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Dragon capsules

February 3, 2014

The current version of the SpaceX Dragon capsules, including the one that historically became the first commercial spacecraft to dock with the ISS, are designed to splash down in water, like those used in NASA’s Mercury, Gemini and Apollo programs and like NASA’s new Orion capsule.

The next generation of Dragon capsules are designed to land on solid ground. Currently in development at SpaceX, they are fitted with eight “SuperDraco” engines, which are a powerful new variation of the Draco engines used by the Dragon RCS (Reaction Control System). Like the Dracos, they use non-cryogenic propellant: monomethyl hydrazine fuel and nitrogen tetroxide oxidiser. However, they’re much more powerful, each capable of delivering about 67 kilonewtons of axial thrust, for a total of about 534kN. These engines will enable the Dragon to land propulsively on solid ground, usually back at the original launchpad, thereby saving the time and expense of water recovery and opening up the possibility for Dragon capsules to land on  the Moon, Mars and other worlds with solid surfaces. This is in alignment with SpaceX CEO Elon Musk’s stated purpose of establishing settlements on Mars.

SpaceX Dragon capsule landing on Mars

SpaceX Dragon capsule landing on Mars

DragonRider

SpaceX offer two basic configurations for the Dragon capsules: cargo and crew. The crewed version is known as a “DragonRider”, and can accommodate up to seven astronauts. These may be used for transporting crew between Earth and the ISS in the near future.

In the Blue Dragon architecture, which is designed for a crew of six, the seventh seat is removed and the volume that it (and a seventh person) would normally occupy is reserved for cargo. This may be last minute personal items from Earth, or samples from Mars. All three DragonRiders in the architecture will be modified to accommodate six people plus storage.

DragonRider capsule with 7 crew

DragonRider capsule with 7 crew

DragonRider capsule with 7 crew

DragonRider capsule with 7 crew

Red Dragon

Red Dragon” is a proposed variant of the SpaceX Dragon capsule currently being investigated by NASA as a low cost alternative for delivering payloads to Mars (Karcz et al., 2012).

Red Dragon will presumably be similarly configured with SuperDraco engines. Alternatively, they may utilise new methane-fuelled “Raptor” engines being developed at SpaceX, which would have the advantage that they could be refuelled on Mars. In addition, Red Dragon will incorporate several modifications necessary for EDL on Mars, including:

  • Removal of systems unique to LEO missions, such as berthing hardware.
  • Addition of deep space communications.
  • Modifications to SuperDraco (or Raptor) engines to suit the Martian atmosphere.
  • Reduction of heat shield thickness, since the atmosphere is far less dense.
  • Algorithms and avionics for pinpoint landing on Mars.

The gravity on Mars is lower, which reduces the acceleration of the capsule towards Mars; however, in the case of direct entry the capsule will be approaching from interplanetary space at a much higher velocity than if it were descending from orbit. Also, Mars’ atmosphere is much thinner (less than 1%) than Earth’s, so it will play less of a role in reducing spacecraft velocity during EDL. However, for the same reason, there is less heating due to atmospheric friction, and therefore less or, perhaps, different heat shield material may be used. The different conditions will affect the forces experienced by the spacecraft, which may require changes to thrusters, heat shield, avionics and other aspects.

NASA have calculated that a Red Dragon capsule will be capable of delivering payloads of up to 1.9 metric tonnes to the surface of Mars. This delivery mechanism has been receiving increasing attention from NASA, being considerably simpler and cheaper than, for example, the sky crane method used to deliver Curiosity. Not only will it be cheaper per kilogram of payload mass, but much cheaper overall.

Another clear advantage is that a landed capsule can be repurposed as a storage unit, shelter or habitat.

Once the Red Dragon technology has been proven as a reliable mechanism for delivery of cargo, this approach may be used to deliver up to seven crew members to Mars surface, simply by using a DragonRider modified in the same way.

Red Dragon represents a near term technology that can enable comparatively inexpensive and functional Mars missions. It’s a fundamental element of the Blue Dragon architecture (hence the name), being utilised for delivery of both crew and cargo to Mars surface.

The Dragon capsules are being designed to land with a high degree of accuracy. From the SpaceX website:

“SuperDraco engines will power a revolutionary launch escape system that will make Dragon the safest spacecraft in history and enable it to land propulsively on Earth or another planet with pinpoint accuracy.”

This ability to land “with pinpoint accuracy” is supported by the Dragon’s GNC (Guidance, Navigation and Control) system. Due to the lack of GPS (Global Positioning System) on Mars, high-accuracy landings must be achieved using alternate methods. However, this problem has effectively been solved. For example, ESA (European Space Agency) have been developing a system known as “LION” (Landing with Inertial and Optical Navigation) that will enable pinpoint landing on the Moon, Mars and asteroids using image recognition of major landmarks (Delaune et al., 2012). Another important development is the Fuel Optimal Large Divert Guidance (G-FOLD) algorithm (Acikmese et al., 2012), able to autonomously calculate landing trajectories in real-time. This was recently tested successfully with Masten Space System’s Xombie VTOL experimental rocket, with the vehicle making a 750 metre course correction in real time. Considering these developments it’s reasonably safe to assume that the Red Dragon will be capable of pinpoint landings on Mars by the time we begin sending them. Because the position of landed base components can be known with precision, a neat, safe and optimised layout of the base can be designed beforehand.

Red Dragon potentially represents a mechanism for delivering cargo or crew to the surface of Mars that is not only repeatable, but affordable. SpaceX currently charge $135M for a Falcon Heavy launch including the Dragon capsule. Making use of COTS and other pre-developed hardware it may be possible to develop and deliver a payload to Mars for under $250M. This is a mere one tenth of the $2.5B Curiosity rover.

Once SpaceX have developed their RLS (Reusable Launch System) for the Falcon Heavy – a goal likely to be achieved within a few years, considering the recent Grasshopper tests, and thus well before the first H2M mission – this price will come down even further.

5-metre capsules

Dragon capsules have a diameter of 3.7 metres. However, the architecture for the Mars One mission, which proposes to send 24-40 astronauts on a one-way mission to Mars, proposes to rely on a larger, 5-metre-diameter Dragon capsule for habitat modules. Although these are yet be to be built or demonstrated, their plan is to land the first two of these on Mars in 2020, which is only seven years from the time of writing.

It could perhaps be inferred that plans exist at SpaceX to have these larger Dragon capsules operational and available within seven years. This is well within the timeline of Blue Dragon. However, SpaceX and Mars One do not have a formal association so there is no real evidence of this yet, and as no information about these larger capsules is currently available, the Blue Dragon architecture does not presently include them. This may change if more information becomes available.

Ice Dragon

NASA have commenced studies of a mission to Mars based on the Red Dragon landing system, which may be flown as early as 2018. Known as “Ice Dragon” (Stoker et al., 2012), it’s being developed in collaboration with SpaceX, and will deliver a science package to Mars including a drill that will penetrate up to two metres into the permafrost to investigate environmental conditions suitable for past or extant life.

There are six objectives currently envisaged for Ice Dragon:

  1. Determine if life ever arose on Mars.
  2. Assess subsurface habitability.
  3. Establish the origin, vertical distribution and composition of ground ice.
  4. Assess potential human hazards in dust, regolith and ground ice, and cosmic radiation.
  5. Demonstrate ISRU for propellant production on Mars.
  6. Conduct human relevant EDL demonstration.
Ice Dragon showing drills. (Human figure shown for scale only)

Ice Dragon showing drills. (Human figure shown for scale only)

Besides the scientific outcomes of the mission, which will certainly be of tremendous value to human missions, one of the most important contributions of Ice Dragon will be demonstration of the EDL capabilities of the Red Dragon capsule.

References

B. Acikmese, J. Casoliva, and J. M. Carson III, “G-FOLD: A Real-Time Implementable Fuel Optimal Large Divert Guidance Algorithm for Planetary Pinpoint Landing,” Concepts and Approaches for Mars Exploration, 2012.

J. Delaune, G. Le Besnerais, M. Sanfourche, T. Voirin, C. Bourdarias, and J. Farges, “Optical Terrain Navigation for Pinpoint Landing: Image Scale and Position-Guided Landmark Matching,” Proceedings of the 35th Annual Guidance and Control Conference, 2012.

J. S. Karcz, S. M. Davis, M. J. Aftosmis, G. A. Allen, N. M. Bakhtian, A. A. Dyakonov, K. T. Edquist, B. J. Glass, A. A. Gonzales, J. L. Heldmann, L. G. Lemke, M. M. Marinova, C. P. Mckay, C. R. Stoker, P. D. Wooster, and K. A. Zarchi, “Red Dragon: Low-Cost Access to the Surface of Mars Using Commercial Capabilities,” Concepts and Approaches for Mars Exploration, 2012.

C. R. Stoker, A. Davila, S. Davis, B. Glass, A. Gonzales, J. Heldmann, J. Karcz, L. Lemke, and G. Sanders, “Ice Dragon: A Mission to Address Science and Human Exploration Objectives on Mars,” Concepts and Approaches for Mars Exploration, 2012.

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