Jim Smith had significant technical roles in the development of the UK’s leading military aviation programmes from ASRAAM and Nimrod, to the JSF and Eurofighter Typhoon. He was also Britain’s technical liaison to the British Embassy in Washington, covering several projects including the Advanced Tactical Fighter contest. His latest book is available here. We asked him if the Angel Interceptor from the much loved 1960s children’s puppet TV show Capatin Scarlet would have worked in real life.

“A while ago now, Hush-Kit asked me to have a look at the Angel Interceptor to see if it was a plausible aircraft. I had a quick look, not knowing anything about the design, or the Captain Scarlet series it came from, and my immediate reaction was favourable, but qualified, along the lines of “Generally looks surprisingly OK, as long as they’re not claiming to do anything silly like hypersonics or STOVL, for example”.

 Well, I don’t need to say any more really, but here goes.

Configuration and stated performance

There is a useful amount of information available on the Angel Interceptor, which is helpful, even though much of it is in old-fashioned colonial units. Noting this, rather than make my comparisons with the Typhoon, I’ll use the F-22 as a comparator.

The Angel Interceptor is a three-surface configuration with a small forward canard, cranked delta wing with turned-down wing tips, and a large tailplane with turned-up tips. Although appearing to be a single-engine aircraft, it has a propulsion system described as “twin turbojet compressors serve the rear-mounted ramjet”. There are some technical issues with this description, but for the moment we’ll assume that we effectively have twin turbofan engines, effectively feeding a common afterburner unit. The F-22, of course, has twin turbofan engines (or given the by-pass ratio of 0.45, these could perhaps be better described as ‘leaky turbojets’) each with its own afterburner and thrust vectoring nozzle.

Among other details, we are told that the weight is 40,000 lb, span 35 ft and length 60 ft. Fuel volume is stated to be 500 gallons, and assuming these are Imperial gallons, this would translate to about 4000 lb fuel weight. The aircraft range is stated to be 25,675 miles, and maximum speed is said to be Mach 3.9. Ceiling is quoted as a surprisingly low 40,000 ft, which makes no sense, considering this is the height quoted for its operating base.

The Angel interceptor is launched from its Cloudbase (flying aircraft carrier) operating base by catapult, and recovered by a pitch-up manoeuvre on to an inclined ramp.

For comparison, the empty weight of the F-22 is about 32,000 lb, and its internal fuel capacity is 18,000 lb. The F-22 has a span of about 45 ft and length of about 60 ft. F-22 ferry range with two external fuel tanks is stated to be 1800 miles, maximum speed is Mach 2.25 (Wikipedia), and ceiling is 50,000 ft.

Performance Issues

From this very quick and limited comparison we can observe some obvious problems. The Angel is, in a very broad sense, comparable with the F-22, having similar operational empty weight of around 35,000 lb, and broadly similar size, although the different planform of the F-22 has greater span. Given this rather broad resemblance, we can see that both the quoted range and the maximum speed of the Angel interceptor look utterly implausible.

Now, it might be argued that there is some magic in the unusual turbo-compressor/ramjet propulsion system, resulting in very high thrust and low fuel consumption. The nearest aircraft in performance terms to the claims of the Angel would be the SR-71, which uses a variable-cycle turbine engine that has been described as operating like a ramjet at high speed. The overall length of the SR-71 engine, intake and nozzle system is about 45 ft, which is somewhat longer that the engine installation on the Angel. The thrust of each engine is 32,500 lb with afterburning, and the unrefuelled range of the SR-71 is stated to be 2982 miles at Mach 3. To achieve this requires ‘more than 80000 lb’ of fuel (Janes all the Worlds Aircraft 1974-5). From this, we can only describe the stated range performance of the Angel as unachievable.

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Even were a ‘magic’ fuel to be available – stated to be ‘coboltide’, it seems implausible that the stated 50+ mile per gallon fuel consumption could be achieved, particularly at high speed. SR-71 data suggests that at Mach 3, that aircraft consumes 27 lb (say 3.35 imp gal) of fuel per mile.  The Angel Interceptor is assuming more than 150 times the fuel efficiency of the SR-71.

The maximum speed quoted is Mach 3.9, which raises real issues for both aerodynamic and thermal heating, particularly since the wing tip pods, the tips of the canards and the outer wings would all lie outside the Mach cone from the aircraft nose at that speed. Essentially, this means that they would experience greater aerodynamic heating and wave drag.

Likely ‘real world’ performance

No thrust rating is quoted for the unusual propulsion system, although one of the internet sources suggests perhaps 50000 lb thrust. If we take this as a working assumption, given the weight and the broad configuration, and assume fairly conventional materials are used, a maximum speed of perhaps M 2.5 might be achievable, but for some very draggy features – particularly the fuselage rocket batteries. Internal carriage of the rockets might be a modification worth examining. The forward canard surface might also be better if relocated to a position on the intakes, like the Mirage 4000.

With these changes, quite respectable speed and manoeuvre performance should be achievable, although a ferry range of perhaps 1500 miles is more likely than the stated 25,000 miles. Very much in line with my initial reaction – quite a decent design assuming no attempt at hypersonics or VSTOL.

Take-Off and Landing

The aircraft is depicted in the show as using a catapult-assisted take-off, and this seems to be a reasonable approach, given it is supposed to be operating from a ‘base’ maintained at 40000 ft altitude. Cutaway drawings show no means of achieving thrust-assisted flight, let alone either STO or a vertical landing.

The landing is not vertical, but instead is as a pull up to a stalling attitude, with forward momentum taking the aircraft on to land on an inclined ramp. While it might be possible to maintain controlled flight in a high-powered jet aircraft with a high nose-up angle – this is, after all, a party piece at many airshows – we should not forget that those are at low altitude, not the 40000 ft of the Cloudbase.

The stalling speed of an aircraft in level flight is given by the expression:

Vstall =17.2 x Square Root (Weight/(CLmax x Sigma x Wing Area))

Where Vstall is in knots, weight is in pounds, sigma is the ratio of the air density to the density at sea level, and the wing area is in square feet. (Aerodynamics for Naval aviators). CLmax is the maximum lift coefficient.

From this useful equation, and making a few assumptions about weight, wing area and CLmax, we find that in level flight, at sea level, and assuming a landing weight of 35000 lbs, and a CLmax of 1.8, we get

Vstall = 91 kt

Which is reassuring as it suggests an approach speed of about 118 kt, which appears reasonable.

However, at 40000 ft, Sigma = 0.25, and repeating the calculation, we find the straight and level stall occurs at 181 kt, suggesting an approach speed of about 235 kt, which is clearly untenable.

But, I hear the reader say, what about landing in a stalled condition on to a 30 degree ramp, with the engine thrust offsetting the weight.  

At 30 degrees incidence, using full thrust (assumed to be 50000lb), the wing only has to provide 10000 lb lift, the remainder being balanced by the engine in the high alpha approach. In these circumstances

Vstall = 97 kt

If we make a small allowance for controllability, the approach speed might be 120 kt, and the controlled crash would still be at an unmanageable 100 kts or so.

My advice would be “Don’t try this at home, folks!” unless Cloudbase is not only sustaining itself at 40,000 ft, but also cruising at about 100 kt.

Looking at one of the relevant episodes, it is apparent that while Coudbase does have engines for changing its location, it does not appear to be in motion during the landing sequence. Operating from a static Cloudbase simply makes no sense, because you won’t be able to land back on board. A conventional carrier landing from an approach speed of 200 kt+ is not going to work. The alternative of pitching up to 30 deg to land on a ramp at 100 kt will not work either.

If Cloudbase were moving at 100 kt or so during the landing sequence, then a conventional carrier landing using arrester wires would be possible, and would be a more flexible and less dangerous solution than the inclined ramp. It would, however, require a very different undercarriage arrangement.

Other Aspects

The Angel Interceptor is supposed to use long-range radar-guided air-to-air missiles as a primary weapon, and also to have a gun, or a directed energy weapon. No issues in principle with the choice of weapons, except to note that the physical space available for a radar to detect and track targets is entirely inadequate, and the extremely finely tapered nose has a shape which would not provide a suitable radome either.

One thing the extremely tapered nose would be good for is in reducing wave drag. The pointed nose acts as an Aerospike, forcing the conical shockwave from its tip forward, and largely keeping it from intersecting other aircraft components, at least up to approximately Mach 2.0 . Aerospikes are not often used, but an example can be seen on the nose of the Trident nuclear missile.

The undercarriage of the Angel Interceptor is located in pods on the tips of the sharply down-swept wings. Although one is tempted to wonder whether this is all done for visual effect, and whether a conventional retractable undercarriage would be a lower drag solution, the down-swept wings might actually be useful.

Coupling between the lateral (roll) and directional (yaw) dynamic behaviour can be problematic in relatively slender aircraft at high speed and high altitude. Otherwise known as ‘inertia coupling’, managing this behaviour requires careful attention to lateral and directional stability, and additional fin area below the axis of the aircraft, or reducing the dihedral effect of the wing has been found to be helpful.  So, the down-swept wing tips, used to carry the undercarriage pods, are likely also to be useful in managing ‘inertia coupling’.

The pilot is ‘loaded’ into the aircraft from below, pre-connected to her seat, with a transparent shield arrangement presumably ensuring protection from the low temperature and low-pressure environment at 40000 ft. The seat arrangement is ejected upwards in the event of an emergency. Perhaps a lost opportunity to feature a downward ejector seat, as used on the Vultee XP-54 ‘Swoose Goose’, which also used this cockpit access method.

A lighter solution would surely be to run up steps to enter the cockpit in the normal way, but this would have to take place on the maintenance deck, which would greatly increase the time taken to launch the aircraft.

Angel Interceptor – Good or Bad?

Judged as a conventional Mach 2.2-ish fighter, operating from land or from an aircraft carrier (with modified undercarriage) – not at all bad. Reasonable credible performance and manoeuvrability, moderate range, and a number of interesting features. The radar would be hopeless; the missile installation looks draggy, and the undercarriage somewhat suspect.

Judged as a system with the full claimed capabilities – hopeless. Basing at 40000 ft simply makes landing arrangements implausible, The claims for range, and the use of the exotic ‘coboltide’ fuel, leave the design open to ridicule, as does the claim to be able to fly at Mach 3.9, and to have any kind of STOVL capability.

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The propulsion system description is sketchy, but, if considered as two military jet engines feeding a single afterburner might be workable. The alternative approach of switching from gas turbine thrust to ramjet thrust is another possibility, but the internal layout is not set up for that approach.

My verdict – Good fun; interesting original features. Given some modifications, such as a decent radar and internal weapons bays, potentially a good conventional fighter, but unable to operate as depicted in the show, or deliver the claimed range and maximum speed.”

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