I am happy to introduce yet another guest blog post. This time written by Copenhagen Suborbitals co-founder Peter Madsen. Here you go ...
Copenhagen Suborbitals works with ballistic spaceflight. That means we do not try to design hypersonic rocket airplanes -- e.g., Virgin Galactic or XCOR Aerospace -- but have chosen to depend on the use of a ballistic missile.
Why? Building rocket airplanes means you must master the science of ultra-high-speed aerodynamics AND the science of rocket engineering at the same time.
A rocket-powered airplane is still an airplane and must somehow comply with legal regulation in the field of homemade experimental aircraft. Finally, rocket airplanes typically do not have the performance for ground start -- so you also need a carrier aircraft like the converted B-52 bomber used by NASA to launch its X-planes.
Can you see the problems mounting?
A ballistic missile is suddenly a simpler, much more affordable, single-stage solution to flying out of Earth's atmosphere. The performance can be increased far beyond that of a winged space plane. And that's it.
Cheaper, faster, more simple -- only one technology to master.
So how does one build a massive ballistic missile? First, we change the word into rocket, since missile essentially means weapon, and we make peaceful rockets.
There are three optional technologies with a lot of myth and misunderstanding attached to them.
I am happy to say that some of the people today working at Copenhagen Suborbitals have gained practical experience over the years with all three kinds of rocket technology in use today. It must also be stressed that a rocket engine is the product of a well-equipped metal shop. So, when you have the metal shop, what type of rocket is most practical to somebody like us?
Solid rockets:
Myth: The simple, straightforward way to rocketry.
Totally wrong. Modern solid propellant rockets are highly advanced giga-sized pyrotechnic devices built in highly specialized, costly factories.
The solid propellant rocket, as invented by the Chinese hundreds of years ago and as used today in the firework displays, shares little but its principle with the giant solid fuel boosters of the space age. Today's high-performance composite propellants are the product of high-tech polymer chemistry -- and, even so, extensive quality control is needed to obtain reproducible, non-explosive performance. Most important: One oxidizer is almost always used -- ammonium perchlorate. In short, this is not available by the tonne to us. It has no other major use but rockets and fireworks, and our country has no industry worth mentioning in that field. Finally, solid propellant rockets are, after all, just oversize fireworks and as such off limits for legal reasons.
Rocket amateurs around the world do build solid propellant rockets with sizes up to about 10 kg propellant. In rare cases, larger. But nobody even imagine casting propellant grains by the ton. The legal, safety and cost issues would be overwhelming.
I tend to say the solid propellant rocket might be simple but the factories building them, are not.
Liquid propellant rockets:
Myth__: __Complex, more demanding than solid.
Totally wrong. Liquid-propellant rockets can be built at any slightly converted tractor factory.
The alternative technology is the conventional liquid propellant rocket. The first of its kind, the German V2 missile, was a machine built from steel and aluminum. Its propellants were a simple combination of 75% ethanol and liquid oxygen. A V2 rocket did not contain any components that would not be easy or even rather cheap to make in a well-equipped metal shop of today. Its performance (in its 1944 version) is well above what we would need to send a capsule like TYCHO DS into suborbital space. In fact, components that would be very heavy and expensive by 1943 standards are now cheap, off-the-shelf components found in any smart phone today. The guidance platform is an example. By 1944 it was a masterpiece of precision mechanics, weighing 35 kg, today the same performance can be purchased as small electrical component of a fem grams. The cost is perhaps a million times less.
We have tested a series of sub size V2 type rocket engines and they work fine. Sometimes we were able to refuel and restart the test engine every 45 minutes. Building it needed no tool unavailable in our old-fashioned metal shop. However, they are sometimes tricky machines. A tiny failure of timing the ignition and propellant valve opening -- just before its 12th test -- resulted in violent engine explosion. The phenomena is known as a "hard start" and it has destroyed many rocket engines and launch vehicles over the years. It is caused by uncontrolled propellant accumulation in the engine prior to ignition.
Nothing special in experimental rocketry, but it would nice if the propulsion system simply lacked this sensitivity to error.
Hybrid rockets:
Myth: Since nobody uses hybrids, it must be an inferior technology.
Totally wrong. Hybrid rockets offer big advantages if safety and simplicity is important.
In a hybrid rocket the liquid fuel component -- like alcohol -- is replaced with a big rubber tube lining the combustion chamber wall. Incredibly this means that the fuel is cooling the chamber - and that it does not need to be pumped or forced into the combustion chamber. On top of this feature - the fuel can only burn as fast as heat transfer can evaporate it. So, a hybrid rocket has a sort of built-in safety feature: You can't get a hard start.
There is also only one liquid to be pumped and controlled. The engine can be turned on and off, or even throttled, by controlling the oxidizer flow. Like its liquid propellant sister, the oxidizer may be liquid oxygen. which is cheap and available almost all over the industrial world.
The rubber fuel grain is a much simper thing than a solid-propellant fuel grain because it can all be rubber. In a solid propellant motor, some 70-85% of the propellant is solid oxidizer and metal power fuel, leaving little room in this explosive mixture for the fuel binder. This problem has haunted solid fuel rocket scientists for more than half a century.
Imagine a wheel barrow full of ammonium perchlorate salt and aluminum powder. Then you get a cup of glue. You must somehow mix this, make it liquid and poor it into a motor case. Any spark and you will be on your way to the heavens in another way than planned.
But it means nothing to a hybrid rocket engineer, who just has to cast something like a big car. It sounds almost to good to be true: a non explosive rocket, burning a cheap and easily available propellant that can be built from iron and low-tech ship-building aluminum alloy.
And it's almost true.
We have built hybrid rockets from 62 mm in diameter to 640 mm diameter -- and with propellant masses of more than a ton. We have measured specific impulses exceeding that of the V2 liquid propellant rocket, and we have had no explosions. Ever. In fact we have never observed an engine malfunction that would be lethal to our future astronaut. Hybrids tend to fail with little drama -- a point on the motor case starts glowing, and sparks and flames comes out -- but no devastating explosion happens. And you can always shut it down if any thing looks abnormal.
In the next part of the propulsion blog we will enter the hybrid rocket and see what it can do, how it functions and how it's built.
Greetings
Peter Madsen
Peter Madsen started Copenhagen Suborbitals with Kristian von Bengtson in 2009. Peters area covers all aspects of the launch vehicle development. He has been working with rocket engines of all types since age 16. However - along with the rockets he has designed and operated three manned diesel electric research submersibles from 2001 till 2008. Peter has volunteered to pilot the CS DIY spacecraft on its first manned flight.
