workshop hours and experiments – to ensure that whatever ”can happen” is something we are in control of!
It is not “more important” than maintaining equilibrium between the heat produced in the chamber and the effectiveness of the coolant flow to maintain the strength of the nozzle and the combustion chamber inner wall. Nor is it “more important” than testing the software controlling the servo motors adjusting the jet-vanes for a straight flight. Nor is it “more important” than making sure that the toilet-facility in HAB works optimally and matches the flush volume to the sewer hose capacity. It’s just IMPORTANT, like everything else in “Rocket Science” is, in all of its mindbogglingly complexity.
Murphy and Sun Tzu in unison says: “You have to choose your battles, so the guaranteed outcome of the victory significantly exceeds the guaranteed loss.”
Everything has a price. Before we make a decision – like design modifications of an injector design – we must be extremely confident that the benefits of the change significantly exceeds the costs and disadvantages arising from the design change. Everything is interconnected. If a design changes anywhere, it inevitably has an impact in many other places.
It is not enough to just think in 3D, but it’s a good start. You also have to think about, for example, how pressure and temperature conditions will change over time. An excellent example is the use of gun-cotton, which proves to be unsuitable as a pressure-generating propellant in, for example, ejecting the parachute. Due to the special way gun-cotton burns, it will not work in the same way in a near-vacuum as under atmospheric pressure at ground level. This is believed to be the primary reason we lost “Sapphire” to the Baltic Sea’s briny deep that day in 2014.
Gun-cotton should be wrapped in a mortar-like structure, so it can work at low pressure, which adds additional complexity and weight to the project. Test with gun-cotton at 1 and at 0.2 atmospheres: https://www.youtube.com/watch?v=QnDZ_cO5Ln4
There are plenty of reasons why we are inspired by so much of what the “adults” in the industry have tried, tested and flown. If NASA has spent millions of dollars on some tests and written a 200-page report with test results that tells us that this or that is too dangerous, unsafe or for some other reason just doesn’t work, then we have not grasped much of the concept of “due diligence” if we still try to go that way.
Likewise we let ourselves get happily inspired when other hobby enthusiasts develop and test some device or another that could solve one of our challenges. So, if a “competing rocket club”, who are building a giant rocket to send astronauts, satellites and cargo into orbit, decide that their rocket motors will all be started with a wooden stick with a sparkly fountain stuck up in the engine, then it’s fine to let it inspire us, right?
So we still use a wooden stick, stuck up in the combustion chamber, fastened with plastic strips. It doesn’t sound very innovative, but the existing model has actually arisen through a large number of iterations. There are innumerable, subtle tweaks made during the many, many tests to the structure, of its size, the choice of materials, assembly methods and much more. The fact that we ended up with a device like the thing that was used to light up the 32 monster engines on the “space object” that transported the Danish astronaut Andreas Mogensen into orbit for his stay on the ISS recently, is something that we’ve only really paid attention to in recent days. So our choice isn’t THAT bad after all, is it?
The off-the-shelf, professionally-made igniters had the unfortunate feature of blowing directly into the aluminum injector, with the risk of burning through this in short time. A solutions exercise was undertaken and Martin started some inspiring experiments with testing various “deflectors”. Whether it was the material, the method of manufacturing or just the thickness of the material that faulted, we never found out, but they burned through, unfortunately. I then made some three-legged, rather thick, conical deflectors on the lathe that spreads out the igniter’s magnesium sparks in front of the injector, so this didn’t get burned through.
They are known inside Copenhagen Suborbitals as “Smurf-barstools” and proved to work well enough that they could at least be used to stabilize engine starts during last summer’s tests. They have since been reduced in size as the off-the-shelf pyrotechnic firework fountains have been replaced by something smaller but still “adequate”, which gives our delicate jet-vanes a better chance for survival.
For a series of BPM-5 tests I constructed an ingenious contraption, called a “Meincke-sled” (sometimes it’s hard to find nicknames for our “gadgets”, so they often ends up being named after the person who came up with the idea or used a lot of time working on it). It is a rail assembly on which a cart that grips the rails is attached and on which the “Wilson-stick” (!) with it’s igniter is strapped to, securing that it will exit the engine in a controlled fashion.
You can clearly see in the photos how much complexity this construction has added to the setup. Not to mention how much it gets in the way of service and inspection. Incidentally, it seems to actually not even work particularly well, as the wooden stick simply flaps like a riding crop on it’s way out, partly caused by the “Meincke-sled”‘s large inertia (one can almost sense its mass from the pictures. Sorry, but I had to use what we had in stock!) so of course we have now found a completely different solution! On the other hand, I enjoy the fact that it doesn’t bear my name.
The rest of the long discussion about this tiny corner of our many challenges will have to wait for the second part of this blog, where I will let our readers learn more about what the “adults” in our industry does, what other enthusiasts are experimenting with, what we have figured out for ourselves, and a little bit about some of the other things we fiddle around with behind the curtain when the stage lights go out, the microphones are turned off, the audience has gone home and the last taxi has driven off!
For the time being, thank you for your attention!