The first thing that's obvious about this airframe design is that it doesn't look much like a conventional rocket.
There are several good reasons for this, but the design wasn't arrived at without a lot of head-scratching and trying to get a
tubular rocket design to work. The problems all stem from the mission objective that the complete craft needed to be recoverable and re-usable
(by a small set of people without access to ocean recovery ships), which in turn made it necessary for the craft to be
a SSTO (single stage to orbit) design that would survive re-entry and be able to find its way back to land autonomously.
All of these constraints added up to make a conventional rocket non-viable, but as it turns out most of the problems - and a few more besides - can be solved by using a 'multi-phase' aerodynamic design.
SSTO is more easily achieved if you don't have to carry all of the oxidiser with you all of the way up, and to this end two specially designed centrifugal (Whittle type) jet engines are employed to help loft the craft subsonically to a design ceiling of around 35kft.
Once there, the aerodynamic surfaces are centred and the aerospike engine takes over to drive the craft asymptotically to a circular Low Earth Orbit (LEO) and a final velocity of 7.8 km/s.
For the re-entry and recovery phase, the underside of the craft is presented to the atmosphere and stability is effected by reaction control High Test Peroxide (HTP) jets. Horizon has a dihedral angle of 10°
for additional aerodynamic stability and the leading surfaces and underside are coated in a natural cork based ablative material which both insulates and carries away the energy of the re-entry plasma
(although how the eroded surfaces affect the performance of the wing once back in flight has not yet been modelled!).
The underside of each wing is completely planar to avoid hot-spots and distribute the heating evenly over a large surface. To do this, is has been necessary to mount the jets above the wing and carefully design the landing gear doors and radar altimeter/sensor ports so that they do not present any gaps when closed.
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Horizon airframe, oblique rear view.
Image Credit: NAs / Peter Jones
Horizon airframe, front view.
Image Credit: NAs / Peter Jones
Horizon airframe, landing simulation.
Image Credit: NAs / Peter Jones
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Once in subsonic airflow, the jets are restarted and horizon can guide itself to the nearest landing point of choice.
A key aspect of this design is the ogee delta wing. It allows the aerodynamic shock wave to pass around the wing tips as the transition from subsonic to supersonic atmospheric flight occurs and
in contrast to more normal delta wing design, during landing at high angles of attack the high-sweep front portion allows vortices to be generated that stick to the aircraft body and create lift over the main wing. This allows landing at lower speeds than would otherwise be necessary and removes the needs for canards at the front that would present a problem during re-entry.
Aside from all of this, being able to take off and land on existing airstrips has enormous benefits in terms of getting permission for a launch to happen in the first place. Not only that, but since testing can be more easily broken down into stages that are recoverable should something
go wrong, it is less likely that everything would be lost than an all-or-nothing-light-the-blue-touch-paper-cross-the-fingers-and-retire-to-the-bunker approach.
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