Pictured at right is one of the great inventions of the twentieth century, the Lego block. The basic blocks, patented in 1958 and still compatible with pieces made today, allowed kids to build and rebuild their own toys - and the building and redesign itself was the play activity. Over the years Lego added new pieces - different dimensions, gears, axles, wheels, figurines, and so on - all compatible with earlier designs, and each new design enabling another infinity of possibilities for play. The results can be astonishing.
These stepping stone technologies are very similar to the iterations of Lego block designs. Each stepping stone allows a broad range of new capabilities, and builds on the prior capabilities developed. And just as a single Lego piece by itself is not particularly impressive, the development of these stepping stone technologies by themselves are not nearly as lofty a goal as "Apollo on steroids". Instead the primary goal of these technologies is to provide logistical (and hence economic) leverage and jumpstart the space industry, enabling sustainable human expansion through cislunar space and then the rest of the solar system: more Bang for the Buck Rogers.
I have tried to categorize the stepping stone technologies below. For some of these stepping stones it makes sense to wait until other stepping stones are in place before beginning major work and bending metal. Others can be started right away or are already being worked on by NASA and/or the space industry. This list is not exhaustive, but I figure it's a good starting point for discussion if nothing else. There are non-technological stepping stones, too, but that's a topic for another blog post.
from Earth to Low Earth Orbit and back
- reusable liquid-fueled unmanned glide-back auxiliary boosters - These would replace the current strap-on solid rocket motors which provide an extra boost to rockets while in the atmosphere. Having these strap-ons helps deliver a bigger payload to orbit than possible with the core rocket by itself. Making them liquid-fueled means quicker turnaround time. UAV technology has come a long way over the last ten years as a result of warfare, but it can also be used to pilot these glide-back boosters. Having them glide back instead of splashing down eases recovery. Reusing them allows categorization of patterns of wear and highlights faults for further iterations, as well as spreading production costs over multiple launches.
- recoverable / reusable rocket first stage - SpaceX is already hard at work on this and plans on doing it with the Falcon 9. Splashdown recovery is more difficult than a glide-back strapon stage, but no more difficult than recovering a shuttle SRB. Reuse of this stage should reduce the cost of access to orbit as long as refurbishment costs are low and turnaround time is reduced.
- low maintenance thermal protection system - This is a key to the economical re-use of reentry hardware. It was also a big part of the cost of operating the shuttles. If the thermal protection system could be robust enough to withstand dozens of reentries before replacement, or was cheap and easy to replace each time, the turnaround time and manpower required would be greatly reduced.
- intermodal transport interface - load a shipping container inside the frame (which also houses solar panels, radiators, GN&C), spin and vibration test it, add a faring, put it on the next available rocket, and go. This enables orbital access to the existing worldwide supply chain. Once a design is up to TRL-11, shipping cargo to space will require much less handling, have higher efficiency, quicker throughput rate, and lower cost. (The intermodal transport container is itself a stepping stone technology, conveniently already in widespread use.)
- tractor (tug) - This would be the cargo workhorse of cislunar space. It would have everything a regular satellite has (propulsion, guidance, navigation, control, power, temperature regulation, communications, propellant tanks, perhaps a robot arm) - the only thing it would lack is a payload. Instead it would couple itself to other orbital assets and perform tasks like proximity operations, transporting propellant to geosynchronous satellites, acting in lieu of an astronaut in teleoperated procedures, and many other tasks. Its function is similar to that of a farm tractor, semi-truck, or tugboat.
- bus - These vehicles would never land, only change orbits and dock. They wouldn't need to deal with the stresses of ascent or reentry, wouldn't need landing gear, wouldn't need aerodynamics. It could be as simple as an inflatable habitable volume (like the Bigelow modules), propellant tanks, and a tractor (snapped together like Lego pieces, perhaps?)
- lunar lander - This would only travel from lunar orbit (perhaps at L1?) to the lunar surface and back. There might be different types of landers for different sized jobs. These would be refueled at a propellant depot in Lunar orbit
- pod - "Open the pod bay doors, HAL." A pod is a one-man spaceship with a spherical pressure vessel and several remote manipulator arms. Such a craft would allow an astronaut to wear a minimalist space suit for emergencies or very temporary sorties, but spend most of their EVA activity in relative comfort and better protected than in current spacesuits, and eliminate the need for prebreathing. EVA times could be measured in days instead of hours.
- better spacesuits - NASA is already working on this with the astronaut glove prizes, but there is a huge design space to explore. Spacesuit improvement should be a never-ending project, with new milestones set as previous ones are met. And since space is a fairly big place, different environmental conditions (surface gravity, atmosphere) occur which preclude a single design.
- consumables depots - This includes propellant depots (storing liquid Oxygen, liquid Hydrogen, RP-1, Hydrazine, N2O4, Xenon... market demand will sort out the specifics) and depots of other fungible fluid consumables (water, Nitrogen, vodka, whatever the market demands). At first only a few propellants would be stored, but as the industry builds the demand for the other consumables will increase. The existence of the first depots will themselves drive up the rate of rocket launches (of tankers of various capacities filling the depots) and reduce the cost per payload kilogram for destinations beyond LEO. Eventually depots would be established in Geosynchronous Earth orbit and the Lagrange orbits (probably starting with L1).
- 4-, 6-, 8-, 12- or 20-sided universal docking nodes - (the numbers chosen are the number of faces on the Platonic solids) A universal node - able to connect habitable volumes in a geometric pattern with a common interface - is sorely needed if we are to build large habitable structures in space. The current six-sided nodes on the ISS might be considered this stepping stone if the design gets published. ITAR stands in the way of the most basic Lego block.
- bus stations / hotels - Habitable volumes with multiple available docking ports, these are likely to be closely associated with propellant depots. Bus stations would be used for transferring people from one mode of transportation to another. Hotels would themselves be orbital destinations. These could be several Bigelow modules connected by universal docking nodes.
- maintenance facilities - Entropy increases. Stuff breaks down. If you can't fix it, you have to replace it or do without. A maintenance facility would have a storehouse of spare parts and the necessary tools and equipment to repair at least the critical items.
- hangars - If you're fixing stuff in orbit, eventually you'll need to work on something in a shirtsleeve environment which is too big to fit through an airlock. You wouldn't bring a bus back to Earth to repair and relaunch, you'd just fix it in the hangar. A large substantially-leakproof hangar bay door poses some significant technical challenges. This is one stepping stone that will require other stepping stones in place.
- drydock - At some point we will want to assemble very large craft from smaller components. Some kind of large frame with several robot arms on rails would make this a whole lot easier.
- substantially-enclosed life support system - The more enclosed the system is, the less resupply is needed. Being able to recycle CO2 and water and food with an artificial ecosystem eliminates a logistical nightmare and enables very long duration missions far out in the solar system.
- artificial (centrifugal) "gravity" - So far, we know a lot about living in 1 gee (Earth's surface gravity), and have learned about some debilitating effects of long-term exposure to zero gee, and how to mitigate some of those effects. We know absolutely nothing about the effects of long-term exposure to 1/6 gee (the Moon) or 0.38 gee (Mars). We don't know if a baby can develop normally in anything less (or more) than one gee. Many of the side effects of weightlessness would be eliminated if orbital habitations are rotated to produce an artificial centrifugal "gravity". Perhaps this could be accomplished by having the habitation attached to a counterweight by a long tether, and the whole thing rotated. Again, we don't know much about the long-term effects of high angular velocity, so there's lots to be learned here.
- improved radiation shielding - Outside the protection of Earth's magnetic field, the danger from solar events and cosmic rays increases enormously. We need to develop better radiation protection for long-duration missions.
- advanced robotics / teleoperation - Robotics will always be an integral part of space operations. This work is already going on, and like spacesuit improvement will likely remain an indefinitely-continuing project.
- orbital assembly - The ISS taught many lessons about orbital assembly - NASA is far more experienced at this than they are at rocket design. The assembly stepping stone will evolve along with the drydock stepping stone. Personally, I'd like to see modules click together like Legos (not exactly like Legos, but interfacing easily, mix and match as needed).
- orbital maintenance - Whether it involves bringing a crippled satellite in for repairs or fixing it remotely, or just doing minor repairs on a spacesuit, this is a critical cost-saving task.
- orbital fabrication and construction - Eventually we will be shipping raw materials to Earth orbit (from the surface of the moon, or from Near-Earth Asteroids) and then making them into something useful "on-site", such as constructing extremely large (kilometer-scale) rotating habitats. The earlier we figure out how to do things like make I-beams in freefall, the better.
- in-situ resource utilization - producing things like Oxygen and propellant and water from materials found on the Moon, Mars, or asteroids are absolutely critical to reducing the cost of all operations in space and reducing the dependence on a supply line from Earth.
- momentum exchange tethers - These have the potential to provide a propellant-less change in trajectory for orbiting bodies and are definitely worth further examination
- electrodynamic reboost - Again with the tethers. This time, interaction between the Earth's magnetic field and an electric current induced on a long tether can raise the orbit of the tether (and whatever it is attached to). Instead of using propellant to fight orbital decay, electrodynamic reboost steals an iota of the energy of Earth's magnetic field (and solar energy to produce the electric current) to magnetically repel the orbiting tether.
- aerobraking - On a high-velocity return to Earth, aerobraking - temporarily dipping into the atmosphere to bleed off speed - is a propellant-minimizing way of slowing down. It's just like skipping a stone on a pond, with each successive skip at a slower speed. If you can go from a parabolic orbit to a low-eccentricity orbit without using propellant, you're ahead of the game.
- nuclear thermal propulsion - If we are to travel throughout the solar system, chemical rockets aren't going to cut it. Propellant accelerated by the heat from a nuclear reactor can achieve much higher exhaust velocities than by combustion, leading to higher ISP (gas mileage).
- cislunar positioning system - GPS is fine if you're close to the Earth, but far enough out and you'd need some fancy astrogation and starfinders. Satellites at the Lagrange orbits could function as the cislunar equivalent of GPS, easing navigation throughout cislunar space.
- lunar positioning system - as we return to the moon we will need a constellation of positioning/communication-relay satellites orbiting the moon for exactly the same reasons we have them orbiting the Earth.
- x-ray pulsar positioning system (XPPS) - X-ray pulsars are natural broadcast signals all over the sky and far from the solar system. We may be able to use those properties to determine the position and velocity of an object anywhere in the solar system with fair precision. This would greatly simplify solar system navigation - and it is mostly a software problem.
- cislunar traffic control - There are already thousands of satellites and many times that number of debris objects orbiting the earth. As the traffic in low earth orbit and cislunar space increases, some traffic control system will have to grow up alongside the increasing traffic - other wise, as time goes on, collisions will become a greater and greater hazard.
- microwave power beaming - Being able to move energy from one place - say a large solar array - to another (like the Earth's surface or another satellite) absolutely requires power beaming. It is a key to opening up a space-based energy industry that could rival oil or coal or nuclear power on Earth.
- low-maintenance nuclear power plants - If all goes well, eventually we will be moving far out into the solar system, where the sunlight is dim, or perhaps to the equator of the moon with its two-week nights. In these cases, solar power may not be practical. Nuclear power plants that can operate with minimal maintenance open up those areas where the sun don't shine.