One of the product areas that Altius has been looking at over the past months is the idea of small reusable reentry vehicles for bringing samples back from the ISS and eventually other destinations like Bigelow’s facilities. A recent blog post over at NasaWatch contained a presentation about some NASA JSC internal technology development ideas, and one of them was actually very similar in several aspects to the concept for a Reusable Micro-Reentry Vehicle (R-μRV) we’ve been batting around at Altius. If you’re reading along in the JSC presentation, we’re looking at page 18.
Key Idea: Biting Off One Piece at a Time
I won’t get into the business side of the discussion in this blog post, but we at Altius feel there’s going to be a real need for frequent small package deliveries to and from an orbital research station like ISS that can’t be met entirely using larger vehicles like Dragon or Boeing’s CST-100. However, trying to solve the whole problem at once is a lot riskier and requires a lot more up-front capital. Like the JSC team, we realized that splitting the delivery and return phases of the mission could allow us to tackle a much smaller subset of the total problem space first.
The concept we independently came up with is illustrated in this picture excerpted from the JSC presentation:
- The μRV is launched inside one of the ISS cargo delivery vehicles (Progress, HTV, ATV, or either of the COTS vehicles), and is brought inside the station like all other pressurized cargo. This allows us to leverage existing proximity operations and rendezvous and docking capabilities at first, and initially avoid the challenge of getting approval for station rendezvous and docking from the ISS Visiting Vehicles group. The theory is that getting approval for departure should hopefully be much easier than approval for arrival.
- The μRV is loaded internal to the station, and then exits the station via the Japanese Experiment Module (shown below). The JEM has a small “equipment airlock” is designed for passing experiments and hardware out of the main Kibo module onto a sort of “back porch” Exposed Facility. It has a slider table (which you can see inside the airlock on the right), that you can attach things to from the inside of the ISS, and then when the door is closed and the airlock vented, the slider table can extend outside the station.
- The μRV departs the station propulsively using a non-toxic propulsion system. Both JSC and Altius came to the same solution of using a Tridyne (well technically Nitro-dyne) warm-gas system. “Tridyne” is a gaseous mixture of dilute amounts of hydrogen and oxygen in a suitable buffer gas (Helium or Nitrogen). The idea is the hydrogen and oxygen are at such low concentrations that the mixture is not flammable in air, and the two components cannot ignite due to the buffer gas, except in the presence of a suitable catalyst. The resulting “flame” is cooler than even a H2O2 monopropellant thruster, with corresponding worse performance–somewhere in the 120-140s vacuum Isp. While you wouldn’t want to use this for a high-delta-V mission due to poor achievable propellant tank to propellant mass ratios and low Isp, it is probably adequate for the small station departure and deorbit burns (100-150m/s according to the presentation). The complexity of the system ends up being even less than a hydrazine monopropellant system (no need for heaters, pressurization systems, etc) and avoids toxicity issues.
Those were the main ideas our concept had in common with the JSC approach.
Some of the key differences in approach revolved around the size and TPS approach. The concept we were looking at was significantly smaller (roughly NanoRack sized) that would use a derivative of the microcontroller-based avionics system we’re developing for the Ventions contract. We were also planning on using a robust, low-maintenance, reusable TPS system we’ve been investigating with HMX and Ventions instead of more traditional ablative TPS options.
There were some other innovations we came up with, which we aren’t ready to talk about yet, but the main point of this post was that it was nice to see that JSC personnel think that the general approach of handling the μRV inside the pressurized volume of the ISS looks workable from a safety standpoint. I had originally been concerned that even Tridyne might not be feasible for use in a system that had to be moved around inside the ISS pressure volume, but it’s good to get some validation that that is an acceptable approach.