Langley engineers got as far as a full-scale mockup of the HL-20. In 2005, SpaceDev, now a subsidiary of Sierra Nevada, licensed the technology from NASA. It was SpaceDev founder James Benson who christened the project “Dream Chaser.” (Voss, who admits he didn’t love it when he joined SpaceDev in 2007, says the name has grown on him.)
Dream Chaser is a significant advance over the HL-20, boasting numerous features the Langley group probably never could have envisaged. For example, the navigation system permits autonomous reentries and landings: The crew could be completely incapacitated and Dream Chaser will bring them home safely.
And the vehicle will be made with carbon composite—a weave of epoxy and graphite fiber—that is half the weight of aluminum and twice as strong. Though Dream Chaser would be the first manned spacecraft made of composite, Boeing’s 787 Dreamliner passenger jet employs carbon composite in nearly half its airframe and has been flying commercially since 2011. The material is also used in unmanned spacecraft. Composite is fashioned in molds, produced much like plastic. Sierra Nevada says it could stamp out duplicate spaceplanes relatively quickly and affordably, each with a 25-mission lifespan and low-cost servicing between flights.
Perhaps the most groundbreaking technology is its propulsion: two hybrid rocket engines, which burn a non-toxic mixture of synthetic rubber and nitrous oxide—yes, laughing gas. Hybrid engines propelled SpaceShipOne, and now Sierra Nevada is developing them for both Dream Chaser and Virgin Galactic’s SpaceShipTwo, which is scheduled to begin carrying paying tourists into space this Christmas. While hybrids aren’t a new idea, they have never been used on a manned spacecraft for orbital missions. Because of their simplicity, safety, and reusability, Sierra Nevada believes they are superior to liquid-fueled rockets, such as those used for the shuttle’s three main engines. The Dream Chaser’s engines work by opening a valve that allows the liquid oxidizer, in this case nitrous oxide, into a combustion chamber packed with solid propellant, the synthetic rubber. Once in the chamber, the pressurized nitrous expands and converts to vapor. When an ignition switch in the chamber fires, the rubber combusts, ejecting hot gases through the exhaust nozzle to generate thrust.
Ken Bowersox, an astronaut on five shuttle missions who would later serve two years as a vice president for SpaceX, says, “The nice thing about a hybrid engine is that if the oxidizer just spills out, you’re not necessarily going to get a fire.” Bowersox is alluding to a potentially deadly hazard associated with so-called hypergolic fuels, in which the oxidizer and propellant ignite by chemical reaction: Merely mix them and they go boom. On the shuttle, “hypergols” fueled secondary engines and thrusters responsible for on orbit maneuvers. “With hypergolic propellants, if you get a leak in one tank, they are pretty corrosive—the stuff is nasty—and it could do damage to the equipment that’s near it,” explains Bowersox. “You can get into a situation where you get a fire or an explosion.”
Hybrid fuels are safer to handle too. “We call them ‘green’ propellants,” says Lindsey. “The hydraulics on the shuttle were driven by auxiliary power units that used hydrazine, which is a dangerous chemical. If you ever watched the shuttle land, it would be probably 30 minutes before anybody could walk up to the vehicle. People would be in special suits to protect them. Our vehicle, when you land, you can walk right up to it because we don’t have any of that on board.”
Not everyone shares Sierra Nevada’s enthusiasm for hybrid engines. In 2007, an explosion killed three employees at Scaled Composites, the company building SpaceShipTwo, during a test using nitrous oxide. The cause of the detonation remains a mystery.
“The oft-repeated claim that hybrids can’t blow up is a canard—they can and do,” says Gary Hudson, president of the Space Studies Institute in Mojave, California, and an expert in reusable rocket design who has founded several commercial space companies. “One problem some hybrids have is ejecting a chunk of fuel near the end of the burn that can momentarily block the nozzle throat, leading to a pressure spike that can result in a case rupture.” Says Bowersox, “A hybrid doesn’t worry me more or less than any other rocket. Any type of engine is intolerant of sloppiness.”
Still to be resolved: whether Dream Chaser’s hybrids can eliminate “black zones”—periods during flight where an abort would entail losing the vehicle and possibly its crew. During the shuttle’s first two minutes off the pad, solid rocket boosters helped lift it. “We could never lose one and survive,” Lindsey says. “They both had to fire all the way to the end of their trajectory.” Mango puts it this way: “If something happened in those first two minutes [on the shuttle], we didn’t have any way of really saving the crew.” But unlike the shuttle, which had an expendable external tank—the big orange beast—feeding fuel to its main engines, the hybrid rockets on Dream Chaser will be self-contained. If a launch malfunction occurs—even during countdown—Dream Chaser can ignite its hybrid engines and escape catastrophe. “Our [commercial crew program] requirements say you must be able to always abort and get to a landing site,” says Mango. “Because of its design, [Dream Chaser] is able to do that right from the pad.” In this scenario, explains Lindsey, “the hybrid rocket motors would fire and run us up to roughly 20,000 feet. We would come off that, go over the top and into a glide to land back at the Shuttle Landing Facility” at NASA’s Kennedy Space Center.
The scenario seems unlikely, given that the Dream Chaser’s launch vehicle, the Atlas V, “has 104 [missions] in a row with no failures,” Lindsey says. Assuming all goes well, Dream Chaser will arrive in low Earth orbit with two fully fueled engines. In addition to its activities with the International Space Station, Dream Chaser will retain enough power to take on other tasks. This is something the shuttle could never do: At 70 miles high, it ditched its external fuel tank, at which point the main engines became nothing more than dead weight. As for what, specifically, Dream Chaser will accomplish with all that extra fuel, “I’m thinking military applications,” says PoliSpace’s Muncy. The vehicle could also be used for satellite repairs, servicing the Hubble Space Telescope, or any “activities that should be done with a human in the loop,” says Mango. Tourism? “It’s one of our future missions,” says Voss. “We’d like to use the Dream Chaser as an orbital tourism vehicle. It can carry seven people for several days. And there is a ton of room [inside] to play around, to fully experience the micro-gravity environment and floating—the kinds of things that people would want to go to space for.”