Spacetruckin’ with SpaceX

A private firm develops the first commercial launch vehicle and transport capsule, then proves they can carry payloads into space.

With the retirement of the Space Shuttle, NASA has had to rely on launch vehicles from Russia, Japan, and the European Space Agency to transport crews and supplies to the International Space Station (ISS). But that’s about to change, thanks to Elon Musk and the folks at his Space Exploration Technologies Corp. (SpaceX), Hawthorne, Calif. They recently managed to successfully send a cargo-laden transport spacecraft, the Dragon, to dock with the ISS using a Falcon 9 launch vehicle. The Dragon was then packed with equipment and gear for a return trip to Earth where it was recovered intact.

SpaceX engineers designed and built the Dragon and Falcon 9 using as much commercial off-the-shelf (COTS) hardware as possible. Here are some of the technical details.

The Dragon
The reusable Dragon capsule measures 14.5-ft high and 12 ft in diameter. It can hold 350 ft³ in its pressurized cargo bay and 495 ft³ in the unpressurized trunk.It took SpaceX engineers four years to design and fly the Dragon, a free-flying reusable spacecraft. And two years ago, it became the first privately developed vehicle to return safely from orbit.

Dragon consists of three major components: the nose cone, capsule, and trunk. The nose cone protects the capsule and trunk, as well as the top-mounted adaptor needed to dock with the ISS (and possibly other future spacecraft). The nose cone is discarded once the Dragon is safely in space.

The capsule carries the pressurized cargo into space and holds all the cargo for return trips. It also houses the avionics, thruster controls and fuel, landing parachutes, and other support subsystems. Inside, it has lighting and fire detection and suppression equipment. Thrusters mount around the outside of the capsule.

A pair of technicians inspect equipment packed inside the Dragon prior to launch. Credit: SpaceXThe trunk is used to carry unpressurized cargo and supports the craft’s two solar arrays (eight panels each) and thermal radiators for keeping the Dragon cool. Each array is 21-ft long and provides 1,500 to 2,000 W to power the Dragon. There are also four lithium-ion batteries in the capsule that store power for use during launch, descent, and when the spacecraft is not in the sunlight.

The trunk is ejected and left to burn up on reentry prior to the Dragon leaving orbit for its return through the atmosphere for a landing. Without the nose cone and trunk, Dragon is shaped so that it develops lift as it plummets through the atmosphere on reentry. This limits deceleration to about 3.5 g and lets flight engineers more precisely control the spacecraft’s splashdown point.

Dragon drops safely to Earth beneath a trio of 116-ftdiameter parachutes. A pair of drogue parachutes initially deploy when Dragon passes through 45,000 ft, slowing and stabilizing the capsule, Then at 10,000 ft, the larger chutes unfurl. The current version lands on water and is equipped with inflatable collars that deploy to keep it afloat as it awaits recovery.

The robotic arm onboard the International Space Station latches onto the Dragon and gets ready to grapple it into one of the station’s docking stations.While in space, Dragon relies on 18 Draco thrusters for attitude (pitch, yaw, and roll) and control maneuvering. They burn the same mixture as the Space Shuttle’s orbital thrusters — monomethyl hydrazine fuel and nitrogen tetroxide oxidizer — and Dragon carries 2,838 lb of propellent. Each Draco generates 90 lb (400 N) of thrust and the thrust level can be adjusted (throttled). The thrusters can also be shut down and restarted. There is some redundancy designed into the thruster layout, so any two can fail without loss of control.

The thrusters were designed at SpaceX and can be reused, just like the capsule.

The Dragon capsule bears the scars of its is flight into space and back. Credit: SpaceXOne of the keys to making the Dragon capsule reusable is the thermal protection provided by tiles made of phenolic- impregnated carbon, PICA-X. It is based on PICA, a material developed by NASA in the 1990s and used on Stardust, the sample-returning capsule that intercepted a comet. The material was critical to that mission because the return capsule was the fastest man-made object ever to reenter Earth’s atmosphere (28,000 mph).

Engineers at SpaceX refined PICA to make it 10 times less expensive to manufacture, plus the upgraded material is easier to machine. And although a bit of the tiles is consumed each time the Dragon lands, the tiles should endure hundreds of Earth-orbit reentries with only minor degradations each time.

SpaceX engineers can monitor Dragon and Falcon launches from a control center at Cape Canaveral.For command and control, Dragon carries a UHF radio for communicating with crew on the ISS and an S-band radio for communicating with SpaceX and NASA’s ground stations. Data can be sent up to Dragon at 300 kbps, and it can return telemetry and other data at 300 Mbps. Equipment onboard Dragon can both compress and encrypt/ decrypt that data.

SpaceX is already developing upgrades for Dragon, including a more-powerful, upgraded version of its Draco thrusters. Dubbed SuperDraco, they are like their smaller cousins in that they can be throttled and restarted. But each can generate 15,000 lb (67,000 N) of thrust. The plan is to mount eight of these new thrusters into the sidewalls of the Dragon. Then, in case of an emergency on launch, they will fire, carrying the Dragon and any cargo or crew to safety. In the future, they could also be used to let the Dragon land on solid ground when it returns from Space.

The company also plans several Dragon variants: DragonLab: This version will carry pressurized and unpressurized equipment and experiments, with the pressurized ones recoverable upon reentry. It could also deploy relatively small satellites in low-Earth orbits (LEOs). It will be able to provide a temperature/humidity-controlled environment, as well as communications between the ground and the equipment. And, if needed, the hatch can be opened (then closed) in space so that sensors can take samples or have unobstructed views. DragonLab flights are currently planned for 2014 and 2015.

DragonRider: With the addition of an escape/safety subsystem, life-support equipment, and onboard controls, not to mention seats, this Dragon will carry up to seven astronauts. The obvious mission is to take crews to and from the ISS, and for one to stay docked at the ISS as an emergency lifeboat. It’s estimated this new manned vehicle will be able to take astronauts into space for about $20 million per seat. This might seem high, but Russia currently charges $62 million per seat on Soyuz spacecraft.

Red Dragon: This unmanned variant is being developed for a one-way trip to Mars. It will carry up to 2,200 lb of equipment, which would most likely be used to search for water. It is hoped that Red Dragon will be able to descend to the Martian surface by using its own drag to decelerate and rely on SuperDraco thrusters to land safely. Such a mission could take off in 2018.

The Falcon
A Dragon capsule sits atop a Falcon 9 launch vehicle next to it gantry. The Falcon launchers were the first privately developed rockets capable of delivering payloads into a low-Earth orbit.The muscle used to push Dragon into orbit comes from SpaceX’s Falcon 9 medium-launch vehicle, one in a planned family of launchers. The two-stage rocket uses conventional liquid oxygen and kerosene (RP-1) for fuel. Engineers stuck to a two-stage design to simplify manufacture and add reliability, two key factors in commercializing space travel.

During takeoff, the launcher is secured onto the pad after ignition. This safety measure lets ground controllers ensure all engines are firing properly and other subsystems operating normally before letting the rocket lift off.

The first stage is designed as a graduated space-grade aluminum monocoque. It is lightweight yet strong enough to support its own weight, which simplifies ground handling. Tanks in the second stage are friction-stir welded out of aluminum-lithium, the aluminum alloy with the highest strength-to-weight ratio. According to SpaceX, this alloy is better at reducing weight than any superalloy or composite compatible with liquid oxygen. And to simplify manufacture, the tanks onboard this stage are machined, which minimizes the number of welds.

The first stage is powered by one or more of SpaceX’s Merlin engines. The Falcon 1, for example carries one Merlin; the Falcon 9 carries nine.

After the first stage has done its job, the interstage, a carbon-fiber/ aluminum core structure between the two stages, jettisons it. For redundancy, two subsystems separate the two stages: separation bolts and a pneumatic pusher. A parachute returns the first stage to Earth, and similar to the Space Shuttle’s solid-rocket boosters, it lands in the ocean to be retrieved and reused or recycled.

The second stage is a shorter version of the first stage, so it can be built using much of the same tooling, material, and manufacturing techniques, which keeps down costs. Although only a single Merlin engine powers this stage, it can be stopped and restarted several times.

The long-term plan is to recover and reuse both stages. So they are covered with a layer of ablative cork, which burns off in case reentry gets too hot. Each stage carries parachutes for soft splashdowns, and are made of anodized materials that resist saltwater corrosion and simplify recycling.

The Merlin engine, which SpaceX designed, is based on several proven engines. The fuel injector, for example, uses a pintle design (much like the nozzle on a garden hose with a pin that moves in and out to control flow), rather than a manifold with hundreds of smaller holes. This same design was also used during the Apollo Mission on the Lunar Excursion Module. Although this design somewhat limits performance, the pintle injector cuts manufacturing costs. Still, the Merlin generates 115,000 lb of thrust (512,000 N).

A single-shaft turbo pump sends high-pressure kerosene through the walls of the combustion chamber and exhaust nozzle before being burned in the combustion chamber. This cools the engine, letting it create more thrust. High-pressure kerosene also serves as hydraulic fluid for various hydraulic components. And on the second stage, exhaust from the turbo pump is metered out side through a nozzle for roll control during flight.

The Falcon 9 can take 23,000-lb payloads to LEO, or 8,800 lb to a higher geostationary transfer orbit, making it a medium-lift launch vehicle. It can also complete its mission even if one of its Merlin engines fails on the launch pad.

SpaceX is developing a new rocket, the Falcon Heavy, which uses 27 Merlin engines to give it 3.8 million lb of thrust. This should let it take 117,000 lb of cargo into orbit.SpaceX is now working on the Falcon Heavy, a heavylift vehicle that will carry 117,000-lb payloads into LEO, more than twice the payload of the Space Shuttle. Its first stage will consist of three Falcon 9 first stages, with two of those stages in side-mounted boosters, much like the Shuttle. Flacon Heavy’s 27 Merlin engines will give it 3.8‡million lb of thrust. And it’s said that several engines can fail without affecting the mission. A protective shell will surround each engine to limit damage in case of a fire or rupture. Engineers also had the foresight to design the first stage of the Falcon 9 with the strength and integrity to handle this future task.

Falcon Heavy will be the first rocket to cross-feed propellant from the boosters to the central core. So after the boosters do their job and separate, the core will still have almost a full load of fuel. This feature, which can be disabled for low-weight mission, should give the Falcon Heavy the performance of a three-stage launcher. All of this means that the rocket will carry twice the load of a Delta IV at only a third of the cost.

Engineers at SpaceX have also kept possible manned missions in mind. The new rocket meets or exceeds NASA human-rating standards. For example, it has structural safety margins that are 40% higher than anticipated flight loads rather than just 25%, and avionics are triple redundant.

© 2012 Penton Media, Inc.