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In the near future, drones replace military fighter jets that require human pilots. These drones will be travelling at many times the speed of sound. Highly sophisticated drones will be able to evade enemy radar.

The problem when drones are used for espionage, in jungle or natural environments, is their supersonic flight will often leave behind a vapor-cone which poses serious noise pollution to the wild animals and native people in the region.

  1. How can I mute the sonic boom while the drone is travelling at supersonic speeds? (muffler actually in this case)

  2. The drone can achieve high speed maneuver to evade enemy fire at low altitude in order to unload package within specific area with great accuracy, will this revolutionary weapon reduce cost and casualty?

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This question asks for hard science. All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed. See the tag description for more information.

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    $\begingroup$ If the drones are being used for espionage, why are they flying so low? Surely that would limit their field of view and time over the target? $\endgroup$ – Steve Bird Aug 18 '15 at 6:13
  • $\begingroup$ Also, the lower they fly, the easier are they to spot by naked eye $\endgroup$ – Pavel Janicek Aug 18 '15 at 6:14
  • $\begingroup$ @SteveBird that's my Q2, I intended for these drones to dive into the enemy territory and drop markers/transmitters and then evacuate. (the package must fall as close proximity to the target as possible) $\endgroup$ – user6760 Aug 18 '15 at 6:21
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    $\begingroup$ It might be simpler to have your markers/transmitters dropped from high altitude and use smart guidance technology to get them on target. $\endgroup$ – Steve Bird Aug 18 '15 at 6:24
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    $\begingroup$ This looks like a case of the xy problem. Try to describe what you are trying to accomplish (and what limitations you actually have) rather than how you think it should be done. $\endgroup$ – Mourdos Aug 18 '15 at 9:50
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Drones replace military fighter jets that require human pilots.

So far, so good. If we have not a third world country, you need to take into consideration that an enemy can and will jam remote control, so an reserve system (AI) is necessary. If you want to get a live broadcast, the enemy will also easily find the drone because senders can be easily located.

Highly sophisticated drones will be able to evade enemy radar.

That means a stealthy approach.

These drones will be travelling at many times the speed of sound.

Why ? Because it seems to be cool ? What exactly is your reason to move fast ? If you want to use stealth for spying, supersonic flight is detrimental.

  • First problem: Moving fast means much stronger air resistance. You are wasting fuel which drastically reduces the amount of time your spy drone is available. Furthermore supersonic flight means turbojet which is louder, has a strong heat signature and eats more fuel than a carefully designed propeller.

  • Second problem: Supersonic flight will always produce sonic shock waves which are easily detectable. People can hear them, a wide range of instruments (acoustic, Schlieren) can register them. Even if there is technology available to dampen shock waves (interference cancelling), this won't dampen it enough to be unnoticeable.

  • Third problem: You cannot get a good recoinnaissance if you moving too fast. Try to get a good photo while being in a fast train and compare that with taking a photo standing still.

  • Fourth problem: Using radar stealth means that your drone must be shaped and equipped properly. Say what, neither the form nor radar-absorbing materials like high speed. The form causes much too air resistance and RAMs dont like heat caused by friction. Both stealth planes F-117A and B-2 are moving in the transsonic range. The other possibility is flying extremely low, but this is vulnerable to OTH radar systems from far range and from anti-aircraft systems in moderate range.

  • Fifth problem: Your "revolutionary" weapon already exists and it is called "cruise missile". Why do people need a drone if they can fire a cruise missile directly ?

Explain why you need high speed.

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  • $\begingroup$ I think that calling the use of a tmahawk "espionage" stretches the term a bit ... ;-) Other than that: good answer! $\endgroup$ – Burki Aug 18 '15 at 11:38
  • $\begingroup$ @Burki The cruise missile is referencing to: "The drone can achieve high speed maneuver to evade enemy fire at low altitude in order to unload package within specific area with great accuracy, will this revolutionary weapon reduce cost and casualty?". $\endgroup$ – Thorsten S. Aug 18 '15 at 11:41
  • $\begingroup$ I was not trying to say your point was invalid. Technically, it should have been directed towards the OP, since his drone is a lot more multi-purpose (and thus at least appearing impractical) than would typically be the case, i guess. $\endgroup$ – Burki Aug 18 '15 at 11:46
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Muting Sonic Boom

This is an incredibly hard question and there is no answer, yet. Various techniques have been attempted but not have ever completely eliminated the sonic boom. The physics just won't let you. Better to go sub-sonic.

Loiter Time

Check out these loiter times for unmanned drones:

MQ-1 Predator: 14 hours

MQ-9 Reaper : 14 hours (with munitions), 30 hours (without munitions)

Note that both drones achieve these loiter times by flying slowly. The A-10 achieves significantly longer loiter times over the F-16 simply by going slower. The faster you go the more energy it takes to overcome drag at higher speeds, ie, the faster you go the faster you burn fuel and the shorter a time you can stay airborne.

The drag equation indicates that the faster you go, that drag increases exponentially.

$F_D\, =\, \tfrac12\, \rho\, u^2\, C_D\, A$

$F_D$ is the drag force, which is by definition the force component in the direction of the flow velocity

$\rho$ is the mass density of the fluid

$u$ is the flow velocity relative to the object

$A$ is the reference area

$C_D$ is the drag coefficient – a dimensionless number|dimensionless coefficient related to the object's geometry and taking into account both skin friction and form drag.

Also note that $\rho$ decreases as altitude increases, thus explaining why the fastest planes always fly as high as possible.

Evading Enemy Fire

Unless you devote a large portion of your drone's sensors when flying at low altitude to detecting incoming air threats such as man-pads, SAMs or AAA fire, you're drone is going to be sitting duck. Devoting sensors to threat detection means that you have less space for mission sensors. The Reaper and Predator drones cited above never fly below 3000 meters over enemy territory because that makes them vulnerable to man-pads, SAMs and AAA. A drone specifically designed to counter these threats is not a recon drone, but a SEAD drone.

Alternative Implementations

A drone version of the SR-71 intended to fly incredibly high and incredibly fast might be very compelling but that doesn't fit the OP's requirements.

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