Sometime this month, an Antares medium-class launch vehicle carrying a Cygnus spacecraft, both products of Orbital Science Corp., Dulles, Va.,will lift off from NASA’s Wallops Island Flight Facility on Virginia’s east coast. It will put the Cygnus in a low orbit (about 150 miles above the Earth) and on its way to rendezvous with the International Space Station (ISS). Cygnus will deliver up to 4,400 lb of cargo to the station, and then haul away 4,400 lb of trash that will burn up on reentry.
Orbital Corp.’s Antares/Cygnus is the second privately owned spacecraft delivering food and supplies to astronauts onboard the ISS. The other is the Dragon, carried aloft by the Falcon 9, two spacecraft designed by Space Exploration Technologies Corp. (SpaceX), Hawthorne, Calif. Both the Antares/Cygnus and Falcon/Dragon spacecraft are critical parts of NASA’s plan to privatize delivery of cargo to the space station after the U.â S. retired its Space-Shuttle fleet in 2011. Since then, NASA has relied on Russian spacecraft to transport crew and supplies to and from the station. Now Orbital can begin making good on the eight flights (44,000 lb of supplies) it agreed to do for NASA for $1.9 billion.
Antares
The 132-ft-tall Antares rocket, which was originally dubbed Taurus II, stands as tall as a 13-story building. It can put payloads weighing up to 11,000 lb into orbit and was constructed using proven components and technologies to be a cost-effective, relatively low-risk method of carrying commercial, science, and defense payloads in space.
The launcher gets its takeoff power from dual AJ-26 rocket engines from Aerojet Rocketdyne, Sacramento. Each can generate over 360,000 lb of thrust. The engines are refurbished leftovers from the Soviet Union’s efforts to put a man on the Moon. Originally called the NK-33, the 1970-era engines were designed to power the N1 rocket Soviet engineers built to take cosmonauts to the Moon and back, much like NASA’s Saturn V. But the N1 was more powerful than the Saturn V. With 30 NK-33s — each measuring 12-ft tall and 6.5-ft in diameter, weighing 2,700 lb — the N1 could generate 11.3 million lb of thrust.
The NK-33s were too late, however, and the Space Race was over before they could be used. So the Soviets switched their efforts to developing the Energia heavy-lift rocket engine. In the meantime, they stored away a couple hundred complete NK-33 engines in a warehouse. In the mid 1990s, Russia sold about 35 of these engines to Aerojet for $1.1 million each.
Aerojet engineers modified the engines to make them compatible with U.â S. propellants (liquid oxygen and rocket-grade kerosene). The engineers also updated the engines’ wiring harnesses, electronics, and improved the steering subsystem. Aerojet says they have enough engines to supply Orbital’s Antares for the duration of its NASA contract, but fewer than two dozen after that. To keep supplying Orbital’s Antares, Aerojet is trying to get the Kuznetsov Design Bureau, the company that built the NK-33s, to restart production and build AJ-26s. (Aerojet is also working with Teledyne Brown, Huntsville, Ala., to build an upgraded version of the AJ-26, one that will have 500,000 lb of thrust, for use in NASA’s Space Launch System.)
The drawback of using two older engines is that Antares must carry over 390,000 lb of liquid oxygen and 140,000 lb of kerosene to keep the two engines firing for the first 4 min of flight, when it will be 66 miles above the Earth.
Although the first Antares stage burns liquid fuel, the second-stage-engine — either an ATK Castor 30B or 30XL — burns solid-rocket fuel. Solid rocket fuel is considered easier and safer to use and store than liquid fuel, but supplies less thrust for similarly sized engines. A solid-fuel engine can’t be shut off or restarted, or throttled. So once ignited, they must burn at full thrust until they consume all the fuel.
The Castor 30 engine, like the AJ-26, has a long heritage. It can trace its technological heritage to the first-stage engine in the Peacekeeper intercontinental ballistic missiles deployed from 1985 to 2005. In fact, Orbital Science is also reusing other Peacekeeper hardware in its new four-stage launcher, the Minotaur IV.
After the first two Antares flights, Orbital wanted to use Russian RD-0124 engines on the second stage. Instead, Orbital opted for a stretched version of the 30B, the Castor 30XL. It measures 19.5-ft tall, 6 ft taller than the 30B, but with the same diameter. It tips the scales at nearly 58,000 lb, about twice the 30B’s weight. The extra fuel the 30XL carries lets it burn for 156 sec, roughly 30 sec longer than the 30B. This should let Antares carry almost 6,000 lb more cargo on its last six scheduled NASA flights.
The Antares can also be outfitted as a three-stage launcher with one of two different engines powering the third stage. The extra stage can put satellites into more-precise orbits, or orbits that take more thrust to reach to such as geostationary orbits.
A two-piece, 32-ft-tall fairing sits atop the Antares, protecting any payload such as the Cygnus from vibrations and thermal and aerodynamic stresses. The two shells weigh 2,134 lb and consist of an aluminum honeycomb core covered with layers of graphite-epoxy composite. The fairing is jettisoned soon after the launcher makes it through the atmosphere to lower the rocket’s overall weight and increase its performance.
Cygnus
On the launch pad, Cygnus sits inside the fairing. It’s then carried 155 miles above the Earth and released into spaced within 10 min after liftoff. As it circles the Earth at 17,500 mph, it unfurls its two solar arrays (three panels each) of gallium arsenide, enough to provide 3.5 kW of power. The capsule then autonomously uses its GPS, inertial navigation, and LIDAR, a laser-based ranging and azimuth device, to locate and plan its approach to the ISS.
When Cygnus and the ISS are in proper position, the capsule makes a series of burns with its Japanese-built IHI BT-4 engine and up to 100 lb of thrust to get itself within 40 ft below the ISS at its orbiting altitude of roughly 220 miles. Cygnus also carries 32 reaction-control thrusters, each with 6 lb of thrust, to maintain the correct attitude.
At this point, the crew onboard the ISS use the Canadarm to grab the Cygnus and grapple it into position onto the Common Berthing Mechanism at Node 2 on ISS’s Harmony module. An air-circulation subsystem, lighting, fire detection, and pressure monitoring inside Cygnus make it safe for astronauts to enter and unload cargo.
The standard Cygnus holds 664 ft3 of pressurized cargo weighing up to 4,400 lb. The larger, extended Cygnus carries 954 ft3 weighing 5,953 lb. Both capsules will take trash on their reentries into the atmosphere where they will burn up. Cygnus was never designed to return to Earth.
Cygnus has enough power to remain in space for up to two years before or after docking with the ISS. This will be useful on missions in which the capsule carries experiments into orbit and remains there as it transmits data back to Earth as the experiments progress.
Resources: Alliant Techsystems Inc., Orbital Science Corp., Space Exploration Technologies Corp.