The Space Reviewin association with SpaceNews

Dragon approaching ISS
Dragon, seen here flying near the ISS last week, could support research into artificial gravity and other techniques to support a humanMars mission. (credit: ESA/NASA)

DragonLab-g: an early step to Mars and beyond

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Dragon’s (so far) successful flight has opened many opportunities in space. Its small size, low cost, and plans for frequent flights allow it to play a role in many facets of human exploration in space, including to help determine if humans can live on Mars.
Instead of trying to adapt the human body to microgravity, why not simply generate artificial gravity along the way?

While current efforts in space are being slowed by debates over costs, preferred methods, and destinations, there are bigger concerns for long-term space habitation. Winds of political will change for better or for worse in the span of a couple years, but two constant challenges await anyone who ventures beyond the atmosphere for a long period of time: the lack of gravity, which changes physiology with especially strong impacts on muscles and bones; and radiation, which raises the risk of cancer and has other effects over time.

The lack-of-gravity question has been the focus of NASA’s research since the end of the Apollo program. Through years of on-orbit research in the form of exercise routines and attempted medical interventions, the space agencies of the world have tried to understand the human body’s reaction to microgravity, and to find a way to counteract it.

There are those who say research so far has been trying to answer the wrong question. Instead of trying to adapt the human body to microgravity, why not simply generate artificial gravity along the way? As early as 1966, astronauts (Gemini 11 and 12) did just that, attaching tethers to their Agena docking targets, then backing away and spinning the combined spacecraft end over end. Gravity levels were low, to the point where astronauts could not feel any (artificial) gravity but a pen could be seen accelerating in the expected direction. There were some unexplained tether dynamics during deployment and spinup, but the overall approach worked. Future spacecraft such as Apollo, Skylab, Shuttle, and ISS were not built for such demonstration, and interest in the field had waned anyway.

An example of the use of artificial gravity for a mission to Mars would involve a crew capsule separating mechanically from the stage that sent it on its way from Earth. The craft would remain connected to its upper stage by a tether. Using onboard thrusters, the crew capsule would maneuver itself away from the upper stage while inducing an end-over-end spin in the system. Eventual rotation speed depends on the tether length, difference in mass between the crew capsule and the upper stage, and the desired gravity field. The system would continue to spin while en route to Mars. As it approached the Red Planet, the crewed section would separate from the upper stage, allowing precise control of the craft for Mars orbit entry and landing.

While anyone can demonstrate artificial gravity by swinging a bucket of water in a circle without spilling it, in space travel there are several questions that the Gemini missions did not answer:

  1. Best spin-up method: The most significant step for any rotating system is spinning up. The crew capsule and upper stage start out mechanically connected in preparation for the launch stresses, and end up spinning to generate artificial gravity. The transition between these two states is not simple, and needs to be demonstrated and optimized.
  2. Tether dynamics: Anyone who’s worked with rope in a dynamic system knows how variable they can be. Simple changes, like the spring constant of the line,can change the way a system responds in many ways. Deployment mechanisms, damping systems, and methods for operating a tethered system with one active control unit need to be evaluated.
  3. Tether design: Tethers have seen some use in space missions over the years, but none has flown with the required strength for this type of mission. Another consideration is the tether’s toughness. One micrometeorite hit could separate a crewed vehicle from its counterweight if a single-strand tether was used.
  4. Power: It is likely that most or all of the power supplied to the vehicle will be provided by solar panels. Solar panels require proper solar pointing to provide power.
  5. Velocity change maneuvers: While any spacecraft sent to Mars would have a very precise trajectory, changes may be required as its course is refined. One of the more intriguing aspects of tethered systems is how they would change their velocity along the three axes of motion. Some methods have been suggested, and some have been demonstrated on a small scale in the lab, but more information is necessary.
  6. Attitude change maneuvers: Related to velocity change maneuvers, attitude changes can impact power and communications for the crew. If a tethered system with a single active side is assumed, attitude change maneuvers may impart a change in velocity as well.
  7. Communications: A rotating craft will have difficulty maintaining consistent data rates with the ground. Certain antennas do not require pointing directly at their target, but the tradeoff is low communications rates. Other, more efficient, antennas require more precise pointing but allow much greater data flow.
  8. Separation dynamics: As the crew approaches Mars, they will need to leave their upper stage behind, giving their capsule the maneuverability and simplicity necessary for Mars operations, but returning them to microgravity. A tether under tension contains a lot of energy, and the simple act of spinning means that the crewed vehicle will be sent off in a straight line the moment a tether is cut while spinning.
While many people decry the retirement of the Space Shuttle, it wasn’t suitable for analyzing gravity or radiation questions.

The radiation question has also been a focus of space agency research over the past decades. However, the radiation environment of LEO (low Earth orbit), where most of this research has been performed, is different than what crews traveling beyond LEO will experience. The International Space Station (ISS) is well protected from some radiation dangers by Earth’s magnetic field. That field extends beyond LEO but loses most of its effectiveness beyond geosynchronous orbit. Getting the answers to the correct radiation questions will involve travel and long-term exposure beyond Earth’s magnetosphere.

Some questions that need to be answered related to the radiation issue include:

  1. What shielding method best balances mission needs and crew safety?
  2. What are the effects of long (six months or more) exposures to the solar system radiation environment?

While many people decry the retirement of the Space Shuttle, it wasn’t suitable for analyzing gravity or radiation questions. Returning to small spacecraft allows such analysis to begin again. The craft that recently flew to the ISS is also the craft that can support both. That craft is named Dragon, and its derivative DragonLab. These spacecraft, and their second stages, provide an almost off-the-shelf solution to two of the nagging problems of long-term spaceflight. (Efforts to contact SpaceX about this idea have not been successful, although the company has been a bit busy lately.)

Part of the beauty of this proposal is the stepwise approach it allows, while also offering the possibility of meeting mission objectives with a minimum number of flights. While any of these missions could be the start, one sample progression of missions includes:

  1. DragonLab-g1, LEO, uncrewed: The Dragon capsule launches on its own. The payload can be as simple as an automated flight control system (as flew in December 2010 and to ISS), or can have animals added to it to demonstrate changes the differences between animals’ response to a partial gravity environment and a microgravity environment. The craft achieves orbit, separates from the upper stage save for a tether connection, performs its artificial gravity demonstration, and returns to Earth.
  2. DragonLab-g2, LEO, crewed, ISS proximity: This mission is useful if a flight is desired before SpaceX brings their crew escape capabilities online. The DragonLab capsule flies to the ISS, keeping the second stage attached. The assembly is berthed to the ISS, per a typical cargo mission, with the exception that the second stage is still attached. Some ISS crewmembers enter the capsule, the assembly separates from the ISS, performs its gravity demonstration, then returns to the ISS to drop the crew off. The capsule can then be used for downmass or any other activity a Dragon capsule is capable of.
  3. DragonLab-g3, LEO, crewed, solo operation: DragonLab launches with a crew on board. Once in orbit, they perform their gravity demonstration, and return to Earth when complete.
  4. DragonLab-g4+, orbits higher than LEO, uncrewed or crewed: It is assumed that these missions will not stop at the ISS, because they require the DragonLab to be in LEO with an upper stage containing propellants. The missions are similar to earlier versions, only the primary location for the gravity demonstration is at a higher altitude than LEO, allowing tests of longer-term radiation effects on tissues while exposed to a gravity field. Example destinations include a high Earth orbit (guaranteed return to Earth), or an Earth/Moon Lagrange point for missions that want the flexibility to set how long they will fly.
The cost of DragonLab missions put them in reach of companies or even wealthy individuals who want to have a lasting, positive impact on humanity’s expansion into the cosmos.

Modifications are required to flight hardware to enable these missions. Two obvious ones include strengthening the solar panels on the Dragon spacecraft and a method of rapidly passifying the Falcon 9 second stage. The solar panels are meant to be deployed in zero gravity, and only need to withstand accelerations provided by the Draco thrusters during flight. Exposure to any gravity field will likely require some sort of structural change. The Falcon 9 second stage requires passification to prevent the eventual explosive loss of propellants. This would be particularly important for operations near the ISS. Current orbital debris mitigation requirements may have driven these changes already, though timing them to take place early in a mission, perhaps with crew on board an attached Dragon capsule, will likely be new.

As stated before, artificial gravity research hasn’t been a priority with any space agency since the 1960s. It’s likely that an agency might not abandon decades of research in one direction to start off in another. This may be irrelevant since the cost of DragonLab missions (several tens of millions of dollars, though a dedicated mission would have to foot the bill for a Falcon flight into orbit as well) put them in reach of companies or even wealthy individuals who want to have a lasting, positive impact on humanity’s expansion into the cosmos.