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The A-10 Warthog is built around a 30 mm cannon that fires depleted uranium rounds, 65 of them per second.

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I heard from someone who worked on the flight controls a long time ago, that when the cannon is fired it provides more thrust than the engines. Is this true? What are the effects of this?

Update

The original question was answered by @Gypaets: thrust of the cannon is comparable to thrust of one engine. The wikipedia article on the GAU-8 cannon mentions that effects of thrust of the cannon are well accounted for by placing the firing barrel at the centre line of the air frame. Ron Gordon's answer reveals some interesting side effects from spooling the cannon up and down when operating - the aircraft bucks. It's a big cannon.

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Image source

Koyovis
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    I've heard it produces more thrust than a single engine, not both. – DeltaLima Jul 01 '17 at 11:08
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    mandatory link: https://what-if.xkcd.com/21/ (scroll towards the middle for the section on the GAU-8/A) – Federico Jul 01 '17 at 11:32
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    It's possible the thing you're forgetting is the cannons only produce force momentarily, the engines produce force continuously. – Fattie Jul 01 '17 at 15:43
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    If the canon produces thrust equivalent to one engine, for 200 ms, then this is equivalent to power down an engine during 200 ms. No big deal I suppose, the vertical stabilizers should be able to cop with that, specially if the canon is not remote for the longitudinal axis. – mins Jul 01 '17 at 19:03
  • @mins They fire for longer than 200 ms. – Koyovis Jul 01 '17 at 22:22
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    @Fattie Where did you get the impression I'm forgetting that? – Koyovis Jul 01 '17 at 23:03
  • @Koyovis true. But if you can't get it done in 200ms, then you're into spray-and-pray, in which case, I wouldn't count on another 600ms closing the deal. – Harper - Reinstate Monica Jul 02 '17 at 00:17
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    I would think the real problem is not engine power, it is stall speed. The way I see it as a non military bystander is that the A10 is supposed to optimize attack time - so on a run it will come in low and SLOW. Slow means there is not a lot of reserve before hitting stall speed AND it means engines likely are not running full power. This makes it a balance act of keeping speed low but above stall speed, and balancing fire and engine output to just dance in the optimal speed range. – TomTom Jul 02 '17 at 06:05
  • It's not thrust, it's drag. –  Dec 08 '17 at 06:06
  • @jjack There is nothing dragging. It is thrust, pointed backwards. – Koyovis Dec 08 '17 at 06:08
  • @Koyovis It's not propelling the aircraft in the intended direction. In fact it opposes the intended direction of motion of the aircraft. So it is non-aerodynamic drag. –  Dec 08 '17 at 06:11
  • @Koyovis And also, why would a thrust reverser be called such. –  Dec 08 '17 at 06:12
  • @jjack It's called a thrust reverser because it reverses the direction of the thrust. Thrust is not bound to a direction of motion. A helicopter rotor produces thrust directed upwards, while it flies forwards. An aerobatic aircraft sliding backwards has still engines producing thrust, but now in the opposite direction of motion. Let me reverse the question: why is it called a thrust reverser and not a dragger? – Koyovis Dec 08 '17 at 06:34
  • @Koyovis We're talking about an airplane here with the classical definitions of thrust and drag. –  Dec 08 '17 at 06:36
  • @Koyovis Call it a meaningful way, since it's an own peculiar force. Call it "Cannon Recoil Force". Just because there is a thrust force on the cannon doesn't mean that this is a correct term for its effect on the airframe. –  Dec 08 '17 at 07:03

3 Answers3

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The A-10 uses a GAU-8/A. According to Wikipedia:

The average recoil force of the GAU-8/A is 10,000 pounds-force (45 kN), which is slightly more than the output of one of the A-10's two TF34 engines (9,065 lbf / 40.3 kN each). While this recoil force is significant, in practice a cannon fire burst only slows the aircraft a few miles per hour in level flight

Your statement is only partially true: two engines have more, but one engine has about 10% less thrust than the recoil force of the cannon.

The recoil force of MiG-27's GSh-6-30 was even larger: 60 kN.

As pointed out in a comment by @tj1000 the recoil of the GAU-8/A wasn't such a problem as the smoke from its shell propellant. If you're looking for the video mentioned in the comment (test pilot ejection due to engines out after firing ammunition with a new flash suppressor) this is the link (12:08-13:59).

Gypaets
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  • Can it still fire the gun when flying on one engine? – Koyovis Jul 01 '17 at 12:11
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    @Koyovis I suppose it is more or less like airebrakes: it reduces the total energy, so yes you can still fly but not maintain a leveled flight at constant speed. – Manu H Jul 01 '17 at 12:33
  • And perhaps struggle with the yawing moment. – Koyovis Jul 01 '17 at 13:18
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    The big problem with the GAU-8 and the A10 during development wasn't recoil, it was the smoke from the cannon shell propellant blowing the engines out. If you've ever seen the oft repeated video of a pilot punching out of an A10, and the aircraft then crashing into a desert, that's what happened: during a test, the cannon smoke blew out both engines, and the pilot couldn't get them restarted. – tj1000 Jul 01 '17 at 15:45
  • @tj1000 Thanks for the comment! I found the video and added a link to it. – Gypaets Jul 01 '17 at 19:38
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    I first saw that film around 1980, in Air Force ROTC training. This was during extended testing of the A10. When the pilot got below 2000 feet and the engines still hadn't restarted, they told him to get out. They showed that film as it was shot on a high speed camera (to record the cannon firing), and it was a good example of an ejection. They pointed out that as the pilot didn't have his head completely vertical, he did get a cracked vertebrae from the ejection, but no permanent damage. – tj1000 Jul 01 '17 at 22:38
  • That's a great video. – Koyovis Jul 02 '17 at 07:02
  • @tj1000: I'm surprised that would be enough to flame out the engines, given that, even if they are sucking in smoke, the vast majority of what they're ingesting is still air. – Vikki Aug 27 '19 at 19:30
  • @Sean That's an interesting point. Would you like to ask a separate question about what needs to happen to flame out such a jet engine? – Gypaets Aug 27 '19 at 20:04
  • The GAU-8 is in line with the engines, insofar as airflow goes, so any gas from the cannon goes straight back to the engines. Given the rate that the GAU-8 fires, and the size of its shells, it appears that it puts out a LOT of smoke. There was a similar problem with the first rotary cannon, a 20mm Vulcan fitted to the F104. The blast damaged the area around the cannon, and seriously reduced engine power. Eventually, they fixed that problem and the Vulcan became standard on a lot of warplanes. – tj1000 Aug 29 '19 at 17:06
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Not only does it affect air speed, but also angle of attack (AoA) and yaw. The gun is well forward of the center of gravity (CG); so add to the recoil vectoring a build up gyroscopic torque as the gun spins up and down. As air speed decreases, maneuvering approaches instability, thus placing a limit on the length of a safe burst determined by available energy (altitude, AoA and air speed.) However, grouping accuracy and ammo conservation usually are more limiting than the effect of gun "drag". Each A10's gun performance is distinctive. Once a burst begins, the aircraft will pitch and yaw in its peculiar manner. On the HUD you might find a wad of gum placed there by the pilot after his first aiming burst to show the direction of push.

The A10 employment scenario has always included battle field loiter to provide air cover, ensure extended engagement coordination, and provide tactical intel directly to the ground commander! Even low and slow, its hard to hit. Most infantry borne G-to-A missiles are IR guided. The A10's exhaust is hidden from below by the horizontal stabilizer; and the RAW/IRW system can automatically launch "angel flares" when the launch is detected, thereby negating any IR guidance system. Meanwhile you have just poked the bear and left your location for immediate response with an array of launch-and-leave air-to-ground guided weapons.

Federico
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Ron Gordon
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  • Are pilots matched with airframes, so that they gain experience with one particular set of peculiarities? Can the A-10 continue to fly the mission on 1 engine? – Koyovis Jul 02 '17 at 02:02
  • Yes. Absolutely. The pilot is a member of the assigned combat team for that aircraft, training, exercising, and deploying together. – Ron Gordon Jul 02 '17 at 08:32
  • @Koyovis: "Can the A-10 continue to fly the mission on 1 engine?" – The A-10's purpose is to fly low and slow over an active battlefield, it is designed for extreme survivability: "The aircraft is designed to fly with one engine, one half of tail, one elevator, and half of a wing missing." – Jörg W Mittag Jul 02 '17 at 10:04
  • Indeed, and there are quite some photo's of battle damaged aircraft that made it home ("You should have seen the other guy!"). But i'm wondering if the A-10 can continue the mission on one engine, or if it is a get home fast scenario. – Koyovis Jul 02 '17 at 13:43
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    Comments right on. Your picture shows how the engines are protected from ground fire by control surfaces that will have to be shot away first. From the technical descriptions of thrust limitations you have elsewhere, an engine failure is usually a good reason for abort. But the exigencies of war dictate risk taking proportional to the objective. The A10 can loiter on one engine, providing surveillance and coordination until help comes. It also carries an array of guided weapons that can be used, if the bomb/nav release systems are still functioning. – Ron Gordon Jul 02 '17 at 16:31
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    @Koyovis - I would assume definitely RTB. The A-10 isn't exactly sporty to begin with. Without a second engine, you lose altitude or jettison stores. AFAIK, that "one engine... half of a wing missing" etc is clean w/o stores or other damage. It's physically possible to do some missions one one engine, but I doubt it's advisable. That's my impression from pilot comments. The survivability features are designed to keep the pilot alive and bring the plane home, not let it keep fighting while shot up. Overall, it's the least survivable USAF fighter, not the most. – Hephaestus Aetnaean Jul 04 '17 at 10:42
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    @HephaestusAetnaean The survivability features are designed to keep the pilot alive and bring the plane home, - sounds like some pretty sensible survivabilitynfeatures to me. – Koyovis Jul 04 '17 at 15:46
  • @Koyovis - Exactly! Sensible indeed. And it's the same mentality with lots of planes, not just the A-10. They're not expected to sponge bullets every mission. People get the idea the A-10 is an impervious flying tank, soaking up 30mm and bouncing bullets. That's not sensible. Everyone remembers this photo (https://en.wikipedia.org/wiki/File:Kim_campbell_damage_a10.jpg), but everyone forgets that that plane sat on the ramp for weeks waiting on repairs. They're aircraft, not tanks. Low n' slow and BRRRRT are romantic, but not very survivable in a high-threat environment. – Hephaestus Aetnaean Jul 04 '17 at 16:38
  • @Federico The last bit added has no direct relevance to the question. It is interesting info though, and should belong in a comment. – Koyovis Dec 06 '17 at 12:06
  • "and the RAW/IRW system can automatically launch "angel flares" when the launch is detected, thereby negating any IR guidance system." Modern IR-guided weapons are designed to ignore flares. – Vikki Aug 27 '19 at 19:27
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As Gyrates points out, no. The recoil from the GAU-8 doesn't completely counteract the thrust of both of the A10's TF-34 engines, but it will definitely slow the plane down. That is a factor before firing the cannon - is your airspeed sufficient to withstand a slowdown?

The same holds true of WW2 fighter aircraft with multiple heavy weapons - firing at slower speeds could stall the plane.

One airplane that did battle serious recoil issues was the WW2 B25 variant that mounted a 75mm cannon in the nose. Apparently, the recoil from the cannon was quite severe, and definitely impacted airspeed.

AC130 crew report that firing the side mounted 105mm howitzer will boot the tail of the plane around quite a bit.

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tj1000
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    Well said. I would think - as a non pilot, jsut from some logic - that part of the problem is that on an attack run the A10 would come in low and SLOW - to give more time for the run. Which means there is not a lot of reserve before hitting stall speed. And the engines possibly are not working at full power either. Which means the question is really "does the gun slow down the plane significantly". Engine power does not really matter in a scenario I describe. – TomTom Jul 02 '17 at 06:03
  • Fortunately, most A10 attacks with the gun are made with the plane in a slight dive - partially to keep the target centered, and partially to keep a reserve of speed - acceleration is quicker when a plane is already diving. I don't think the GAU8 has to stay on target for very long to destroy whatever it is aimed at. – tj1000 Jul 02 '17 at 19:08
  • Yeah, but I think someone envisioned a battlefield with a tank battle going on and an A10 slowly going over the battlefield obliterating a dozen or so tanks. Not that this is the case these days ;) – TomTom Jul 02 '17 at 19:10
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    Oh, yes! two A10s, 23 tank kills, one day. .https://theaviationist.com/2012/12/20/warthog-in-action/ And its not just the plane!! These guys used a dirt strip to refuel and reload. We practiced ground turn around on the A10 bring it down to less than 15 minutes without peacetime safety restricts prohibiting loading bombs and fuel simultaneously. In this scenario, the pilot is the guy pump his on JP fuel. – Ron Gordon Jul 05 '17 at 01:09
  • A figure I saw was that the 8 M2's on a P47 would put the equivalent of around 400hp in reverse. Not enough to stop the plane, but definitely a factor. If the aircraft was close to stalling in a drastic maneuver, firing the guns could push it into a stall. – tj1000 Aug 29 '19 at 17:10