SupeDraco

Advantages Of Liquid Rocket Propellants

The thruster we will see in action on SpaceX’s next launch on March 1st will be the Draco. The Draco thruster is the smallest engine in the SpaceX fleet but don’t let the size fool you, it packs 90 pounds (400 N) of thrust. The Draco is a liquid propellant thruster that uses Monomethyl Hydranzine. There is an oxidizer needed with a liquid rocket engine and SpaceX uses Nitrogen Tetroxide, the combination of orbital propellant and oxidizer that were used for the Space Shuttle.

SpaceX went with a liquid fuel rocket because, while the thruster design is more complex, the advantage is variable thrust meaning the amount of fuel and the fuel burn rate can change during flight. Liquid fuel rocket engines can not only be throttled but are able to shut down and be restarted. Having so many options for throttle and restart are helpful in a redundancy situation. It also aides in maneuvering the precise approach required for to berth with the International Space Station (ISS). The combination of the propellant and the oxidizer keeps the fuel stable allowing the Dragon capsule to be berthed to the ISS for up to a year, providing a life boat of sorts for our cosmonauts.

The Dragon qualification unit being outfitted with test Draco thruster housings. Photo by SpaceX.
The Dragon qualification unit being outfitted with test Draco thruster housings. Photo by SpaceX.

The Draco thruster was first tested in 2008 where it underwent a 10-minute continuous fire, shutdown and then restarted and fired for an additional minute. It completed qualification testing in April 2009 completing 42 firings with over 4,600 pulses of varying lengths. The test time was 50 minutes on a single thruster chamber. Draco thrusters were on the Dragon spacecraft, and fired, during the COTS 1 demo mission SpaceX conducted for NASA in December 2010. During the SpaceX CRS-1 mission in May 2012 the Draco engines fired over 2 1/2 days as it climbed higher in orbit and caught up with the ISS.

There are up to 18 Draco thrusters on the Dragon spacecraft, depending on the mission. The thrusters are used for steering, attitude control, thrust back towards earth to initiate de-orbit burn, and re-entry trajectory. So the Draco thrusters preform a several minute de-orbit burn and then pulse, on the order of 10 milliseconds, to control attitude during re-entry trajectory. The are aligned in a dual-redundancy on all axes so any two can fail on any axis and the Dragon will still be fully functional.

Illustration of the Dragon capsule engaging the launch escape engines. Image by SpaceX.
Illustration of the Dragon capsule engaging the launch escape engines. Image by SpaceX.

The SuperDraco is currently being developed as SpaceX prepares for NASA’s Commercial Crew Program. The SuperDraco will replace the current version of the Draco engine. Eight SuperDraco engines will be built into the side of the Dragon spacecraft and will produce up to 120,000 pounds of axial thrust. This advancement is getting the Dragon equipped for an escape system for astronauts. The launch escape engines use the same propellant that would’ve been used for orbital maneuvering. When in use for escape, the SuperDraco engines preform a deep throttle and propel the Dragon away from the rocket. Escape can take place all the way to orbit. The SuperDraco is also planned to be used for propulsive landings of the Dragon spacecraft.

 

 

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