The Other Guys
NASA needs a space taxi. The likely pick is SpaceX—but don’t count out Colorado-based Sierra Nevada.
- By Michael Behar
- Air & Space magazine, August 2013
University of Colorado Boulder
(Page 4 of 5)
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.”
At Sierra Nevada, McMillan sets me up in the flight simulator’s commander seat, the control stick on my left. He flips a switch and suddenly Dream Chaser is pitched forward in a steep dive, doing 300 knots at 12,000 feet, with Cape Canaveral approaching fast. The spaceplane is hyper-responsive; the slightest twitch can knock it off axis. McMillan, who is sitting at a computer station a few yards away, is talking to me through a headset. He warns, “She’ll roll quickly, like a fighter jet.”