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. In his life he has provided technical assessments of the YF-22 and YF-23, we wondered what he would make of the Lippisch P 13, an exceptionally ambitious wartime concept for a supersonic interceptor of bizarre appearance powered by a coal-fuelled ramjet. “Hush-Kit has asked me for an assessment of the extraordinary Lippisch L13a. A bold and innovative concept, which, although it did not get to fly, was wind tunnel tested, and was also trialled in glider form. Before I give my thoughts on the aircraft I’ll start by quoting verbatim the Wikipedia entry:

"(https://en.wikipedia.org/wiki/Lippisch_P.13a):
Lippisch P.13a
From Wikipedia, 

Lippisch P.13a

Model of Lippisch P13a at the Technik Museum Speyer
Role Interceptor
Designer Lippisch
Status Project
Number built 0
Developed from Lippisch DM-1
The Lippisch P.13a was an experimental ramjet-powered delta wing interceptor aircraft designed in late 1944 by Dr. Alexander Lippisch for Nazi Germany. The aircraft never made it past the drawing board, but testing of wind-tunnel models in the DVL high-speed wind tunnel showed that the design had extraordinary stability into the Mach 2.6 range.[1]

Contents
Design and development[edit]
As conventional fuels were in extremely short supply by late 1944, Lippisch proposed that the P.13a be powered by coal. Initially, it was proposed that a wire-mesh basket holding coal be mounted behind a nose air intake, protruding slightly into the airflow and ignited by a gas burner. Following wind-tunnel testing of the ramjet and the coal basket, modifications were incorporated to provide more efficient combustion.
The coal was to take the form of small granules instead of irregular lumps, to produce a controlled and even burn, and the basket was altered to a mesh drum revolving on a vertical axis at 60 rpm. A jet of flame from tanks of bottled gas would fire into the basket once the P.13a had reached operating speed (above 320 km/h), whether by using a rocket to assist takeoff or by being towed.
The air passing through the ramjet would take the fumes from the burning coal towards the rear where they would mix under high pressure with clean air taken from a separate intake. The resulting mixture of gas would then be directed out through a rear nozzle to provide thrust. A burner and drum were built and tested successfully in Vienna by the design team before the end of the war.
It is not known what armament would have been carried by the P.13a; the MK 103 cannon would have been too heavy and large for such a small aircraft and it is possible that one or two large-calibre machineguns would have been used.
At the end of the war even the prototype DM-1 test glider had not been finished when it was captured by American forces. The Americans ordered Lippisch's team to complete the glider, and it was then shipped to the United States where it was test-flown. According to the National Advisory Committee for Aeronautics the results were positive[2] and lessons learned were incorporated into NASA's research aircraft of the 1950s and on.
Film footage exists which shows a gliding test of a scaled-down model of the P.13a. These tests began in May 1944 at Spitzerberg, near Vienna.[3]
Variants
• Akaflieg Darmstadt/Akaflieg München DM.1 - AKA Lippisch DM.1 A scale flying wind tunnel glider version of the proposed Lippisch P.13a
• Lippisch P.13b - a further development of the P.13a, which never came beyond the drawing board.
The P.13a was completely unrelated to the 1942 project for a high-speed bomber aircraft, but similarly named P.13.
Specifications (P.13a, as designed)
General characteristics
• Crew: one
• Length: 6.70 m (22 ft 0 in)
• Wingspan: 6.00 m (19 ft 9 in)
• Height: 3.25 m (10 ft 8 in)
• Wing area: 20.0 m² (215 ft²)
• Loaded weight: 2,295 kg (5,060 lb)
• Powerplant: 1 × Kronach Lorin coal-burning ramjet

Performance
• Maximum speed: 1,650 km/h (1,025 mph)
• Range: 1,000 km (621 miles)
• Wing loading: 115 kg/m² (24 lb/ft²)" 

The configuration, propulsion system, and claimed performance are all of note, as is the fact that a glider version is known to have been tested. The maximum speed quoted was probably first exceeded by the Fairey Delta 2*, which established a world speed record of 1132 mph on 10 March 1956, just over 63 years ago, and 12 years after the DM 1 glider trials supporting the L 13a program. (*editor notes: the Bell X-1A appears to have done this earlier, but this was air-launched)

Other similar aircraft

The aircraft which perhaps most closely resembles the Lippisch in configuration is the Payen Delta, an example of which is to be found in the Musee de L’Air in Paris. This flew successfully, powered by a Turbomeca Palas engine of only 330 lb thrust. Projected developments included a jet trainer, the P 56 Jockey, to have been powered by a 1640 lb thrust Viper engine.

Another little-known French aircraft of similar configuration, the Gerfaut, was the first European jet aircraft to exceed the speed of sound in level flight, and did so without the use of an afterburner. It too was intended to be developed with the use of an afterburning ATAR 101C engine.

The Fairey Delta 1 was originally designed as a vertical take-off fighter (!), but was modified to become a delta-wing research aircraft, first flying in 1951.

A small aircraft, with a wing span of only 19.5 ft, rivalling the tiny Payen Delta’s 17 ft and quite similar in size to the P 13a. For comparison, the Chipmunk, which I used to enjoy flying, has a wingspan of 31 ft.

The Boulton Paul P111 was built to investigate the delta wing in transonic flight. Powered by a Nene turbojet of 5100 lb thrust, it was just subsonic in level flight, and able to reach supersonic speeds in a shallow dive.

The XF-92A was the pre-cursor to the F-102 Delta Dart, and in many ways resembles a turbo-jet powered P 13a. A larger aircraft, with a span of 31 ft, it first flew with an Allison J33 with 5400 lb max thrust, but even when fitted with an afterburner and delivering 8200 lb thrust, its maximum speed was Mach 0.95.

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Assessment of the P 13a

How to assess the bold and innovative Lippisch P 13a? Readers who have seen my earlier discussion of fictional aircraft (the F-19, Firefox and Stingbat) will be aware that I am prepared to take a pretty open-minded view about the most radical configurations. In the case of the little P 13a, it seems to me necessary to look at the following aspects: propulsion; configuration; stability & control, and performance, and through examining these aspects seek to form a rounded view of the design.

Propulsion

This is the most challenging area to assess. Whoever heard of a coke-burning ramjet propulsion system for a supersonic aircraft? Well, we must not just reject the unfamiliar out of hand. After all, the Germans had a long history of highly innovative piston engines, had by this time flown both jet and rocket-powered manned aircraft, had deployed operationally a cruise missile powered by a gasoline burning pulse jet, and had fielded an operational liquid-fuelled tactical ballistic missile, the V2.

To get a feel for the plausibility of a carbon-pellet-burning and natural-gas-initiated ramjet, I checked out the energy-density that might be available. For natural gas, this is about 10% better than gasoline, and for coal around 25% worse. In the system as a whole, the ramjet is likely to be lighter than a turbojet, but this will be offset by pressure storage for the gas, and the rotating drum and storage for the carbon pellets. Apart from a general conclusion that the thermodynamics of propulsion systems should be left to the experts, my overall feeling was that a functioning system might be achieved.

For the moment, in the spirit of exploring the P 13a further, I will assume a viable propulsion system exists. This is a tried and tested way of assessing systems with new technologies. One simply assumes that the technology will work as advertised, and you are then assessing the best possible outcome.

Of the similar aircraft considered above, it is worth noting that the Fairey Delta 2, considerably larger than the Lippisch, requires an Avon engine with 10,000 lb thrust, and the F-102, capable of 825 mph, required 17,200 lb thrust. To achieve a similar thrust to weight ratio to the Fairey Delta 2, the P 13a would require a thrust of about 3750 lb.

At the tropopause, Mach 1 is about 660 mph, and the claimed max speed of the P 13a at the tropopause would be equivalent to Mach 1.55, just in the acceptable range for the simple pitot intake shown.

Configuration

From the perspective of a concept looking to achieve a design speed of around 1000 mph, or Mach 1.55, drag for this design is going to be a significant problem. There are two main issues: firstly, the wing thickness-to-chord ratio appears far to high to achieve supersonic flight, let alone 1000 mph; and secondly, the configuration was clearly designed when the understanding of wave drag was in its infancy.

The XF-92A revealed the consequences of a failure to understand wave drag all too clearly. The designers had access not only to trials of the DM-1 glider, but to Lippisch himself, and to wind tunnel test data from NASA Langley showing the DM-1/P 13a wing thickness was too large, and would generate high transonic drag. Flight test revealed the inability of the XF-92A to fly at supersonic speeds, even with the installation of a more powerful engine.

The resolution of this problem was the discovery and articulation of the Area Rule by R T Whitcomb and R T Jones at NACA Langley and NACA Ames respectively, although the principle had earlier been established by Junkers in Germany. As depicted in the illustration accompanying the Wikipedia article, there is no way this configuration would have been supersonic in level flight.

As indicated in the Wikipedia article, payload-range, and armament in particular, is also a concern. The aircraft is very small indeed, and the quoted range looks a little unlikely. Against the principal threat towards the end of the war – US bombers and their escort fighters, an armament of two machine guns seems unlikely to be an effective weapons capability, despite the claimed high speed of the aircraft.

Stability and Control

DVL wind tunnel tests are cited as showing stability of the design up to Mach 2.76. These results would have been regarded as plausible at the time because German capability in supersonic wind tunnel testing was world-leading at the time. However, even were these results to be accurate, transonic testing would be required to reveal the large increase in wave drag which, in my view, would limit the design to subsonic flight.

US testing of the XF-92A showed some good qualities, particularly the relatively slow approach and landing speeds made possible by the leading-edge vortices formed over the wings at high incidence and low speed. A transonic pitch up issue, leading to loads of 6 to 8 g was discovered, as well as the use of wing fences as a means of flow control which made this problem manageable. Wing leading-edge camber, later applied to the F-102, is likely to have both improved high speed stability and reduced drag.

Powered flying controls had been discovered by the US to be necessary to control aircraft in the transonic reason. These are likely to have been fitted to the XF-92A, and were certainly fitted to the F-102, but this need had almost certainly not been identified by the designers of the Lippisch P 13a.

Performance

As shown in the Wikipedia article, it seems clear the Lippisch P 13a would be very unlikely to be supersonic in level flight. Although I cannot pretend to have a deep understanding of the carbon and gas fuelled ramjet, it seems to me similarly unlikely that the claimed range of 1000 km/621 miles could have been achieved, mainly because of the extremely small size of the airframe.

The development history of the DM-1 –  XF-92A – YF-102 – F-102A  series shows conclusively that a thinner, more sophisticated wing, incorporating leading edge camber, and the application of a coke-bottle (pun intended) fuselage incorporating lessons learned from the Whitcomb/Jones Area Rule would be required to attain anywhere near the claimed speeds.

Overall Assessment

Much to my regret, I have to find the Lippisch P 13a, as the Mythbusters TV program would have said, “Busted”, or at least “very implausible”. The claimed performance could not have been achieved, and the weapons carried were likely to prove ineffective. The claimed supersonic stability is not relevant because it is unlikely those speeds could be reached.

In addition, the domains of wave drag, transonic pitch up, and roll-yaw coupling were unknown and therefore unaddressed. These issues were encountered in the XF-92A and YF-102 and eventually resolved in the F102A.

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