The VentureStar may now be nothing more than a memory, but it nearly became part of NASA’s future ambitions – playing a role as part of a commercial fleet and even an option for the US Air Force. However, controversial key decisions – and a major test failure – during the construction of the technology demonstrator, the X-33, saw the program cancelled just as the first vehicles were being constructed.



X-33:

Taking shape at Lockheed Martin’s Skunk Works facility, the X-33 was intended to be a 1/3 scale prototype of a fully-operational RLV (Reusable Launch Vehicle) called the VentureStar, designed to dramatically lower the costs of launching payloads into space. However, the official reasons for its demise fail to portray the true story.

Testing and construction had been going well during the assembly of the scaled down version of the VentureStar, with the launch facility at Edwards Air Force Base in California completed – along with 40 percent of the X-33.

Advances had been made with the thermal protection system (TPS) which doubled up as the aerodynamic shell of the vehicle.

The XRS-2200 Linear Aerospike main engines were on target to become the next generation of liquid-fuelled propulsion systems, although Rocketdyne’s decision to use Narloy-Z – a heavy copper alloy – saw changes to the flight control surfaces due of an aft-heavy center of gravity. This would prove to be a major – and defining – problem later in the process.

Other minor issues had continued to be ironed out until a critical test highlighted previously noted concerns, one that would add years to the target date for the first test flight, ultimately leading to be the end of the program – the failure of the composite LH2 (liquid hydrogen) tank during testing.

However, this was no surprise to those working on the program, with new information now showing that engineers and designers had protested at the very moment they were informed of a management decision to build a composite LH2 tank.

Such was the scale of the initial protest, the go-ahead was given to build the LOX (liquid oxygen) tank out of the same aluminium-lithium alloy that is currently used on the external tanks for the Space Shuttle, a small but important victory for the protesting engineers at the time.

The LOX tank passed testing and was installed with plumbing and electronics around the front third of the vehicle’s structure.

Yet still, the LH2 tank – a large multi-lobed structure – was to be the single most challenging project for engineers involved in the X-33 program, even if they had been able to use Al-Li alloys.

Building one out of composite materials proved to be – as protests noted – even more of a challenge, with very little experience with large-scale composite structures within the workforce undertaking the fabrication of the tanks.

A five-foot long tank was built and tested at NASA Lewis (now NASA Glenn), but the larger structure for the X-33 was going to be a step too far. Skunk Works’ designers of the LH2 tank had heard from engineers that storing liquid hydrogen in a pressurized composite (notably material IM7/997-2) tank with the hollow honeycomb walls was simply doomed to failure – but their advice was ignored.

Concerns grew with engineers of the tank, as problems in the LH2 tank fabrication stages during late 1997 and 1998 saw Alliant Techsystems personnel try to find solutions, personnel who also lacked experience with composite tanks. This led to the first tank – fabricated by Alliant in Sunnyvale – found to have debonds and delaminations – being sent back to try and fix the problems.

A second LH2 tank appeared to be in a much better shape and was shipped to MSFC (Marshall Space Flight Center) in Huntsville, Alabama for testing. The failure of the tank during testing at MSFC was still predicted – and occurred on November 3, 1999, during the fifth stage of testing.

Ironically, engineers – predicting the impending problem – had a solution already at hand. By filling the honeycomb walls of the tank with closed-cell foam, air wouldn’t be able to enter the structure and liquefy.

This idea had to be rejected, due to the 500 kg of extra weight being added to the aft, further affecting the center of gravity, which was already having serious fallout on the design due to the heavy engine ramps.

Thus the tank was doomed, as predicted.

‘Damage was discovered Wednesday (November 3) evening to one wall of the X-33’s composite liquid hydrogen tank currently undergoing cryogenic and structural loads testing,’ noted an MSFC press release at the time. ‘The damage was discovered while viewing the tank over video monitors, approximately two hours after the completion of a test cycle which appeared to be nominal.’

Such was the consensus that a failure was the only outcome for the composite tank, MAF (Michoud Assembly Facility) engineers had already started the process on having their own Al-Li LH2 tank ready for fabrication, pre-empting the call for a change of plan.

Faced with a project failure, Lockheed Martin and X-33 NASA managers gave the green light to proceed with the fabrication of the new tank. Ironically this new tank weighed in less than the composite tank – disproving one of the reasons for going with a composite tank in the first place.

While the aluminum LH2 tank was much heavier than the composite tank in the skins, the joints were much lighter, which was where all the weight in the composite tank was, due to the multi-lobed shape of the tank requiring a large amount of surrounding structure, such as the joints. Ironically, the original design of the X-33 on the drawing board had the tanks made out of aluminum for this reason – but the cost played a factor for the potential customer base.

Then the hammer blow, as despite the project now appearing to be back on track, with the move towards testing of the new LH2 tank, the much-respected former NASA director Ivan Bekey appeared in front of the Subcommittee on Space and Aeronautics, Committee on Science, at the US House of Representatives. His testimony on April 11, 2001, on NASA’s FY2001 budget request ‘Aero-Space Technology Enterprise,’ proved to be the final blow for the X-33 VentureStar.

His address to US lawmakers stressed that the X-33 had to continue with composite tanks, thus making the project doomed to failure.

‘The principal purpose of the X-33 program is to fly all the new technologies that interact with each other together on one vehicle so that they can be fully tested in an interactive flight environment,’ said Bekey during his testimony. ‘If that is not done, the principal reason for the flight program disappears.

‘Even though the thermal protection system and the engine would be tested, the structure and its interaction with the tanks and support for the thermal protection system would not be tested. Since the biggest set of unknowns in this vehicle configuration have to do with the structure-tankage-aeroshell-TPS-airflow interactions, it is my belief that to fly the vehicle with an aluminum tank makes little sense from a technical point of view.

‘Worse yet, the flight of an X-33 with an aluminum tank will increase the difficulty of raising private capital for a commercially developed VentureStar from the merely very difficult to the essentially impossible.

‘What I would recommend is that NASA and Lockheed Martin face up to the risks inherent in an experimental flight program and renegotiate the X-33 cooperative agreement so as to delay the flight milestone until a replacement composite tank can be confidently flown.

‘Both NASA and Lockheed Martin should make the investments required to build another composite tank and to absorb the program costs of the delay, because only then will the X-33 program be able to meet its objectives. To do anything less is flying for flying’s sake, wastes the funds already expended, and makes little sense.’

X-33 workers were said to be stunned by the comments of Bekey – still to this day a highly respected authority on space transportation – on his call to look once again at the composite tanks, now believed to be impossible to correct without a complete redesign of the entire vehicle – and then not assured.

By early 2001, the program was officially canceled – five years and $1.5 billion down the line. Official reasons for the cancellation was a disagreement over extra funding from both industry partners, NASA and Lockheed Martin. However, the recommendation of the composite tank to keep costs down to prospective commercial interest was the main reason given to workers.

Not only was the program canceled, but all the successful new technology was laid to rest along with the death of the X-33. The metallic TPS developed by BF Goodrich is still seen by some engineers as one of the most impressive parts that made up the X-33.

Four XRS-2200 Linear Aerospike main engines were constructed (two for testing and two for flight). One complete engine is still around today, displayed at NASA Stennis, at least two others were disassembled.

Two LOX tanks were built over the lifetime of the X-33 construction, both are mothballed at NASA Glenn. The partially built vehicle, thought to be still in a storage hanger at Edwards Air Force Base, was actually disassembled.

All this despite interest from the Air Force in resurrecting the project, with Lockheed Martin high-flyer Cleon Lacefield in charge of the effort to re-start the program on at least one occasion. Each time the Air Force made requests to take the X-33 project as their own, they found the opportunity denied at the highest level of US government.

Even when armed with Lacefield’s final comments on the X-33, comments which gave full support to the Al-Li, added to by support from NASA Stennis on the engines, the Air Force – now trying to have their own VentureStar flying by 2012 – found the door of the White House firmly closed shut on any possibility of resurrecting the project.

Rumors of a final attempt by the Air Force came just last year, only to for them to hear the same answer, likely ending their interest in the vehicle and also the last remaining monument to the X-33 – the state-of-the-art launch facility at the Phillips Test Range, Edwards Air Force Base. Again, it was the White House that vetoed any new evaluations of the X-33.

No part of the X-33 technology will play a role in NASA’s architecture being developed for the return to the Moon, with the agency deciding to use the conservative – yet experience-rich – shuttle derived and Apollo approach.

The X-33 VentureStar was highly criticized as a chunky flying fuel tank with little to offer once the satellite launch business diluted between several regular and proven launch systems around the world.

But for those that worked on the X-33, the pure complexity of the new system – and the chain reactions felt from issues and bad decisions such as the heavy engine ramps to the resulting need for the low weight of the LH2 tank – to the lack of options open to use a solution for the LH2 tank failures, turned her from a potential leap in space vehicle technology, to one that became a $1.5 billion white elephant to the tax payer.