Private suborbital flights involve carrying passengers to the Kármán Line (an altitude of 100 km, or 62 mi—the officially accepted definition of “space”) before returning safely to Earth. The demands on a propulsion system designed for suborbital-flight vehicles are not the same as those on propulsion systems for orbital space flight. The systems being developed now are enabling manufacturers to achieve lower-cost, safe, reliable reusable vehicles without government-sized budgets.
Of the three basic types of rocket engines—solids, liquids, and hybrids (which usually combine a solid and a liquid)—XCOR Aerospace has chosen a liquid engine, the XR-5K18, using a proprietary blend of liquid oxygen and kerosene, as the main propulsion for its Lynx suborbital vehicle.
According to Dan DeLong, Chief Engineer at XCOR, this type of system was chosen for high performance and safety. Having run about 2700 rocket engine firings without an explosion, engineers always assume the next one will.
“Liquids are small enough to enclose with a blast shield,” DeLong said. “If an engine does blow up, it can be contained within it. The FAA has regulations that jet engines on airlines have containment rings, secondary containment systems in case of leakage or a breach. Since rocket engines are less mature than jet engines, XCOR will use containment rings as well. They were present on previous vehicles, but a severe test for the blast shield hasn’t yet been done. We plan to put explosives inside to verify that containment rings do the job.”
To get decent performance, the engine has a pump-fed rather than pressure-fed system. That’s another difference between government and personal space flight: propellant pumps are more akin to using automotive technology than turbo pumps used in government programs. Since XCOR optimizes for long service life and reliability rather than minimum weight, they use automotive technologies to a much greater extent and have crankshafts, reciprocating rods, and pistons, which are also easy to overhaul.
XCOR currently is testing its Lynx manned flying vehicle, which operates like an airplane using wings for takeoff. Its third of this type, the Lynx will be a suborbital reusable launch vehicle with two seats. Its revenue mission is to carry a space flight participant. The first two versions were modified off-the-shelf airframes—subsonic airplanes that were modified with rocket propulsion. Lessons learned from the subsonic airframes as flying testbeds for rocket propulsion technology are currently being built into the Lynx vehicle system design.
Where NASA and the U.S. Air Force are interested in high performance and high reliability through extensive upfront design, and satellite launch vehicles need high performance and low weight, private space flight begins with different requirements. DeLong says that safety, reliability, and maintainability are the key drivers.
XCOR achieves reliability by performing many flight tests.
“By performing less engineering and more testing, we are able to complete more product development cycles,” he said. “Manned reusable vehicles are a lower-cost way to develop, because when you have problems, you get the vehicle back.”
Safety is achieved through lowering the cost per flight, which DeLong thinks may sound funny. His logic trail is “the only way to get safety is with good reliability, which means lots of flight tests,” he said. “But the only way to test often is if you can afford to. Achieving low cost per flight is a big safety feature. XCOR will complete 50 to 100 vehicle test flights before putting it on the market. Compare that to the space shuttle, which had four test flights before being declared operational.”
DeLong noted that another difference between big government programs and private space flight is the requirement that private space vehicles use nontoxic propellants. Although the highly volatile substances used in government programs can produce higher performance, they can also be ingested into skin and have higher environmental impacts. The propellants in XCOR’s propulsion systems have low toxicity, lower environmental impact, and are less expensive, helping achieve a lower cost per flight.
According to DeLong, XCOR believes it is important that the engineering team is developed at the same time as the hardware. It is also important that the same people who design do the testing, which is also different from big government. When design and test groups are so large, only those doing the testing learn about the problems.
The Lynx propulsion system is currently well into the development and test process.
“The main XR-5K18 rocket engines have been run 30-40 times, and the pumps are coming along,” said DeLong. “The engines are still running in pressure-fed mode and pumps in their own pump test stands, so we haven’t run the main engine on its pumps yet, but we expect to close the loop later in 2012.”