On underground runways, SR-71s, and heat & blast effects on concrete.

Factual Questions
1

In all the realms of fiction, there’s a certain something you can be sure to find over and over again…the underground runway.

That is, an aircraft runway built underground inside a cavern or a bunker. (ou know what I’m talking about.

Now, my question is…what would really happen if you tried to fly a jet out of one of a runway like this?

Let’s say the runway-tunnel…is 3,500 feet long, 400’ wide, and 200’ tall. There’s a single enterance/exit at one end of the runway, which is, of course, 414’x200’. (I’m ignoring, if only for the setup, any doors, ventilation systems, or hanger space.) The walls, ceiling, and runway are made of concrete.

Now the aircraft I want to try and launch is a Lockheed SR-71. (Johnny Quest and G.I. Joe never settled for Cessna-172s.) It’s two engines, by my figures, produce 34,000 lbs of thrust, afterburning. The exhaust gas temperature from each engine is something like 2000 degrees fahrenheit. (According to my flight sim, 1,200 degrees on the runway)

Now…what happens to the runway-tunnel if I, parked in my Blackbird near the closed end of the tunnel, go to full throttle?

Noise, I assume, would be tremendous. But what else would happen? Would the engines melt or burn the walls—and does concrete burn? I thought I saw some sizzling on a Wings documentary on experimental jet VTOLs—? Would there be any weird air pressure effects?

And if anything really disastrous would happen—explosive overpressure, walls bursting into flames, etc.—could there be an engineering workaround?

Ranchoth

2

Although you probably have something on the limitations of such a base … The base itself is not just SciFi. Taiwan has one. I am sure they can’t be alone in it. There was a recent thread about what would happen if China attacked Taiwan with some info on it.

3

Isn’t there also a runway/air base built into the side of a mountain in Prishtina, Kosovo? This air base was the one the Russians illegally snatched and caused the crisis involving Wesley Clark when he was the commander of the NATO forces in Kosovo.

4

Well, trick number one is that you don’t kick in the afterburners that close to the back wallThe second thing is you’re going to need a vent at the back end of the runway.

Hardened aircraft shelters have a big, huge door at one end and an exhaust vent at the other. If the engines are running, both ends of the shelter have to be open. The front has to be open to let in enough air for the engines. The back has to be vented to get rid of the exhaust.

The vents are built such that there’s no direct path through them into the shelter - the exhaust can get out, but shrapnel from a bomb blast would have to make a couple of right angle turns to make it. Also, the vents open upwards, so that blasts from the ground can’t through at all - anything that is going to get through your vent would have to come from an airburst.

The tunnel over the runway is going to have to be either very large or else it will have to be vented as well. The pressure in a tunnel can build up when something big and fast blasts through. The German ICE trains are a good example. They are limited to around 250KMH in the tunnels. The trains push a wave of compressed air out of the tunnel ahead of themselves that has dirt and sand in it. I’ve been told that standing near tunnel mouth when an ICE comes through wil get you sandblasted quite nicely - and if you don’t watch out, you can get sucked under the sumbitch as it passes. I’ve also been told that the ballast (the gravel between the railroad ties) is covered in some epoxy or resin to keep from being blow out. So, vent your tunnel or else make it big or else you are going to have overpressure problems in the tunnel when the Blackbird takesoff.

Switzerland has air force bases built under mountains, so I’m sure they’ve got a good grasp of how to do it. They also build hangars for the fighter planes underthe mountains. Weird shit. Fighters hanging from the ceiling because floor space is at a premium. I’ve never actually been in one, but I saw pictures in some of the magazines we had when I was in engineering in the USAF.

JRE, who many years ago worked on air base layouts in the civil engineering department of Lindsey Air Station, Germany.

5

Well, they launch missiles from silos, so obviously the problems of melting concrete aren’t insurmountable.

From the description of your tunnel (200' tall), it appears that you expect the plane to actually take off while still underground and thread a 200' x 400' opening at 250+ mph.    Which leads me to suspect that your major problem is going to be laundering the pilot's shorts after each mission.
6

If the concrete is in danger, then you could do what they do with the launch pads at Cape Canaveral: Pump water in ducts through the concrete.

I think you are going to have to have at least part of the run way outside of your tunnel. Cleaning bills aside, you need nearly a mile (5000 feet) of runway for an SR-71 to get off the ground. No doubt it could be done. I just don’t see how you can expect that sucker to hit the end of the tunnel and zoom on up at the 5000 foot mark.

Also, it seems that the SR-71 takes off at a lowly 200 knots. I don’t think you’ll need the afterburners for that, so that’s one less problem.

7

3500 ft is a pretty short runway for jet powered aircraft, without catapults, I think.

My big question is: How do they land? How do they “go around” in the case of a “botched” approach?

-Butler

8

Not to hijack this thread, but most modern silo based missiles are “cold-launched”. They are pushed out of the silo by a huge burst of gas and the engines fire only when clear of the silo. I’m pretty sure an old-fashinoned Titan type missile silo (as seen in Star Trek:Generations) was meant to be a one shot affair.

I’m guessing even if with dimensions of 200’/400’ the effects of the wing vortices bouncing off the walls and ceiling at unpredictable angles could cause significant turbulance, possibly enough to send the plane out of control. I don’t think the “venting” at both ends is any help here.

9

Actually, I have managed to take off from a 3500 foot runway in an (simulated) SR-71…and without afterburners. The trick being that the runway led up to the edge of a 700 ft. cliff, where the Blackbird could make a “dip” after leaving the runway, to build up speed.

If I applied the afterburners about halfway down the runway, the dip needed is markedly reduced. With full afterburners from the start, it wasn’t even needed at all. And with smaller aircraft, or aircraft with a much lower takeoff speed, it’s surprisingly easier.

(All that was without the aerodynamic effects of flying in a tunnel, however.)

Landing wasn’t that hard…Slowing down enough within 3500’ without an arrestor hook, on the other hand, could be very hard. It could be done—most of the time, in most aircraft I tried—but wasn’t a picnic.

Of course, if you need to “go around,” you’re up the proverbial creek. (Which is, I imagine, a good arguement for having two underground runways. At least one would be useable if the other is clogged with twisted, flaming wreckage, and tons of spilled fuel.)

10

Really? I’ve never read a story which included an underground runway.

Anyway, there’s nothing particularly hard about it. It depends on how much you want to hide the exhaust. All you need to do is have a deflector that moves the blast wherever you want it to go. If you can’t have the heat signature spotted, you could build a large chamber behind the runway, and let the exhaust blow into that. You could also build various types of piping, expansion chambers, and other fun stuff to dissipate the energy.

Have a look at big rocket launch, like the Saturn V. The blast actually goes underground into a chamber below the rocket. And the amount of thrust coming out of that would absolutely dwarf the thrust of an SR-71. Really, I don’t think dealing with the exhaust is much of an issue at all. If you can let it escape to the atmosphere, you could just deflect it through vents like they do with some underground missile launches. If you look at some video of those, you’ll see the missile lifting out of the silo, and you’ll see exhaust blowing out of the silo at angles to the rocket - the exhaust is deflected up from underneath.

The main practical issues with an underground runway are more mundane - what do you do when the wind isn’t blowing in the runway’s direction? I’d hate to exit the runway tunnel only to hit a 35 knot crosswind, or find a 30 knot tailwind and suddenly lose a whole bunch of energy. This would be essentially artificial wind shear, and wind shear can bring down airliners.

Also, unless your runway is cut into a mountain, it’s going to have a pretty good incline, and that will require more runway to get the plane up to speed.

Finally, once you take off you’d have no place to land. Unless you plan on landing back through the hole, and that’d require the winds to be just right and a pilot with nerves of steel. Also, I’m not sure an SR-71 can stop all that short.

11

[QUOTE=Mort Furd]
If the concrete is in danger, then you could do what they do with the launch pads at Cape Canaveral: Pump water in ducts through the concrete.

[QUOTE]

If your are reffering to the water that starts spraying on the launch pad seconds before launch, it is not to prevent things form melting, it is to dampen the noise made by the engines so if does not bounce off the pad and damage the shuttle.

http://www.floridatoday.com/news/space/stories/2004a/spacestoryN0508SHUTTEST.htm

Unless you are reffering to something else.

12

You can test an F-14 at full afterburners indoors so I don’t know why you couldn’t build an underground runway for an SR-71. I was in on a hush house test at Miramar when I was in the navy. It’s a sturdy concrete building to be sure but it isn’t especially atom bomb proof or anything. It has two huge ducts to redirect the exhaust blast through the roof so it’s a little more bearable to people in the vicinity.

The plane is kneeled as it would be on a catapult and the holdback point on the back of the nose strut is attached to section of very heavy anchor chain attached to a padeye - yes, that’s part of where my nick came from - in the concrete deck. The holdback device is remarkably small, only a few square inches area, considering it holds the full force of two P&W TF30 engines and it’s actually designed to come apart at a specific amount of force. It’s loud as hell even with earplugs and a helmet but honestly it isn’t the loudest plane I’ve been around.

An SR-71 has bigger engines but it isn’t the space shuttle. Building such a runway wouldn’t be cheap but it should be a straightforward engineering task.

13

The plane would have to get out of an unvented cave pretty quickly or exhaust reingestion is going to be a real problem. Not only are the engines going to use up the available oxygen pretty quickly, but they’ll start sucking in their own hot exhaust, which will get recompressed and even hotter, and it becomes a battle to see if they stall from lean blowout before they overheat and stuff starts to break.

The underground bases in Switzerland and Taiwan do not have the runways buried, just the taxiways, AFAIK. Planes are towed out to the runways, which can consist simply of straight, wide sections of public road, as the Swiss bases are. I understand it’s not unusual for tourists to puzzle at the sight of straight roads hacked through the mountains, apparently unnecessarily, until they’re told that if they went exploring they’d find an artificial cave holding a squadron of fighters nearby.

14

If you’re doing part of your takeoff run underground, there’s also the vibration to factor in - those engines spit out shockwaves you wouldn’t believe, especially with the afterburners lit. You wouldn’t want to bring the ceiling down on yourself because you shook the concrete apart…

15

A visitor’s description of the Swiss Air Force base at Meiringen:

16

Nope. Not referring to a spray at all.

Here’s a quote about cooling:
http://www.aviationnow.com/content/publication/awst/20020826/avi_news.htm

The pads where they launched the moon shots were water cooled from within (and probably from the outside too.) that or else I don’t remember properly what I was told when my family visited Cape Canaveral many years ago.

17

Thanks, my mistake.