This page is related to aircraft lift-off, staging and landing operations. The return capabilities are discussed too.
Lift-off will not be simple. The engines we are designing are optimized for high-altitude flight, and may not be able to provide the required thrust at ground level. Some additional acceleration mechanisms would then be required:
- using the rocket's engine for a short time. The problem is then how to carry the additional propellant required for this lift-off operation. An additional disposable tank could be mounted on the rocket or on the aircraft, jettisoned after this burn. The engine may allow throttling, reducing the thrust and propellant consumption, while still providing the required delta V for lift-off.
- push-on boosters, either with propellers or miniature solid rocket engines, that would stay on the ground when the plane lifts-off.
- some kind of pulling cable on the runway
Staging and risk evaluation
In the subsonic flight variant, staging will occur when the highest service altitude has been reached, which will probably happen when the fuel has all been used.
Possible staging mechanism:
- aircraft is attached to the rocket by some pins, the rocket is released then fired with no possible abort. A self-destruct mechanism must be added to the rocket, because if the rocket engine fails to ignite, the rocket would turn into a ballistic air-to-surface missile.
- aircraft is attached to the rocket by some pins, the rocket is fired before being released. Risks are lower, but the attachment may be complicated to design, since the thrust of the rocket would be acting on it.
- rocket is attached to the aircraft with a rail on which it can roll. The rocket engine would ignite before separating from the aircraft, providing a way to return to the ground without exploding if it doesn't ignite, by gliding. The rocket engine's thrust is what would make the rocket separate, which is probably the safest and simplest way to make it.
The self-destruct could be required in all cases, because there are lots of other issues than separation.
Return to earth mission
What should be done with the "aircraft" when it has separated with the rocket? What remains is mainly the wings and the tail, probably linked together by spars. For the N-prize, the aircraft should be reusable to deduce its cost from the project's cost. For other applications, this is not mandatory. The engines could be complicated and expensive to build, to it may be important to return them safely to earth, if they have a lifespan of at least two flight durations.
With two engines mounted under each wing, the returning aircraft would be hard to land properly. There is no landing gear, and the engines are the lowest part of the aircraft. If it is placed on the floor, the engines and the tail support the wings. A landing gear could consist of wheels deeply embedded in the engines and in the tail for its simplest form, or of a deployable gear stored in the wings or in the attachment between the wings and the tail.
List of strategies for aircraft return:
- free fall:
- crash on ground (no safe return)
- parachute deployment triggered by the altitude or by timer after separation (safe return is not guaranteed for aircraft, and unlikely if the rocket has not separated)
- airbag, MER style. If used without parachute, the wings at least will be damaged (no safe return in all case). If used in combination with the parachute system, airbags could prevent the engines from being destroyed at touchdown (safe return for aicraft, no safe return for non-separated rocket).
- controlled gliding, requires the centre of mass to allow it and probably some balancing weights to have it at the right position; requires and independent battery to operate the plane actuators for the return trip. The landing is then the issue:
- controlled crash at zero vertical speed, like a miniature remote-controlled aircraft, without landing gear. If autonomous, requires to be lucky with the return terrain, or to be able to return to the runway with unpowered flight, possibly by recording wing directions on the way up. With no landing gear, landing would be on the engines (no safe return of the vehicle). If remote-controlled, the terrain may be better.
- landing at zero vertical speed, with a landing gear. It still depends on the landing terrain and the ability to find and go to a runway, but engine could be spared (safe return possible). If remote controlled, it depends on the ability to fly to glide to something close enough to a runway.
- controlled crash, upside down. Having the aircraft upside down may not damage the engines, but damage the tail instead. This could be complicated to do autonomously, and would still depend on the landing terrain (safe engine return possible).
- powered flight could be used to return the complete vehicle to a runway and allow for a safe return on non catastrophic rocket failure and no aircraft failure. If autonomous, that requires a complicated control system to return to runway. If remote-controlled, that looks more easy and could be the safest solution for abort or safe return. Requires one of the following propulsion systems:
- continuing turbofan operation, requires more fuel in the aircraft
- electric motor with a propeller, requires additional battery
If the aircraft has to return to ground autonomously, a control card has to be put in it, managing several sensors and actuators. Some of the sensors would also have to be in the rocket. This redundancy could allow the vehicle to return to the ground safely if something wrong happens with the rocket's control system.
The remote-controlled operation will probably be developed for early tests of the aircraft prototype, and can thus be left in the final vehicle for return and especially landing.
If recovery is required, a beacon should be put aboard the aircraft.