Extreme Spec: Desert

What: Assa Abloy Megadoor
Where: Stratolaunch Systems Hangar, Mojave Desert USA

View looking across the full 128m width of the Stratolaunch Systems hangar – with the transluscent Megadoor to the left
View looking across the full 128m width of the Stratolaunch Systems hangar – with the transluscent Megadoor to the left

Legendary business magnate, aviator and OCD  recluse Howard Hughes would not be happy. Although the 1947 maiden flight of his huge H-4 Hercules, at little more than a mile, was its only one; the 97m wingspan of the ‘Spruce Goose’ officially remained the longest in the world – until now. In the USA’s arid Mojave desert Stratolaunch Systems, a firm half-owned by Microsoft co-founder Paul Allen, is to build a one-off carrier aircraft that can piggy-back commercial payloads into space. In its massive hangar, the company is about to start sewing together two Boeing 747 fuselages in parallel, with six huge engines, creating – with a wingspan of 117m and weight of over 540 tonnes – the largest commercial plane in the world. 

Based at Bakersfields’ Kern County Airport, Stratolaunch’s ‘very large’ ­‘T Hangar’ opened recently, two months ahead of schedule. The 8600m2 facility, built by specialist contractor Wallace & Smith, uses over 1500 tonnes of structural steel, formed into thin beams, to give a 128m clear span; leaving 5.5m either side of the carrier aircraft to allow it to be guided in.

‘The main benefit of the vertically dropping Megadoor is that sliding horizontal doors would necessitate a rectangular building, which increases the volume and the cost,’ says Assa Abloy project manager Pierre Varlamoff. ‘With the T hangar, the portion that needs to fit the wings is wide, but also lower. It even narrows for the front of the dual fuselage – hence the “T” – which all makes for cost savings,’ he adds. 

The doors perform well thermally as the Megadoor’s  translucent PVC fabric has an air gap between the outer and inner layer. However, Varlamoff says the problem is not about thermal conductance through the fabric, but around it. ‘Peripheral heat transfer around the mullions is the main problem,’ he says. ‘The Megadoor has a system where the descending curtain envelopes the side guides, pulling tight against the side seals and jambs so there’s no infiltration of desert air and dust.’ With extremely sensitive monitoring equipment for the prototype aircraft inside the hangar, dust would always be a prime concern, especially here. This may be why they’re used at Cape Canaveral as well. 

‘The doors effectively provide a hermeti­cally sealed environment,’ says Varlamoff reassuringly. Now that’s something Howard Hughes might have been pleased about.


Megadoor on the T hangar

The Megadoor’s vertical PVC-coated polyester blinds in the open position – just before the vertical mullions are retracted into the roof
The Megadoor’s vertical PVC-coated polyester blinds in the open position – just before the vertical mullions are retracted into the roof

The Stratolaunch’s seven Megadoors open and close using a Venetian blinds principle, performing the task in less than two minutes. The hangs of PVC coated polyester are run through at 2m horizontal intervals with aluminium bars that pull the fabric tight as it drops. These bars run down between 590mm square aluminium mullions attached to cables that are connected to a motorised winch in the head steel, allowing them to retract as the door opens. The tallest central door here, at 20.7m, is also the widest at 19m. Door size is a cost issue; the widest dimension as a single leaf is currently 36m. The self-sealing jamb detail was essential to ensure a dust-free internal space in this challenging environment. As part of the air conditioning strategy, four large exhaust fans have been installed at low level within the Megadoor. The double skin PVC coated polyester has other benefits beyond thermal performance. ‘With only 10% pigment to the fabric, there is a high level of light transference, which saves on operational energy,’ says Varlamoff. ‘It’s also a soft, diffuse light, which is great for the mechanics who are working there,’ he adds.


Assa Abloy Megadoor S1500

Opening speed: 0.15 – 0.25 m/sec
Size range: Any
Windload: Any

Electrical system Power supply: 3-phase 400V-50Hz (other options available on request)
Control voltage: 24V AC
Fabric data: PVC coated polyester
Unchanged pliability: -35°C to +70°C

Tensile strength: 2700/2500 N/5 cm acc. to DIN 53354, EN ISO 1421.
Resistance to light: 6-8 (scale 0-8) acc. to BS 1006, ≥7 acc. to ISO 105-B02.
Resistance to UV-rays: UV-stabilised
Flame resistance: Yes – acc. to SIS 650082, ASTM E84-94 class A, DIN 4102 B1

Surface treatment Bottom profile and mullions: Aluminium: Sandblasted and primed