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In a previous question the P&W J 58 engine for the SR-71 was discussed. The engine was a new and innovative design combining the characteristics of a turbojet and a ramjet.

Before flying with such a new design I would think they would want to test it on the ground as much as possible to ensure the safety of the pilot and the aircraft. Otherwise you're sending a guy up with things that should work but so far only on paper. (I know that's what test pilots signed up for but they don't want to be reckless about it.)

J-58 ground test
J-58 ground test. from Wikimedia commons

From what I know a pure ramjet won't even operate at zero airspeed. What I'm wondering is how much can be learned from ground testing on such a radical new design, or any supersonic design for that matter? Can supersonic conditions be replicated on the ground?

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    $\begingroup$ There are supersonic cars which operate on the ground. But I suppose you are talking about testing engines on the ground? And the J-58 could operate on the ground because it could do subsonic operations too (takeoffs, landings, and refuelings). $\endgroup$ – SMS von der Tann Apr 17 '16 at 20:27
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Yes, supersonic conditions can indeed be replicated on the ground for purposes of airframe and engine testing.

Arnold Engineering Development Complex (AEDC)

The U.S. Air Force and NASA both operate supersonic wind tunnels, as do Lockheed, P&W, and others. The most advanced of these facilities are probably those at Arnold Engineering Development Complex on Arnold Air Force Base. I won't mention any specific numbers from memory because they've likely changed since I worked there and I can't remember exactly which ones were classified/FOUO/otherwise-not-public, but let's just say they're pretty good.

AEDC has test cells designed both for testing jet engines as well as for testing scale models of airframes (or full scale airframes if it's a small enough airframe, such as a missile.) Additionally, it has facilities for testing upper-stage rocket motors. Lower-stage rocket motors don't need the facilities at Arnold, as those are designed to operate in the atmosphere and can just be lit up out in a desert somewhere.

According to Wikipedia (which I believe is sourced from publically-available information,) Hypervelocity Wind Tunnel 9 can generate air flow of Mach 14 with simulated altitudes from sea level to 173,000 ft.

AEDC Tunnel 9 with Test Article
AEDC Tunnel 9 (with test article) Source: Wikipedia

In case that's not enough, G Range can fire projectiles at roughly the speed of low Earth orbit. However, this one is kind of stretching the definition of "supersonic," since it's typically pumped down to such negligible air pressure that Mach number isn't really defined before a shot at that kind of velocity. Still, as the wiki article indicates, it's capable of shots at up to 1.7 atmospheres, so it definitely counts as legitimately supersonic on those shots.

AEDC G Range
AEDC Range G Source: Wikipedia

Holloman Air Force Base

In addition to wind tunnels and other facilities at AEDC, the U.S. Air Force also occasionally uses rocket sleds, such as the one at Holloman Air Force Base, which currently holds the speed record for any type of non-air/space vehicle at 6,416 mph / 10,326 km/h (Mach 8.5.)

Holloman High Speed Test Track
Holloman High Speed Test Track Source: Wikipedia

Land Speed Record Rocket Sled at Holloman
Land Speed Record Rocket Sled at Holloman Source: Wikipedia

Holloman "Speed Limit" The "Speed Limit" at Holloman Air Force Base
Source: some guy's website, which got them from publically-released images from Holloman AFB

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    $\begingroup$ To be fair, that sled ran inside a helium tube (okay, some thin plastic sheeting draped over a stick frame - it was destroyed as the sled passed through it - but still an environment in which a jet engine would have some trouble) and the front-wheel hardware was almost completely ablated away at the end of the run. Not one of your more practical test beds, really, but 'twas fun to watch. $\endgroup$ – Stan Rogers Apr 18 '16 at 12:36
  • $\begingroup$ A) You worked there? That had to be fun! B) I'm sure that those silver blocks at about 5 o'clock and 10 o'clock in the first picture are for a variety of sensors, but I'd have to imagine that their rather blunt leading edges would have some sort of impact on airflow at Mach 14!!! Sheesh! $\endgroup$ – FreeMan Apr 18 '16 at 12:57
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    $\begingroup$ An extra +1 for the speed limit sign! Dang, that didn't work... :( $\endgroup$ – FreeMan Apr 18 '16 at 18:19
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    $\begingroup$ @FreeMan I originally saw it on a t-shirt and was pleasantly surprised to find that the sign was actually there. :) Good luck pulling over the guy violating it, though. Also, good luck violating it. $\endgroup$ – reirab Apr 18 '16 at 18:45
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    $\begingroup$ My favorite episodes of Mythbusters were the ones that they used the rocket sled. It's just so awesome to watch something move that fast. Especially when they added a second stage. Just when you think nothing could go that fast the second stage kicks in and... DAMN! $\endgroup$ – TomMcW Apr 18 '16 at 19:22
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It is a struggle, but it can be done. For the J58, Pratt&Whitney had a test facility at West Palm Beach which would simulate conditions at speed and altitude. From this site:

J-58 Engine Testing in Afterburner at test cell A-1 at Pratt and Whitney’s West Palm Beach facility.

The picture actually shows test cell A-1 at Pratt and Whitney’s West Palm Beach facility. […]. This test cell was actually an altitude simulation cell used for testing purposes. (We had a additional cell used for sea level runs for motors which were overhauled onsite also.) It used a non-afterburning J-79 as a slave motor. The exhaust of the slave was introduced to the inlet of the J-58 through a series of valves thereby simulating the speed, temperature, and density of the air at the inlet normally seen during flight. In this particular picture the motor is running at sea level as indicated by the inlet screen.

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    $\begingroup$ This answer inadvertently becomes the answer to this question. $\endgroup$ – FreeMan Apr 18 '16 at 13:04
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    $\begingroup$ Also, I just realized that the orange section at the end of that engine does not appear to be paint, but glowing hot metal! Is that by design? I realize that it's mighty hot at that end of the burner, and that we normally don't see that because of the nacelle, but does the outer part of the exhaust nozzle normally get that hot? $\endgroup$ – FreeMan Apr 18 '16 at 20:27
  • $\begingroup$ @FreeMan I had the same thought, and I wondered if normally it would be more cooled by airflow if it were actually in flight. $\endgroup$ – Todd Wilcox Apr 19 '16 at 15:22
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As shown here, Lockheed Martin has a supersonic wind tunnel capable of generating wind speeds of up to mach 5. A quick Internet search revealed an extensive list of other supersonic wind tunnels, indicating that such tech is fairly widespread. In addition, as suggested by SMSvondertann, jet- and rocket powered cars such as the Thrust SSC have attained supersonic speeds on land.

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    $\begingroup$ My first thought was, "but was it open when they were developing the SR71?". The answer to that is, yes - it was opened in 1958! $\endgroup$ – FreeMan Apr 18 '16 at 12:52
  • $\begingroup$ An important caveat is that high mach wind tunnels can only operate for short times (they can't accelerate the air that fast in real time, and instead function by filling a huge pressure building and then venting it into the tunnel). LockMart's tunnel is rated for 15-110 seconds at mach 5.0-0.3. It gets even worse at hypersonic speeds; wind tunnels can only hit those levels for at most a second or two meaning that R&D needs to involve using rockets to accelerate physical hardware to verify the CFD simulations in flight. $\endgroup$ – Dan Neely Apr 18 '16 at 13:38
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    $\begingroup$ @DanNeely Maybe Lockheed's tunnels have that restriction, but AEDC's tunnels don't. They can (and do) run for hours on end. They also have to call TVA about every half hour to tell them how much electricity they're planning to use when the cells are running. :) Otherwise, they'd literally overload or underload the local power grid. $\endgroup$ – reirab Apr 18 '16 at 18:38
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Not quite on the ground, but some organisations go to quite extreme lengths to simulate supersonic conditions. Take NASA and their supersonic parachute intended for Martian atmospheric entry, also known as the Low-Density Supersonic Decelerator. They recently tested this on a quite startling rig:

-The parachute was dropped from a helicopter
-A 1km steel rope attached the chute to a winch via a pulley
-The winch keeps the rope under tension...
-...and is attached to the the star of the show - a 4-motor rocket sled, designed to accelerate the parachute to extreme speed before it hits the ground.

This rig is worth mentioning not least because the description conjures up an image of a pile of empty ACME boxes!

See here for further information

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    $\begingroup$ ..."meep, meep!" $\endgroup$ – TomMcW Apr 18 '16 at 21:00
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    $\begingroup$ @TomMcW that road runner doesn't stand a chance ;) $\endgroup$ – Gargravarr Apr 18 '16 at 21:09

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