Arif Karabeyoğlu - SPG, Hybrid Rockets
Originally written after an SSI event in 2012:
Learn more about the space propulsion group on their website
On November 8th, the Student Spaceflight Initiative was proud to sponsor a presentation by Dr. Arif Karabeyoglu, Consulting Professor in the Department of Aeronautics and Astronautics at Stanford, graduate of the AA PhD program, and CTO and Co-Founder of the Space Propulsion Group Inc. Arif’s expertise lies in hybrid rocket technology, the technology he believes will revolutionize the space field by solving the launch problem- achieving lower costs, increased safety, and bolstering reliability than either liquid or solid rockets can allow.
Fundamentally, Arif explained, liquid and solid rockets are expensive simply due to the physical nature of the rocket motor. Even though solid rockets can be extremely compact due to the high density of the propellant, the performance of solid rockets is limited due to the lack of high performance oxidizers that are solids at room temperature. Additionally, since the fuel and oxidizer are premixed, any ignition source will cause complete combustion of the propellant which poses huge safety hazards during fabrication, transportation, launch, and abort and drives up cost. Liquid rockets can achieve much higher performance but are vastly more complex systems that: require a lot of maintenance since typically propellants are cryogenic, need turbopumps for the required mass flow rates and pressures are extremely difficult to manufacture, and must actively cool any materials that come in contact with the combusting propellant to keep it from melting. Moreover, while the fuel and oxidizer are not premixed, gas leaks can cause combustible mixtures that pose serious safety hazards and drive costs even higher.
Hybrid rockets, on the other hand, avoid these problems. By using a liquid oxidizer and solid fuel, they can achieve performances similar to and even better than any liquid or solid engine. Since the fuel and oxidizer are in different states, it is much more difficult to cause undesired combustion of the propellant. Moreover, the system is extremely simple because no pumps or active cooling systems are required, only a valve between the oxidizer tank and combustion chamber that can be turned on or off to control combustion is necessary. The safer fabrication can be shown by the fact that the Space Propulsion Group is making 21-inch hybrid rocket motors in house as a very small company in Sunnyvale, CA - within city limits. Additionally, the very low fabrication costs mean that hybrid rocket motors can be tested numerous times at low cost in order to achieve a more reliable system. What this means is that hybrid rockets have the ability to be mass-produced more easily with the same performance, higher safety and reliability, and therefore lower costs, when compared to solid or liquid engines.
Despite all of these benefits, there have historically been two major issues that have held back hybrid rocket technology: low regression rate fuels and major combustion instabilities. While these issues remain today, Arif and SPG are well on the way towards solving them. At Stanford in the 1990s, Arif discovered a new class of fuel, paraffin waxes, that had a regression rate five times higher than any other hybrid fuels. This rate allows the use of simple grain configurations necessary to achieve the performance and reliability large-scale systems require. Moreover, SPG has recently eliminated nearly all of the instabilities in their 10-inch motor and have shown that all of the technology is scaleable with their 21-inch motor. However, Arif emphasizes caution. Despite the success that SPG is having with a low budget, the technology is still under development and rockets are inherently dangerous. “When I push that button [to start combustion], I never fully know what is going to happen” is a common refrain among the rocket experts at Stanford- Arif included. While hybrids can be safer, if shortcuts are taken and people become complacent, people will die, as shown in the Scaled Composites accident that killed 3 people in 2010 when their oxidizer tank exploded during a cold flow.
But nonetheless, Arif is quite optimistic. Completely focused on developing the technology, SPG has been progressing with SBIR grants from NASA, money from the Air Force, and help from the interactions and in-flux of talent from Stanford. When asked about tuning his company for marketability, Arif shrugs and says he isn’t really worried about that. He believes that SPG is the best hybrid rocket company in the world and that if anyone wants frequent, cheap launches, essential to space tourism or other increased activities in orbit, they will have to come to him eventually. For example, while in 2005 when Scaled Composites was looking for a hybrid motor, SPG wasn’t ready, but now in 2012, SPG’s rockets, though smaller, are scalable and have much higher performance and reliability than those used on SpaceShipTwo. Safety is paramount and there is work yet to be done, but listening to Arif, it’s hard not to believe, as he does, that it is only a matter of time.