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Got another mystery.

What are these things (highlighted) that are attached to to the heatshield of the McDonnell Douglas X-33 proposal?

All I can think is maybe OMS?

enter image description here X-33 Proposal by McDonnell Douglas (Source: NASA, www.dfrc.nasa.gov)

Cheers!

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The appendages that you are referring to are most likely gaseous hydrogen thrusters. In the middle is a liquid hydrogen engine.

In 1995 during Phase I of the X-33 program NASA received proposals from Rockwell, McDonnell Douglas, and Lockheed Martin. In 1996 NASA selected Lockheed Martin's proposal.

The X-33 was to be a subscale, suborbital demonstrator of the larger orbital VentureStar spaceplane. Lockheed Martin's X-33 used a liquid hydrogen powered aerospike engine, and would have landed horizontally on a runway, in contrast to the McDonnell Douglas proposal which would have landed vertically. The Lockheed Martin X-33 project ran until the cancellation of both the X-33 and VentureStar in 2001.

McDonnell Douglas had already been working on the DC-X project since 1991 with NASA funding. The DC-X used vertical takeoff and landing, and was powered by four RL-10 liquid hydrogen engines and used four gaseous hydrogen oxygen thrusters McDonnell Douglas DC-X

DC-X test flight DC-X test flight (Source: NASA, via Wikimedia Commons, DC-XA first flight, DC-XA first landing)

Test flights of the DC-X were ongoing in 1995 when McDonnell Douglas submitted their proposal for the X-33. It is not known to what extent their X-33 proposal mirrored the DC-X, but it can probably be assumed that the two designs were similar. The graphic that you showed in your question is one of two graphics on the NASA Armstrong website depicting the McDonnell Douglas X-33 proposal. In the graphic below the vehicle in the center is the McDonnell Douglas proposal.

X-33 concepts (Source: NASA, www.dfrc.nasa.gov)

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