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The X-3, as currently in the National Museum of the US Air Force, has two oil pressure gauges per engine, and one fuel pressure gauge per engine. One of the per-engine oil pressure gauges is labeled as being for the turbojet, the other for the afterburner. (But why would an afterburner need lubrication?)

The preliminary flight operating manual says that the engine instrumentation would be the other way around: one fuel pressure gauge for the turbojet part and one for the afterburner part of each engine, and just one oil pressure gauge per engine. What could it be that the two separate oil pressure gauges per engine measure?

Instrument panel of the X-3 in the NMUSAF. U.S. Air Force photo by Ken LaRock.

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    $\begingroup$ In the event no one comes up with an official source, the best guess is almost certainly that the afterburner nozzle actuators were pressurized by engine oil and you needed to know that the pressure was adequate to avoid problems when selecting reheat with the throttle if the nozzle failed to reshape as required. $\endgroup$
    – John K
    May 29, 2022 at 13:05
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    $\begingroup$ I'd allow for the possibility that the middle gauges should be fuel pressure gauges, and that the have oil pressure gauges installed where they shouldn't be. Not saying that this is the case, but it seems possible. $\endgroup$
    – Ralph J ♦
    May 29, 2022 at 18:39
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    $\begingroup$ @RalphJ, you nailed it. Good instinct :) $\endgroup$
    – egid
    Jun 14, 2022 at 11:03

1 Answer 1

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After some research, Ralph's comment is almost certainly the answer here: during the restoration they installed incorrect (though period-accurate) oil pressure gauges, rather than fuel pressure gauges. I found a copy of the X-3 Stilleto flight handbook on Google Books, and the relevant pages are included in the preview. Here's a snap of the panel image, Fig 1-12, on page 10:

An image of the X-3 Stilleto panel from the declassified flight handbook

It's a bit low resolution, but you can tell (by comparing the gauges, at least) that of those six gauges, the top four are the same (reading "FUEL PRESS"), and the bottom two are the odd ones out ("OIL PRESS") -- the opposite of the photo of the panel currently on display.

Item 1-26 has a single entry for oil pressure:

"Two pressure indicators for the turbojet engines, calibrated in pounds per square inch"

Items 1-40 and 1-41 describe the fuel pressure gauges:

"Two turbojet engine fuel pressure indicators, calibrated in pounds per square inch [and] two afterburner fuel pressure indicators, calibrated in pounds per square inch"

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