An obvious next step in the effort to dramatically lower the cost of access to low Earth orbit is to explore non-rocket options. A wide variety of ideas have been proposed, but it’s difficult to meaningfully compare them and to get a sense of what’s actually on the technology horizon. The best way to quantitatively assess these technologies is by using Technology Readiness Levels (TRLs). TRLs are used by NASA, the United States military, and many other agencies and companies worldwide. Typically there are nine levels, ranging from speculations on basic principles to full flight-tested status.

The system NASA uses can be summed up as follows:

TRL 1 Basic principles observed and reported
TRL 2 Technology concept and/or application formulated
TRL 3 Analytical and experimental critical function and/or characteristic proof-of concept
TRL 4 Component and/or breadboard validation in laboratory environment
TRL 5 Component and/or breadboard validation in relevant environment
TRL 6 System/subsystem model or prototype demonstration in a relevant environment (ground or space)
TRL 7 System prototype demonstration in a space environment
TRL 8 Actual system completed and “flight qualified” through test and demonstration (ground or space)
TRL 9 Actual system “flight proven” through successful mission operations.

Progress towards achieving a non-rocket space launch will be facilitated by popular understanding of each of these proposed technologies and their readiness level. This can serve to coordinate more work into those methods that are the most promising. I think it is important to distinguish between options with acceleration levels within the range human safety and those that would be useful only for cargo. Below I have listed some non-rocket space launch methods and my assessment of their technology readiness levels.

Spacegun: 6. The US Navy’s HARP Project launched a projectile to 180 km. With some level of rocket-powered assistance in reaching stable orbit, this method may be feasible for shipments of certain forms of freight.

Spaceplane: 6. Though a spaceplane prototype has been flown, this is not equivalent to an orbital flight. A spaceplane will need significantly more delta-v to reach orbit than a suborbital trajectory requires.

Orbital airship: 2. Though many subsystems have been flown, the problem of atmospheric drag on a full scale orbital airship appears to prevent this kind of architecture from reaching space.

Space Elevator: 3. The concept may be possible, albeit with major technological hurdles at the present time. A counterweight, such as an asteroid, needs to be positioned above geostationary orbit. The material of the elevator cable needs to have a very high tensile strength/mass ratio; no satisfactory material currently exists for this application. The problem of orbital collisions with the elevator has also not been resolved.

Electromagnetic catapult: 4. This structure could be built up the slope of a tall mountain to avoid much of the Earth’s atmosphere. Assuming a small amount of rocket power would be used after a vehicle exits the catapult, no insurmountable technological obstacles stand in the way of this method. The sheer scale of the project makes it difficult to develop the technology past level 4.

Are there any ideas we’re missing here?