I've very interested in the potential of depots in exploration architectures. It allows reusable orbit-to orbit spacecraft to be used for transits to the Moon or Mars, and allows missions that would require superheavy-class launch vehicles without depots to be staged with nothing more than existing or planned launchers like SpaceX's Falcon Heavy. For instance, on an Apollo lunar mission, the total hardware mass was only around 30 metric tons, but the total initial mass delivered to Earth orbit was 127 metric tons or so. This means that with a depot system for storage and transfer of cryogenic propellants, an Apollo-class mission could have been undertaken with 4 launches of a 30 ton vehicle. The lesson here is interesting particularly because of the capabilities of the Falcon Heavy and Falcon 9 Block II rockets (roughly 50 and 15 tons respectively) and their relatively low costs compared with past vehicles.
However, the need to fill such a depot requires a vehicle to serve the depot, a tanker. This vehicle will have to be launched as payload on a rocket, then be maneuvered to dock with depot and offload propellant. This creates two options. The first is a dumb stage, a tank that is inject to an orbit close to the depot, then grappled by a reusable space-only tug and moved to the depot. This makes the tank light, cheap, and ultimately disposable. The other option is a stage that's smart enough to fly itself, eliminating the need for the tug. However, this makes the stage an automated logistics spacecraft and thus expensive. Thus, if possible, it seems that such as "smart" spacecraft should be reused if possible.
In thinking about reuse of a "smart" tanker, I was inspired by the concepts for using modified Delta IV upper stages as depot components like the image shown above. It seemed like fairing over the tank structure could create a capsule-like shape. Covering this with a thermal protective system could allow a ballistic entry into the atmosphere for reuse. However, this require a lot of modifications, since the Delta IV 5-meter upper stage shown as a depot component in that image isn't intended for long-term use or reuse,. Thus, adding proper maneuvering control and thermal protection might be very challenging. Turning a lightweight tank stage into a capsule might be a bridge too far: too hard, too heavy, too expensive.
However, what about turning an existing capsule into a tanker? SpaceX's Dragon had always been my mental image of the final product due to the steep sidewalls that would be dictated by the Delta IV stage design. However, since liquid oxygen has a density of about 1.2 tons per cubic meter, a tank filling the payload margin for a Dragon would also roughly equal the pressurized volume. With a 16 ton-to-orbit Falcon 9 Block II and the 4.5 ton mass cited for Dragon on Wikipedia and in some SpaceX materials, this leaves room for 11.5 tons of LOX, which would have a volume of about 9.5 cubic meters. this fits nicely in the 10 cubic meters of pressurized volume on-board a Dragon. thus, it's possible that the only work needed to turn Dragon into a Dragon LOX Tanker would be to add insulation and an internal tank inside what is currently the pressure hull, and add systems to fill this tank on the ground and empty it into the depot on-orbit. The primary structure could remain largely unchanged, as could the Draco thrusters and the capsule's control systems. Dragon's thermal protective system has already been designed to be capable of multiple flight, so I think it's possible that after factoring in all the costs, this could be a competitive alternative to expendable "dumb tanks" both per-flight and on the more critical $/kg scale.
|Dragon Pressure vessel being integrated with bulkheads for the un-pressurized hardware section containing thrusters and other equipment. In a Dragon Tanker, this pressure volume would contain the LOX tank.|
As to why I suggest a single-fluid tank (LOX only) instead of carrying both a fuel and LOX, there's a couple of reasons. First, while the idea could work with two tanks (one for LOX, one for fuel), kerosene and methane are both less dense than LOX (70% and 37% as dense respectively) so a Dragon spacecraft carrying a full 10 cubic meters of mixed kerosene/LOX or methane'LOX might not be taking full advantage of the launch vehicle's capabilities, Liquid hydrogen is so much less dense (only 6% the density of LOX!) that it's not even worth thinking about. (On a side note, this low density is why despite the fact that only 16% of the mass of a hydrolox rocket's propellant is hydrogen, the hydrogen tank is usually several times the volume of the LOX tank. See the Space Shuttle external tank, the Saturn IV-B stage used in Apollo, or the Delta IV upper stages used in the depot shown above.) This means I don't think the added complexity justifies the gains.
This is especially true in my mind in light of the fact that for almost all liquid rockets, LOX makes up far more of the mass than does the other fuel. For instance, 100 tons of fuels for a kerolox rocket would consist of only 30 tons of kerosene, while a similar mass of hydrogen/LOX fuels would be 16 tons of hydrogen. A depot storing LOX could serve to "top up" stages launched carrying fuel only whether they burned kerolox, methalox, or hydrolox, while being substantially less complex due to the need to only handle and store one fluid.
Anyway, in short, Dragon is an existing multi-role vehicle that I think might adapt well to serving as a cheap, reusable LOX tanker as part of a depot-based exploration architecture. I think it'd be an interesting trade study to compare in more detail the cost per kg of fuel of a "dumb" expendable tank system versus a modified Dragon Tanker like I've described above, but I don't know that the data do do so is available publicly.