Tag team from Rutan enters space race
Aircraft designer Burt Rutan and his team at Scaled Composites, Mojave, Calif. (www.scaled.com), recently unveiled plans to win the X-Prize by building the first privately financed spaceship to carry a crew of three into suborbital space (62 miles high) and back. The team has to repeat the feat using the same aircraft within two weeks to win the $10 million prize. The plans, which have been closely held company secrets for the past two years, will likely culminate in suborbital flights by the end of next year.
The White Knight
Like NASA's X-15, Rutan's spacecraft will be launched after being carried aloft by a larger aircraft, the recently developed White Knight. The aircraft's twin General Electric J-85 turbojet engines, originally developed in the 1950s and put on T-38 trainers, each develops about 4,000 lb of thrust and uses an afterburner. The plane can cruise at over 53,000 ft, and carry up to 8,000 lb of external cargo and 6,400 lb of fuel. And if a mission calls for better climb performance, maintenance crews on the ground can extend the 82-ft wingspan to 93 ft.
The plane's most striking feature is the array of 16 portholes dotting the cockpit. Each consists of two panes for redundancy. The design reduces weight and manufacturing costs compared to an expansive cockpit canopy, and the round shape minimizes structural loads. The portholes are in the same position on the White Knight as on SpaceShipOne (SS1), which lets the White Knight serve as a flying simulator for the spacecraft. And designers placed them on the space ship such that pilots have a good view of the horizon throughout its mission.
With its high thrust-to-weight ratio and large pneumatically activated spoilers, White Knight has many of the flying characteristics of SS1. Spoilers, or speed brakes, dramatically increase drag and allow for steep descents. White Knight and SS1 also have the same flight controls, environmental systems, trim servos, and electrical components. This lets potential astronauts practice almost every SS1 maneuver, except rocket firing and propulsion, in the White Knight.
White Knight's fuselage is built like a submarine with one pressure vessel inside another. This level of redundancy eliminates the need for spacesuits and pressurization, so pilots fly in a "shirt sleeve" environment. To keep air inside the fuselage breathable, a small cylinder replenishes the oxygen, Sodasorb (a sodium-hydroxide compound) chemically removes carbon dioxide, and a desiccant keeps humidity in check.
To launch SS1, the White Knight will climb to 53,000 ft, which should take about an hour. Then it will drop the spacecraft and fly out of the way. In the future, the plane could see service as a reconnaissance and surveillance platform, a high-atmosphere research and data-relay aircraft, or as a launcher for microsatellites.
After being dropped, the spacecraft will fire its nitrous oxide and rubber-fueled rocket engine, and soar to an altitude of 62 miles. The engine uses nitrous oxide (N2O2, or laughing gas) as an oxidizer and burns hydroxy-terminated polybutadiene, a synthetic rubber, as fuel. Both can be stored safely without special precautions and will not react even when mixed. It takes a significant heat source applied to the fuel and then adding the oxidizer to ignite them. Combustion products include water, carbon monoxide and dioxide, hydrogen, and nitrogen, which are relatively benign compared to by-products from other rocket fuels.
The craft carries nitrous in a filament-wound tank that has much of its surface area bonded to the fuselage. This makes it a structural part of the spacecraft. The large bond area also reduces loading per square inch and isolates the airframe from engine vibrations. Nitrous is loaded onto SS1 prior to launch at 700 psi. This is enough to pressurize the oxidizer tank, so turbo pumps and complicated plumbing are not needed.
The oxidizer tank is reusable, while the fuel casing, which includes the fuel, throat, and nozzle, are good for several short burns or one long, mission-length burn. The engine's ablative nozzle burns away during each mission to save weight and expense.
The flight and rocket controls in SS1 are simple. The rocket, for example, has just two switches: one to arm it, and one to fire it. There are no throttle or fuel controls, though it can be shut down and restarted in flight. A screen displays critical motor parameters while a navigation unit guides the pilot along a preprogrammed flight path. The pilot uses a stick to control flight surfaces. Surfaces are manually controlled at subsonic speeds, electric trim actuators kick in at supersonic speeds, and for altitude control in outer space, the craft uses cold-gas, carbon-dioxide thrusters. Its three-person crew will experience about 10 sec of 5-g acceleration.
Reentry begins after the fuel is exhausted. SS1's twin-boom tail flips up 90°, providing a shuttlecock or "feathering" effect. It puts the craft in a stable, nose-up attitude that slows reentry and reduces aerodynamic heating. It should limit heating to about 1,000°F and let the designers get away with thermally protecting only 20% of the hull. The flip-up tail also reduces workload on the pilot, letting the SS1 glide down in a "hands-off" reentry, according to Rutan. The tails then flip down for a glide-in landing. SS1 should land about 90 min and 35 miles away from White Knight's take off.
The support team
While White Knight and SS1 might get all the glory, Rutan and his team also designed and built a mobile infrastructure which is critical for testing, launching, and recovery. Mission Control, for example, is housed in a trailer tuck equipped with a generator, telemetry radios, antennae, and computers. It also has a duplicate set of SS1's avionics and displays so ground staff can closely watch what's going on. It is driven to all motor tests and flights to monitor and record data. It also sends encrypted data and images over a wireless network back to headquarters at Scaled Composites.
To fill SS1's oxidizer tank, the team constructed a Mobile Nitrous Oxide Delivery System (Monods). It has a well-insulated 1,700-gallon tank, generator, refrigeration unit, and a resistive heater all on a trailer. Nitrous is delivered from a commercial supplier at 300 psi and 0°F, and heated to about 70°F and 700 psi before being sent into SS1.
The team also built a propulsion test trailer to check out the rocket engine during development. It collects data during firings of full-sized rocket engines. Parameters such as thrust and exhaust temperature are measured using strain gages and thermocouples.
To train pilots, Scaled Composites had an SS1 simulator built. It uses a detailed mock-up of the cockpit, complete with video displays outside most of the windows and a projection screen that recreate what pilots will see during boost, black-sky space, reentry, and landing. It is equipped with actual flight hardware and software. All this should give pilots a good idea of what they will see before ever taking off in SS1 tucked beneath the belly of the White Knight.