This time around, we turn our attention from combat aircraft and transports to the weird and wonderful world of support aircraft. The common thread here is information, either gathering it, moving it around, or denying it to the enemy.
AWACS
The US military has a lot of airplanes, and that in turn means it's generally a good idea to have someone coordinating all of them. It would be really helpful if that someone was mobile and had a really nice radar, and shortly after WWII, people realized that they could bundle the whole thing into an airplane, and thus was born Airborne Warning And Control Systems, or AWACS.1 It's an extremely useful concept, and they form a critical component of any serious Air Force.
E-3 Sentry
What would happen if we took a 707 airframe, attacked it with a flying saucer, and then stuffed the back full of people to manage an air battle? The E-3 is pretty much the worldwide standard for such things, and it's an extremely versatile platform, combining the radar with a sophisticated ESM system to give incredible situational awareness. The US only has 15 in inventory, and they are very busy airplanes. So busy, in fact, that they still haven't gotten around to replacing the engines, which are extremely noisy. But it's OK, because the Sentry is in the twilight of its career with the US, and with most of the other countries that operate it, because the 707 airframe is old and terrible.
E-7 Wedgetail
The replacement for the E-3 was developed by applying the same formula to a more modern airplane, although the "UFO attacking an airliner" aesthetic has been spoiled by phased arrays. The E-7 is a 737NG with a pair of fixed phased arrays on the top, designed to fulfill basically the same mission as the E-3. It's currently in service with Australia, South Korea, Turkey and Britain, and on order with the US and NATO (which operates a small AWACS fleet under the flag of Luxembourg).
E-2 Hawkeye
The E-3 is a fine aircraft (unless you live under the approach path at Tinker AFB, as I used to) but it is completely incapable of landing on an aircraft carrier. As a result, the Navy had to look elsewhere for its AWACS needs, and the result was the E-2 Hawkeye, a purpose-built turboprop plane with two pilots in the front and three crew in the back to control an air battle. Despite the much smaller crew than its land-based counterparts, it is an extremely capable airplane. Each carrier operates a squadron of 4, and it has been exported to the French Navy, and to 6 other countries for use from land bases. Astonishingly for a design from the 60s in such a niche role, it remains in production, and upgrades are ongoing.
Command and Control
There are a number of planes dedicated to providing command and control to things that are not combat aircraft. Generally, these are nuclear forces, but not always, and in any case, the plane is generally a converted civilian airliner full of fancy electronics and maybe a command team.
E-4B NEACP
The National Emergency Airborne Command Post is a modified 747, nuclear hardened and set up to provide a survivable command capability for the nation's nuclear forces in the event that the worst happens. Also known as the "doomsday plane" or "Nightwatch", the fleet of four jets is hardened against EMP and can carry a crew of 100 or more to ensure that the US can retaliate in the event of a nuclear war. Early work has begun on a replacement based on the 747-8.
E-6B Mercury
Another product of the US Military's inexplicable love affair with the Boeing 707, the E-6 is operated by the Navy to provide survivable communications with the nation's ballistic missile submarines. It carries a 10 km antenna, which is held vertically by the airplane flying in a circle, and can be used to send low-frequency radio signals to submerged submarines, as well as passing launch orders to land-based ICBMs. The E-6 is in the twilight of its career, and will soon be replaced by a C-130 variant that for some inexplicable reason has been dubbed the E-130J.
VC-25
Better known as Air Force One (although it only carries that callsign with the President is actually onboard), the VC-25 is a plane roughly based on the 747 and designed to carry the most powerful man in the free world and his entourage around. Modifications range from EMP hardening and a truly impressive communications suite to a second fueling point on the right wing so that the press doesn't see the ramp personnel. Currently being replaced by a couple of 747-8s.
E-11 Battlefield Airborne Communications Node
The odd one out in this category, both because it isn't for the nuclear forces, and because it isn't being replaced.2 The BACN is a Bombardier Global Express business jet stuffed full of equipment to move data between different datalinks, allowing various participants a better view of the battlefield.
Electronic Warfare
These are aircraft intended less for moving around information and more for denying it to the enemy by attacking his electromagnetic capabilities.
EA-18G Growler
The US military's primary tactical jamming aircraft, based on the F/A-18F Super Hornet, and intended to shut down enemy radars either with noise or with missiles, allowing other planes to do their jobs. While obviously designed to fly from carriers and primarily used by the Navy, four of the squadrons are "expeditionary" and assigned to support the Air Force, which gave up its dedicated jamming planes back in the 90s. 172 have been built, a dozen of them for Australia.
EA-37B Compass Call
A modified G550 business jet3 stuffed with jamming gear, primarily intended for use against enemy communications. Set up to provide standoff jamming with a massive phased array (the same as used on the G550 AWACS), as it's less suitable to close with the enemy than the Growler, and the bigger platform gives more room for operators and equipment.
RC-135
The other half of the electronic warfare game is signals collection, and the Air Force has ripped the refueling equipment out of a number of KC-135s and filled them with all sorts of sophisticated electronics to collect intelligence. A full overview of all the variants would take a post of their own, but the most prominent is known as "Rivet Joint" and is intended to pick up enemy radar signals for various purposes.
EP-3E ARIES
The Navy's equivalent of the RC-135, but based on the P-3 Orion patrol aircraft. Famous mostly for causing a diplomatic crisis in 2001, when an aircraft on patrol off the Chinese coast collided with an intercepting fighter and had to make an emergency landing on Hainan Island. The Chinese eventually returned the aircraft, but not before they examined the sensitive collection electronics. In the very twilight of its career, with the Navy planning to replace it with a mission package for the unmanned MQ-4C Triton.
RC-12 Guardrail
A modified King Air set up for gathering information. Primarily operated by the Army for tactical signals collection, although other services seem to operate variants for when they need to have a fancy camera in the air at a reasonable cost with a person attached.
Next time, we'll turn our attention to the world of helicopters, which are generally slow and expensive to operate, but have one important benefit: the ability to take off and land vertically.
2 These two are related. There was a big nuclear modernization in the 80s which produced the E-4, E-6 and VC-25, all of which thus needed to be replaced around the same time. ⇑
Comments
I thought the Army wasn't allowed to fly airplanes.
No, the ban in the Key West Agreement is on armed fixed-wing airplanes. This allows both armed helicopters and some level of unarmed airplanes. (It’s not entirely clear where drones fall, as the Army has recently started operating a Predator derivative with Hellfires.) Back in the 50s/60s, they tried to make their own airlift force, but a clarification was issued that that was an Air Force mission instead.
Cynically, if it's not going to involve much money or cool planes, the Air Force is fine with the Army doing it themselves, because someone has to shuttle around the division commander, and there's no reason that person has to be Air Force.
On the 707, I thought that more engines used to be popular on jet planes because it made it more likely that the plane would survive the loss of one engine, a concern which has faded with more reliable jets, but which might have lingered longer in the military, leading to excess fondness for the 707 because of the four engines.
@Protagoras: my guess is it's because most progress since the 707 has been in the direction of lowering running costs (e.g. improved fuel efficiency) rather than improving performance, and as military planes spend less of their time actually flying than airliners, they have less reason to care about that. (Particularly for types that have extensive modifications and/or are used in small numbers, so replacing them would have high development costs per plane.)
This might also be part of why they're now among the few buying 767s rather than 787s.
That's probably a lot of it. And in fairness, the E-3 was built when there wasn't really an alternative to the 707 (or I guess DC-8) in the size range they needed. I'd guess for the E-6 they wanted more engines and also they were already flying E-3s. (Also worth noting that the 757 or 767 would have been quite new at that point.) But I have some history with the 707 family, and don't have to be more than grudgingly fair to them these days.
I think a lot of this (as with the C-130 and to a lesser extent the C-135) is that subsonic aerodynamics don’t have a lot of room left to squeeze from what they knew in the 50s/60s. Most of the incremental improvement seems to have been from the engines - higher bypass ratios and higher turbine temperatures for better combustion.
But the other side of it is that the mission systems are almost unrecognizable.
On the E-7 - can it scan end on, or is there a null spot of lowered performance?
I believe there's a null, but it's smaller than you'd think. That sort of phased array system has become quite common in recent years, with the EC-37 using basically the same arrangement that a lot of countries use on G550-based AWACS. Particularly with modern combat systems/datalinks, you can afford to take the minute or so of no data that you get when turning around, and it greatly simplifies the system.
I have to wonder if its possible to effectively shove an antenna in the wing spar of the aircraft. If so, the only real null would be very small. It would also make it harder for polarization to be an issue.
From what I can gather, the big issue with ECM is knowing what the transmitter is going to do in order to be able to screw with it. Hence the SIGINT stuff. With software-defined radio, you can mimic just about anything as long as you know what you want to produce. The challenge is in knowing what to produce.
I'm uncertain how practical it would be for an aircraft to be able to do that in real-time. That is, intercept a novel signal, decide how to screw with the receiver, and then do it in real-time.
Some things are a lot more difficult to screw with. If you can code/frequency-hop using quality cryptographic algorithms and implementations, there would be no way for an aircraft with ECM to generate a fake return signal before being hit by the relevant pulse. So it could generate a fake return which would suggest that it is further from the transmitter than it really is, but it wouldn't be able to eliminate the initial radar return.
What I don't know is whether you can properly fool multiple radar receivers significantly off-axis from each other simultaneously. This exceeds my knowledge of radio theory.
Fitting the antenna in a wing spar is going to require designing the plane for the job from the ground up, so I don't think that's really plausible relative to free-riding on the development costs of an airliner (unless they decide to replace the E-2, I guess, and even then you have to worry about aerodynamic stuff getting in the way). There's also the problem that the antenna is going to be pointing the wrong way. You're generally going to have your AWACS flying a racetrack with the long axis perpendicular to the direction to the target (because it keeps you at the appropriate distance for the longest time), so you care most about seeing to the sides of the plane. Older AWACS have domes because they were designed in the era when you needed to mechanically scan the antenna, so an all-around view is really helpful (unless you decide you want to imitate Nimrod AEW.3, which is clearly a terrible idea).
Protagoras
No, it was over concerns about losing more than one engine (though having four engines increases the chance of that happening).
Also back then they didn't have engines powerful enough for a large aircraft to be a twinjet.
redRover:
The thing on top allows it to look at the front and back (while the sides are done by the things on the side).
bean:
If you could make the radar air cooled maybe not, but then are you really imitating it?
the army actually has a growing fleet of prop powered ISR planes. It's not very well documented, because the army crams all of fixed wing aviation into a single budget line. but they have a lot of weird isr stuff like this: https://en.wikipedia.org/wiki/BeechcraftRC-12Guardrail
The Italian Air Force is getting EA-37B Compass Calls, alongside the G550 CAEW we have as AWACS.
OTGH in the 50s/60s on of the things the US Army tried to get was the G-91, so that they could do their own CAS.