The future and the past: comparing Dragon and Orion
by R.D. Boozer
|After separation, the Orion’s launch abort system—its shroud and solid rocket booster—is dead weight. It’s thrown away, never to be used again. That’s an expensive waste of valuable equipment.|
The Dragon 2 uses a modern “pusher” escape system. Rocket engines are actually built into the spacecraft with the exhaust exiting near the bottom of the capsule. If the system is used in an emergency escape, these engines will whisk away the human crew to a safe distance from a booster that is about to explode. In that case, the spacecraft will then reenter Earth’s atmosphere and land by parachute. If no emergency situation occurs, the rocket engines will eventually be used to have the spacecraft land at a predetermined point with helicopter-like accuracy.
The escape mechanism on Orion is an upsized version of the Apollo-style “puller” or “tractor” type system used by NASA in the 1960s. At launch, the Orion spacecraft is covered by a shroud, above which is mounted a solid rocket booster. In case of an emergency abort, the solid rocket booster accelerates the Orion away from the booster at high speed. Successful boost to orbit or not, the shroud and solid rocket booster are a potential danger. The shroud needs to separate from the spacecraft after the crew is a safe distance away from the rocket. If they do not separate, they can imperil the crew by preventing a safe reentry, a life-threatening crisis that cannot occur with the Dragon 2.
After separation, the Orion’s launch abort system—its shroud and solid rocket booster—is dead weight. It’s thrown away, never to be used again. That’s an expensive waste of valuable equipment. What’s more, the Orion is designed to make an old Apollo-style splashdown at sea, a strategy that requires an extremely expensive open-sea recovery by a large ship with a full crew plus planes and helicopters to search for the Orion’s chancy landing location. While crewed Dragon missions will initially also splash down in the ocean, they will eventually take a far less expensive approach: retrorocket touchdown at a landing and service facility—a spaceport or launch pad—conveniently located on shore.
Dragon v2 is designed with reusability in mind. Though NASA insists on a new Dragon spacecraft on each NASA sponsored flight, it is possible for SpaceX to reuse a Dragon with little or no refurbishment between flights. SpaceX could use a second-hand Dragon v2 spacecraft for private industry missions to orbit at a fairly low price, since it will be a reused spacecraft.
A Dragon spacecraft being prepared for its pad abort test. (credit: SpaceX)
Some people will say this is really not a fair comparison. After all, Orion is to be a deep space vehicle that is capable of going to the Moon, an asteroid, or Mars, but Dragon v2 is only going to the ISS. I would answer that while the contracted destinations for each spacecraft are radically different, the actual capabilities of the two spacecraft are more similar than one might think.
SpaceX designed the Dragon’s thermal protection system (TPS) to perform far beyond what it needs for orbital reentry. Instead, its heat shield is made to withstand the extreme high heat and stresses associated with an atmospheric reentry from a direct return to Earth from Mars. To accomplish this feat it relies on a modern heat shield substance called PICA that was developed by NASA and improved upon by SpaceX.
|Neither Orion nor Dragon v2 can be used on a crewed interplanetary mission to a near Earth asteroid or Mars by themselves.|
Orion uses the antiquated AVCOAT heat shield material that was used on the Apollo capsules in the 1960s. That system was designed to withstand the lower reentry temperatures and pressures associated with a return from the Moon. This made some sense as far as Orion’s original mission was concerned (that was getting astronauts to the Moon and back), but it makes no sense for Orion’s new role as a spacecraft to go beyond the Moon. In fact, Lockheed Martin (prime contractor for Orion) is unsure whether or not Orion’s old style AVCOAT TPS can withstand the harsher conditions of a faster reentry from deeper space without a significant “enhancement” to the heat shield. In the company’s own words, “Reentry velocities are 11.05 to 11.25 km/s for asteroid missions, vs 11.0 km/s for lunar return. TPS enhancement may be required depending on the ultimate capability of Orion lunar TPS.”
Concerning another issue relating to Orion’s heat shield and missions farther than the Moon, NASA’s own Inspector General related:
“Cracking of Heat Shield. The MPCV applies a material known as AVCOAT, which was also used on Apollo spacecraft, to the capsule’s heat shield to serve as a protective barrier during re-entry into the Earth’s atmosphere. Unfortunately, the material has shown tendencies to crack under thermal conditions similar to those the capsule will experience during the mission in the deep space environment prior to reentering the Earth’s atmosphere… If the heat shield cracking degrades below mission requirements, the Program’s schedule will be impacted while officials develop a solution.”
It should be noted that the asteroid mission referred to in the above-mentioned Lockheed Martin statement would be one where astronauts would be flying far beyond the Moon, but not as far as Mars. This is radically different from the proposed Asteroid Retrieval Mission or ARM, where a robot would bring a small piece of an asteroid to the vicinity of the Moon for astronauts in Orion to visit and bring samples back to Earth. Unlike a mission to a more distant asteroid, there is no doubt Orion’s heat shield can handle an ARM reentry because it would be doing the type of lunar return for which AVCOAT was originally designed.
There is an argument to be made on behalf of Orion. With the addition of a European-built service module, the Orion can support a crew for a far longer time than a Dragon v2. There is no reason that SpaceX cannot develop a service module that would equal or better the capabilities of anything Orion can do, but SpaceX currently has announced no plans to do so.
Neither Orion nor Dragon v2 can be used on a crewed interplanetary mission to a near Earth asteroid or Mars by themselves. Each would require an additional habitat to provide the extra living space, life support systems, and radiation protection needed for such a spaceflight. There is no money in NASA’s budget for such a habitat and currently no firm plans for one.
|As long as short-term regional advantages are given precedence over what is the best deal for the country at large, Orion will continue to be the expensive and obsolete fiscal black hole that it is.|
Even though NASA has contracted SpaceX for Dragon v2 flights to the ISS, there is no reason why it could not be used for crewed missions around the Moon in back. SpaceX CEO Elon Musk says he plans to execute such a flight sometime in the near future. The new Falcon Heavy launch vehicle, scheduled to perform its first launch later this year, will be the most powerful rocket since the Saturn V of the 1960s and more than capable of sending a crewed Dragon v2 on such a moon circling flight.
Considering the billions already spent on Orion and what is projected to be spent in the future (an estimated total of $16.5 billion) versus the small fraction of that amount ($2.6 billion in the latest NASA commercial crew contract) space by NASA for Dragon v2, the capabilities of the latter are much more impressive. The difference in cost is money that NASA could use to aggressively pursue truly up-to-date deep spacefaring capabilities such as propellant depots, solar electric propulsion, the advanced NAUTILUS-X interplanetary spaceship, and more. But the Dragon v2 doesn’t give pork-loving politicians the “bring home the bacon” billions of Orion. As long as short-term regional advantages are given precedence over what is the best deal for the country at large, Orion will continue to be the expensive and obsolete fiscal black hole that it is. Thus we see the reason why Orion’s detractors often say that MPCV should stand for “More Politically Correct Vehicle”.