SI VIS PACEM, PARA BELLUM: L'Horten Ho IX, identificato in base al sistema di designazione RLM Horten Ho 229 e spesso erroneamente citato anche come Gotha Go 229 dall'azienda costruttrice

L'Horten Ho IX, identificato in base al sistema di designazione RLM Horten Ho 229 e spesso erroneamente citato anche come Gotha Go 229 dall'azienda costruttrice, era un aereo multiruolo progettato dai fratelli Reimar Horten e Walter Horten per conto dell'azienda aeronautica tedesca Horten-Flugzeuge nei primi anni quaranta.

Era caratterizzato dall'impostazione tutt'ala tipica dei fratelli Horten e dalla innovativa propulsione a reazione.

Storia del progetto

Nei primi anni trenta, alcuni progettisti tedeschi si interessarono della configurazione tutt'ala come metodo per migliorare le prestazioni dei loro alianti, tra questi i fratelli Reimar e Walter Horten. Il governo tedesco stava fondando molti club di alianti, perché all'epoca gli era proibita la costruzione di aerei militari dal Trattato di Versailles stipulato dopo la prima guerra mondiale. La configurazione tutt'ala rimuove tutte le superfici "non necessarie" e, almeno in teoria, riduce l'attrito con l'aria al minimo possibile. Quindi una configurazione ad ala volante permette, a parità di prestazioni, di costruire degli alianti con ali più corte e quindi più robuste, senza l'attrito causato dalla fusoliera.

Nel 1943, il Reichsmarschall Hermann Göring promosse la cosiddetta richiesta 3x1000 per produrre un bombardiere che fosse in grado di trasportare un carico di 1 000 kg a una distanza di 1 000 km e a una velocità di 1 000 km/h. I bombardieri tedeschi convenzionali potevano raggiungere i centri di comando alleati in Gran Bretagna, ma stavano accusando disastrose perdite da parte dei caccia alleati. All'epoca semplicemente non si conosceva alcun modo per raggiungere questi obiettivi: il nuovissimo motore a reazione Junkers Jumo 004B poteva fornire la velocità necessaria, ma era troppo assetato di combustibile.

Gli Horten erano convinti che il loro progetto di ala volante a basso attrito potesse soddisfare gli obiettivi richiesti. Allora proposero al governo il loro progetto personale (e gelosamente custodito): l'Ho IX, come punto di partenza per il bombardiere. Il ministro dell'aria del governo (Reichsluftfahrtministerium) approvò la proposta degli Horten, ma ordinò l'aggiunta di due cannoni da 30 mm, perché riteneva che l'aereo avrebbe potuto essere utile anche come caccia, vista la sua velocità di punta stimata, che era molto più elevata di qualsiasi altro aereo alleato.

Il primo Ho IX V1, che era un aliante senza motore, volò il 1º settembre 1944. A questo seguì nel dicembre 1944 l'Ho IX V2, che era spinto da uno Junkers Jumo 004 (il progetto originale prevedeva l'impiego del più potente BMW 003, ma al momento questo propulsore era praticamente irreperibile). Göring credeva nel progetto e ordinò una serie di 40 aerei di produzione alla Gotha con la designazione Ho 229 del RLM (ministero dell'aria tedesco) prima ancora che avesse volato il prototipo motorizzato. Il programma non fu fermato nemmeno il 18 febbraio 1945, quando l'unico Ho IX V2 precipitò a terra a causa dell'esplosione di un motore dopo soltanto due ore di volo: infatti, vennero ordinati altri prototipi e 20 aerei di pre-produzione. Il 12 marzo 1945, l'Ho 229 venne incluso nel Programma Jägernot per una produzione accelerata di Wunderwaffen a basso costo.

Caratterizzato da un'impostazione innovativa ed anticonvenzionale, riproponeva l'aspetto dei velivoli tutt'ala progettati dai fratelli Horten, venendo sviluppato inizialmente dalla Horten-Flugzeugen e poi dalla Gothaer Waggonfabrik, l'unica delle due che riuscì a completare i soli tre esemplari, non riuscendo però a superare la fase di prototipo. Pur realizzato essenzialmente con materiali non strategici, in legno e compensato, adottò per la sua ricopertura una speciale vernice in grado di assorbire le onde radio, cosa che, unitamente alla forma, ne riduceva l'individuabilità da parte dei sistemi di avvistamento radar dell'epoca, anticipando molte delle soluzioni adottate per il bombardiere stealth Northrop Grumman B-2 Spirit. Fortemente appoggiato dal Reichmaresciallo Hermann Göring, fu l'unico modello che riuscì ad avvicinarsi alle esigenti specifiche emesse dal Reichsluftfahrtministerium (RLM) per il "Progetto 3000".

Destinato ai reparti della Luftwaffe, grazie alle elevate prestazioni previste, una velocità stimata di 1 024 km/h ed una quota di tangenza superiore ai 15 000 metri, l'Ho 229 sarebbe stato un avversario assai temibile per le forze aeree alleate.

Durante gli ultimi atti della guerra, l'esercito statunitense diede il via all'Operazione Paperclip, che era uno sforzo da parte delle varie agenzie di Intelligence per impossessarsi delle ricerche tedesche su armi avanzate, ed impedire che cadessero in mano alle truppe sovietiche. Un veleggiatore Horten e l'Ho 229 V3, che era quasi completamente assemblato, vennero protetti e inviati alla Northrop Corporation negli Stati Uniti perché fossero sottoposti a dei test. Fu scelta la Northrop a causa della sua esperienza con le ali volanti, dato che Jack Northrop, ispirato dai filmati d'anteguerra dei veleggiatori Horten, aveva iniziato a costruire aerei tutt'ala a partire dal Northrop N-1M del 1939.

Tecnica

L'Ho 229 era di costruzione mista, con sezione centrale in tubi d'acciaio saldati e superfici alari principalmente in legno. Le ali erano fatte con due sottili pannelli in compensato incollati con una mistura contenente tra l'altro segatura e polvere di carbone. Se queste scelte furono fatte per migliorare la caratteristica stealth (ovvero l'invisibilità ai radar), la protezione al fuoco nemico, o semplicemente perché alla fine della guerra la Germania aveva carenza di metalli e gli Horten avevano necessità di rinforzare il compensato per il volo transonico, è ancora motivo di dibattiti. Il controllo del velivolo era affidato agli elevoni e agli spoiler. L'aereo utilizzava un carrello di atterraggio triciclo retrattile, e aveva un paracadute per rallentare in fase di atterraggio. Il pilota sedeva su un primitivo sedile eiettabile.

Nel corso del tempo diversi velivoli aventi linea simile sono stati sviluppati dalla Northrop come il Northrop YB-35, il Northrop YB-49 e il Northrop-Grumman B-2 Spirit.

Versioni:

H.IX V1 Primo prototipo, aliante senza motore, un esemplare costruito e portato in volo.
H.IX V2 Primo prototipo motorizzato, equipaggiato con due motori turbogetto Junkers Jumo 004B, un esemplare costruito e portato in volo.
Sviluppi Gothaer Waggonfabrik:
Ho 229 V3 sviluppo, revisione delle prese d'aria e spostamento in avanti dei due motori per correggere il bilanciamento longitudinale; un esemplare in costruzione catturato dagli alleati.
Ho 229 V4 prototipo della versione pianificata da caccia ognitempo biposto; in fase di costruzione a Friedrichroda, non fu realizzata altro che la sezione centrale tubolare della cellula.
Ho 229 V5 prototipo della versione pianificata da caccia ognitempo biposto; in fase di costruzione a Friedrichroda, non fu realizzata altro che la sezione centrale tubolare della cellula.
Ho 229 V6 prototipo, versione definitiva caccia monoposto; realizzato un mock-up in fase di costruzione a Ilmenau.
Sviluppi Horten-Flugzeugen:
H.IXb (identificati anche come V6 e V7 dalla Horten)
progetti per le pianificate versioni di addestratore biposto o caccia notturno, mai costruito.
Ho 229A-0 solo progetto, versione semplificata destinata alla produzione in serie in tempi ridotti, basato sulla versione Ho 229 V6, mai costruito.

Esemplari attualmente esistenti

L'unico Ho 229 sopravvissuto, il V3, è conservato al National Air and Space Museum Paul E. Garber Restoration Facility di Suitland, nel Maryland. Attualmente non è esposto al pubblico. Alcuni aerei fortemente danneggiati, trovati nelle catene di montaggio, furono distrutti dalle truppe americane per impedire che fossero raccolti dalle avanzanti forze sovietiche. L'Ho 229 venne portato via dall'VIII Corpo della Terza Armata del generale Patton.

ENGLISH

The Horten H.IX, RLM designation Ho 229 (or Gotha Go 229 for extensive re-design work done by Gotha to prepare the aircraft for mass production) was a German prototype fighter/bomber initially designed by Reimar and Walter Horten to be built by Gothaer Waggonfabrik late in World War II. It was the first flying wing to be powered by jet engines.

The design was a response to Hermann Göring's call for light bomber designs capable of meeting the "3×1000" requirement; namely to carry 1,000 kilograms (2,200 lb) of bombs a distance of 1,000 kilometres (620 mi) with a speed of 1,000 kilometres per hour (620 mph). Only jets could provide the speed, but these were extremely fuel-hungry, so considerable effort had to be made to meet the range requirement. Based on a flying wing, the Ho 229 lacked all extraneous control surfaces to lower drag. It was the only design to come even close to the requirements, and received Göring's approval. Its ceiling was 15,000 metres (49,000 ft).

Design and development

In the early 1930s, the Horten brothers had become interested in the flying wing design as a method of improving the performance of gliders. The German government was funding glider clubs at the time because production of military and even motorized aircraft was forbidden by the Treaty of Versailles after World War I. The flying wing layout removed the need for a tail and associated control surfaces and theoretically offered the lowest possible weight, using wings that were relatively short and sturdy, and without the added drag of the fuselage. The result was the Horten H.IV.

In 1943, Reichsmarschall Göring issued a request for design proposals to produce a bomber that was capable of carrying a 1,000 kilograms (2,200 lb) load over 1,000 kilometres (620 mi) at 1,000 kilometres per hour (620 mph); the so-called "3×1000 project". Conventional German bombers could reach Allied command centers in Great Britain, but were suffering devastating losses from Allied fighters. At the time, there was no way to meet these goals—the new Junkers Jumo 004B turbojets could provide the required speed, but had excessive fuel consumption.

The Hortens concluded that the low-drag flying wing design could meet all of the goals: by reducing the drag, cruise power could be lowered to the point where the range requirement could be met. They put forward their private project, the H.IX, as the basis for the bomber. The Government Air Ministry (Reichsluftfahrtministerium) approved the Horten proposal, but ordered the addition of two 30 mm cannons, as they felt the aircraft would also be useful as a fighter due to its estimated top speed being significantly higher than that of any Allied aircraft.

The H.IX was of mixed construction, with the center pod made from welded steel tubing and wing spars built from wood. The wings were made from two thin, carbon-impregnated plywood panels glued together with a charcoal and sawdust mixture. The wing had a single main spar, penetrated by the jet engine inlets, and a secondary spar used for attaching the elevons. It was designed with a 7g load factor and a 1.8× safety rating; therefore, the aircraft had a 12.6g ultimate load rating. The wing's chord/thickness ratio ranged from 15% at the root to 8% at the wingtips. The aircraft utilized retractable tricycle landing gear, with the nosegear on the first two prototypes sourced from a He 177's tailwheel system, with the third prototype using an He 177A main gear wheelrim and tire on its custom-designed nosegear strutwork and wheel fork. A drogue parachute slowed the aircraft upon landing. The pilot sat on a primitive ejection seat. A special pressure suit was developed by Dräger. The aircraft was originally designed for the BMW 003 jet engine, but that engine was not quite ready, and the Junkers Jumo 004 engine was substituted.

Control was achieved with elevons and spoilers. The control system included both long-span (inboard) and short-span (outboard) spoilers, with the smaller outboard spoilers activated first. This system gave a smoother and more graceful control of yaw than would a single-spoiler system.

Given the difficulties in design and development, Russell Lee, the chair of the Aeronautics Department at the National Air and Space Museum, suggests an important purpose of the project for the Horten Brothers was to prevent them and their workers from being assigned to more dangerous roles by the German military.

Operational history

Testing and evaluation

The first prototype H.IX V1, an unpowered glider with fixed tricycle landing gear, flew on 1 March 1944. Flight results were very favorable, but there was an accident when the pilot attempted to land without first retracting an instrument-carrying pole extending from the aircraft. The design was taken from the Horten brothers and given to Gothaer Waggonfabrik. The Gotha team made some changes: they added a simple ejection seat, dramatically changed the undercarriage to enable a higher gross weight, changed the jet engine inlets, and added ducting to air-cool the jet engine's outer casing to prevent damage to the wooden wing.

The H.IX V1 was followed in December 1944 by the Junkers Jumo 004-powered second prototype H.IX V2; the BMW 003 engine was preferred, but unavailable. Göring believed in the design and ordered a production series of 40 aircraft from Gothaer Waggonfabrik with the RLM designation Ho 229, even though it had not yet taken to the air under jet power. The first flight of the H.IX V2 was made in Oranienburg on 2 February 1945. All subsequent test flights and development were done by Gothaer Waggonfabrik. By this time, the Horten brothers were working on a turbojet-powered design for the Amerika Bomber contract competition and did not attend the first test flight. The test pilot was Leutnant Erwin Ziller. Two further test flights were made: on 2 February 1945 and on 18 February 1945. Another test pilot used in the evaluation was Heinz Scheidhauer.

The H.IX V2 reportedly displayed very good handling qualities, with only moderate lateral instability (a typical deficiency of tailless aircraft). While the second flight was equally successful, the undercarriage was damaged by a heavy landing caused by Ziller deploying the brake parachute too early during his landing approach. There are reports that during one of these test flights, the H.IX V2 undertook a simulated "dog-fight" with a Messerschmitt Me 262, the first operational jet fighter, and that the H.IX V2 outperformed the Me 262. However, the Me 262 was considered by many as unsuitable for fighter missions, being slow in turning. Additionally, pilots and aiming devices had not yet adapted to the speed of jet aircraft, forcing pilots to slow their airplanes to accurately fire at bombers, leaving them momentarily at the reach of Allied gunmen.

Two weeks later, on 18 February 1945, disaster struck during the third test flight. Ziller took off without any problems to perform a series of flight tests. After about 45 minutes, at an altitude of around 800 m, one of the Jumo 004 turbojet engines developed a problem, caught fire and stopped. Ziller was seen to put the aircraft into a dive and pull up several times in an attempt to restart the engine and save the precious prototype. Ziller undertook a series of four complete turns at 20° angle of bank. Ziller did not use his radio or eject from the aircraft. He may already have been unconscious as a result of the fumes from the burning engine. The aircraft crashed just outside the boundary of the airfield. Ziller was thrown from the aircraft on impact and died from his injuries two weeks later. The prototype aircraft was completely destroyed.

Despite this setback, the project continued with sustained energy. On 12 March 1945, nearly a week after the U.S. Army had launched Operation Lumberjack to cross the Rhine River, the Ho 229 was included in the Jäger-Notprogramm (Emergency Fighter Program) for accelerated production of inexpensive "wonder weapons". The prototype workshop was moved to the Gothaer Waggonfabrik (Gotha) in Friedrichroda, western Thuringia. In the same month, work commenced on the third prototype, the Ho 229 V3.

The V3 was larger than previous prototypes, the shape being modified in various areas, and it was meant to be a template for the pre-production series Ho 229 A-0 day fighters, of which 20 machines had been ordered. The V3 was meant to be powered by two Jumo 004C engines, with 10% greater thrust each than the earlier Jumo 004B production engine used for the Me 262A and Ar 234B, and could carry two MK 108 30 mm cannons in the wing roots. Work had also started on the two-seat Ho 229 V4 and Ho 229 V5 night-fighter prototypes, the Ho 229 V6 armament test prototype, and the Ho 229 V7 two-seat trainer.

During the final stages of the war, the U.S. military initiated Operation Paperclip, an effort to capture advanced German weapons research, and keep it out of the hands of advancing Soviet troops. A Horten glider and the Ho 229 V3, which was undergoing final assembly, were transported by sea to the United States as part of Operation Seahorse for evaluation. On the way, the Ho 229 spent a brief time at RAE Farnborough in the UK, during which it was considered whether British jet engines could be fitted, but the mountings were found to be incompatible with the early British turbojets, which used larger-diameter centrifugal compressors as opposed to the slimmer axial-flow turbojets the Germans had developed. The Americans were just starting to create their own axial-compressor turbojets before the war's end, such as the Westinghouse J30, with a thrust level only approaching the BMW 003A's full output.

Surviving aircraft

The only surviving Ho 229 airframe, the V3—and the only surviving World War II-era German jet prototype still in existence—has been at the Smithsonian National Air and Space Museum's Paul E. Garber Restoration Facility in Suitland, Maryland, U.S. In December 2011, the National Air and Space Museum moved the Ho 229 into the active restoration area of the Garber Restoration Facility, where it was reviewed for full restoration and display. The central section of the V3 prototype was meant to be moved to the Smithsonian NASM's Steven F. Udvar-Hazy Center in late 2012 to commence a detailed examination of it before starting any serious conservation/restoration efforts and has been cleared for the move to the Udvar-Hazy facility's restoration shops as of summer 2014, with only the NASM's B-26B Marauder Flak Bait medium bomber ahead of it for restoration, within the Udvar-Hazy facility's Mary Baker Engen Restoration Hangar. As of early 2018, the surviving Horten Ho 229 has been moved to display in the main hall, alongside other WWII German aircraft.

Claimed stealth technology

Radar absorbent material (RAM)

After the war, Reimar Horten said he intended to mix charcoal dust in with the wood glue to absorb electromagnetic waves (radar), which he believed could shield the aircraft from detection by British early-warning ground-based radar that operated at 20 to 30 MHz, with a wavelength of 10 to 15m (top end of the HF band), known as Chain Home. This charcoal glue treatment was planned for the never-made production model, however it remained unclear if the V3 prototype had benefited from a preliminary iteration of this technology.

In 2008, a team of engineers from Northrop Grumman undertook electromagnetic tests on the V3's multilayer wooden centre-section nose cones. They tested over a frequency range of 12 to 117 THz, with wavelengths of the order of 10 microns. The cones are 19 mm (0.75 in) thick and made from thin sheets of veneer. The team observed that the "Ho 229 leading edge has the same characteristics as the plywood [of the control sample] except that the frequency [do not exactly match] and have a shorter bandwidth." The team who had assumed the presence of carbon black from visual inspection went on to conclude that the "similarity of the two tests indicates that the design using the carbon black type material produced a poor absorber."" The Smithsonian Institution has since performed a technical study of the materials used on the prototype and determined that there is "no evidence of carbon black or charcoal" thus invalidating the proposed presence of carbon black to explain the slightly different absorbent property of the prototype wood compared to the control sample of plywood used in the Northrop Grumman testing.

Radar cross section (RCS) and shape

A jet-powered flying wing design such as the Horten Ho 229 has a smaller radar cross-section (RCS) than conventional contemporary twin-engine aircraft because the wings blended into the fuselage and there are no large propeller disks or vertical and horizontal tail surfaces to provide a typical identifiable radar signature.

In early 2008, Northrop Grumman paired up television documentary producer Michael Jorgensen and the National Geographic Channel to produce a documentary to determine whether the Ho 229 was the world's first true "stealth" fighter-bomber. Northrop Grumman built a full-size non-flying reproduction of the V3, made out of wood primarily, unlike the original aircraft which had an extensive steel space-frame to which the wooden skin was bolted. The space-frame for the real aircraft was made from steel tubes up to 160 mm in diameter, and provided the entire structure for the centre section of the aircraft. After an expenditure of about US$250,000 and 2,500 man-hours, Northrop's Ho 229 reproduction was tested at the company's RCS test range at Tejon, California, US where it was placed on a 15-metre (50 ft) articulating pole and exposed to electromagnetic energy sources from various angles, using the same three HF/VHF-boundary area frequencies in the 20–50 MHz range used by the Chain Home.

Radar simulations showed that a hypothetical Ho 229, with the radar characteristics of the mockup which had neither metal frame nor engines, approaching the English coast from France flying at 885 kilometres per hour (550 mph) at 15–30 metres (49–98 ft) above the water would have been visible to CH radar at a distance of 80% that of a Bf 109 This implies a frontal RCS of only 40% that of a Bf 109 at the Chain Home frequencies. The US magazine Aviation Week & Space Technology published summaries about Stealth technology; some reports indicate the Horten Ho-IX/ Gotha Go-229 returned radar echo just from the annular air entries to turbines, the nose and canopy, and the wing track binding the inner part of turbine intake to cabin.

Variants:

H.IX V1 First prototype, an unpowered glider, one built and flown (three-view drawing above).
H.IX V2 First powered prototype, one built and flown with twin Junkers Jumo 004B engines.
Gotha developments:
Ho 229 V3 Revised air intakes, engines moved forward to correct longitudinal imbalance. Its nearly completed airframe was captured in production, with two Junkers Jumo 004B jet engines installed in the airframe.
Ho 229 V4 Planned two-seat all-weather fighter, in construction at Friedrichroda, but not much more than the center-section's tubular framework completed.
Ho 229 V5 Planned two-seat all-weather fighter, in construction at Friedrichroda, but not much more than the center-section's tubular framework completed.
Ho 229 V6 Projected definitive single-seat fighter version with different cannon, one captured in production at Ilmenau by US troops.
Horten developments:
H.IXb (also designated V6 and V7 by the Hortens)
Projected two-seat trainer or night-fighter; not built.
Ho 229 A-0 Projected expedited production version based on Ho 229 V6; not built.