What have you got? The expendable core of a flexible launch system, highly compatible with current Space Transportation System infrastructure, which at a bare minimum could be used to launch large amounts of supplies to the ISS, or large upper stages to reboost the station in its orbit.

Plans for using Space Transportation System (STS) components in flexible combinations loosely known as Shuttle-Derived Vehicles (SDVs) is almost as old as the STS program itself. In the post-Columbia pondering of new space program directions, however, it seems time to pick up that all-but-forgotten train of thought.

Let's dub this SDV variation "Shuttle-B," "B" for "Backbone" or simply the "B" that comes after "Shuttle-A" � the current and only flying STS variant, with the Shuttle Orbiter.

Shuttle-B, a modest variation on the current STS, might further be used to launch an unmanned Crew Rescue Vehicle (CRV) to the ISS, be the baseline launcher for a full-up Orbital Space Plane (OSP), or launch large payloads to Earth orbit and beyond.

The RS-68 is an expendable cryogenic engine, designed to be less expensive to manufacture than previous such engines. It has a maximum rated thrust 50% greater than a Space Shuttle Main Engine (SSME) running at 109% rated thrust. The RS-68 uses the same propellants as the SSME, hydrogen and oxygen, at the same feed ratio. Even with its somewhat lower performance level -- a specific impulse (Isp) of 410 seconds compared to the SSME's 452 -- the new engine is a superb candidate for a mostly expendable shuttle-derived vehicle.

There was no such SSME substitute 15 years ago when another concept known as Shuttle-C ("Cargo") was under serious consideration for space station construction and resupply needs. Shuttle-C would have replaced the Orbiter on the Shuttle stack with what was essentially a large cargo container. The SSME was the only engine then available to power Shuttle-C, which meant either devising a new way to recover these expensive engines, or accepting their destruction. Neither option was really tenable; for this and other reasons, the concept was eventually discarded.

Under this new concept new structure is attached to the External Tank where the Shuttle Orbiter is currently mounted. This structure includes attachment points for a potential series of vehicles: cargo pods, orbital space planes, upper stages, etc. The expendable engines are permanently attached to the tank and its propellant lines. Moreover, the Shuttle Orbiter carries the SSMEs, and detaches from the propellant lines as it separates from the tank. (This is a critical operation, especially in the event of a launch abort in which the Orbiter must separate from the tank with the propellants flowing.)

The Shuttle-B carries its payload nearly to orbit, as with the current STS. However, payload separates entirely from the Shuttle-B, which then deorbits and breaks up in the atmosphere like the current tank, but with the expendable engines still attached. Engines on the payload take that vehicle the final step into orbit. (The Shuttle-B's high-value avionics could reside in the payload vehicle for reuse for further economy. They could be removed from an ISS-bound cargo canister for eventual return, or simply made an integral part of any Shuttle-B reusable payload vehicle.)

Along with the RS-68, there is at least one other engine that can fulfill the Shuttle-B role. A promising alternative is Northrop Grumman's TR-106, a cryogenic engine with 650 klbf thrust that has undergone limited static testing at NASA Marshall. While a significant amount of further testing would be required before it could be launched, this pintle-injection engine holds the promise of an even cheaper expendable cryogenic engine in the SSME class: one-half to one-quarter the cost of an RS-68, according to Northrop Grumman.

To be fair, both the RS-68 and TR-106 would need to become man-rated to propel piloted missions. However, for non-crewed flights of cargo or Crew Rescue Vehicles (CRVs) to the ISS, the RS-68 would be immediately usable, as would the TR-106 following basic development. Man-rating these engines in the context of use with an otherwise man-rated system (the STS) would seem to be a much easier task than man-rating an entire Delta IV or Atlas V, each an Orbital Space Plane (OSP) launcher candidate.

Using expendable engines avoids some of the development costs on Shuttle-B that Shuttle-C would have entailed. There would be no need to develop and test a "recovery module" that would house the engines and return them to Earth. Nor would there be a need to recover said recovery module. The alternative scenario of using high flight-time SSMEs is also avoided.

The benefits of using Shuttle-B would include:
• Making use of more than 20 years' experience operating the STS using the Shuttle Orbiter. Processing and launching a Shuttle-B should be well within the experience base and physical configuration of the existing STS launch and ground support systems.
• Leveraging existing assets. Especially from a political standpoint, moving more traffic through the existing STS processing and launch system, using Boeing- or Northrop Grumman-made engines, and probably doing a range of other things with existing components of other aerospace corporations, eases the transition to life after Shuttle-A.
• The continued use of STS launch facilities and personnel would ease the transition from the Shuttle Orbiter (Shuttle-A) to the next piloted vehicles both economically and politically. The vehicles launching from KSC Pads 39A and 39B might become more diverse, but there should be a high degree of overlap in the crews and facilities needed to prepare and launch them.
• The ability of the STS side-mount configuration to launch winged vehicles is well-provien. Launching vehicles like the OSP on top of EELV presents sufficient flight stability concerns that many previous spaceplane concepts added large fins to the launch vehicle. Indeed, planning for the X-37 Space Maneuver Vehicle has included placing this small spaceplane within a launch shroud to skirt the issue.
• Large increases in payload. The Shuttle-C concept of the 1980s was expected to be able to loft payloads 2-3 times heavier than what could be carried by the Shuttle Orbiter. Lofting a cargo canister, Shuttle-B should have a similar performance.
• Better abort options for piloted flights. A manned vehicle mounted on the Shuttle-B would be independent. In the event of a launch abort, the payload vehicle could separate and rocket away from the rest of the launcher, without the present concern of having to disconnect from flowing cryogenic propellant lines.
• Flexibility. Shuttle-B could be used to launch a number of "payload vehicles:" cargo canisters, CRVs and OSPs, large reboost stages for ISS, interplanetary probes with their upper stages. Because the launcher is now independent of the Payload Vehicle, a range of things could be mounted and launched on Shuttle-B's back.
• Experimentation. Once flight-proven, Shuttle-B could be used for non-piloted test flights of STS modifications. For example, attempts to detach and recover the expendable engines in a separate pod could be made to evaluate the potential economies and issues, while risking only engines that would be discarded anyway.

In the process of formulating the Shuttle-B idea and writing this editorial, I discovered another public plea to make use of the STS infrastructure. At a public hearing on the OSP Program in July, Apollo 11 astronaut Buzz Aldrin described a different system, which he said he and other Apollo veterans were studying. In this system, three expendable RS-68s would be mounted on the bottom of the External Tank, and a payload vehicle derived from an Atlas V or Delta IV EELV would be mounted on the tank's side, in place of the Orbiter.

This idea would make use of a long-discussed advantage: the lightening of the External Tank due to reduced thrust structure. While additional, unspecified modifications to the ET and the STS pad would be needed for the bottom-mounted engines, this is also a promising and clever way of putting existing pieces together in useful new ways.

It was fitting that Aldrin should make this plea, for the Apollo infrastructure that was summarily dismantled after the lunar landings might well itself have been adapted for future missions. Imagine if we had continued in the spirit of Skylab (an orbital station adapted from the Saturn IVB upper stage), and evolved Apollo hardware and systems for new piloted and non-piloted missions.

However it might be done, using the existing STS in new ways could provide a near-term solution to the issues of ISS crew rescue and transfer, ISS reboosting and resupply, and heavy launch capabilities for future missions, would make tremendous sense.

Greg Zsidisin is the Founder and Contributing Editor of AstroExpo.com, a space-industry news and reference website.

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