The basic concept of this airborne launch and recovery system is based on the the commonly used In Flight Refuelling (IFR) system via probe and drogue. Instead of hooking up a fuel nozzle the whole UAV is hooked into the drogue, and with a winch it is pulled in a pod under the wing where the UAV is stored while not in use or being refuelled / recharged. For every UAV one pod dedicated, except when a pod is mounted in the payload bay of the mother plane. Multiples of pods can be mounted on existing stations.
Since the properties and size of the drogue are similar to the UAV both react similar to turbulences. In this suggested approach the coupling of the UAV with the drogue is outside of the area of the propeller wash and the down wash of the wing and other disturbances. Further this allows to define a safety zone around the mother airplane to prevent any threat from the UAV to the mother airplane or its crew.
In the example of the C-130 one pod can be mounted inside the fuselage accessible for the crew. Through opening the back door the pod can be brought into position to deploy the UAV. This gives the ability to deploy and store multiple UAVs with one pod and allows access for the flight crew during flight to the UAV and its payload.
The rough closing in for recovery is done by transmitting the position, speed and heading of the mother airplane via an encrypted data link. The UAV uses this information to calculate a possible rendezvous point and adjusts its flight path.
If the pilot of the mother airplane is keeping its heading and speed, the rendezvous manoeuvre is straight forward. To give the mother airplane pilot more flexibility a predictive probability based approach can be used to control the UAV in rough closing in phase. This is very similar to a submitted publication of mine (Predictive Probability Based Collision Avoidance for Unmanned Aerial Vehicles).
When the UAV is close enough a precision guidance system is used to couple. The precision guidance system persists of a infra red (IR) light source (preferable wave length 940nm) mounted in the middle of the drogue and a corresponding IR camera with a high update rate in the probe at the head of the UAV. This allows the UAV to close precisely the probe in the drogue where a coupling mechanism fixes it (Figure 2). The winch then pulls the drogue with the coupled UAV into the pod where the UAV is stored and refuelled for the next deploy (Figure 3).
The UAV can be either specially designed for this kind of application or can be refitted with the probe. The probe can interoperate all necessary systems needed for deploy and recovery. The adaptation of a different UAV can be done by integrating the probe and adjusting the shape of the storage bay in the pod for the corresponding UAV.
To deploy the UAV the winch can either unwind and at a distance release the UAV or it can be directly released from the pod. The pods position and the drag of the UAV prevent any contact with the mother airplane. For a safe deploy even under imperfect conditions or during a dive or deceleration additional drag can be generated by releasing a drag chute prior to deployment.
The necessary systems for this type of in flight deployment and recovery of UAVs includes following Systems:
- The UAV
- Airframe with payload, propulsion and control for normal operations
- High level control for distributed control (coordination)
- IR-Sensor with control for recovery
- Control station for distributed coordination of several Vehicles
- The launch and recovery system (including storage and refuelling)
- Storage bay
- Refuelling / recharging system
- Display for the Pilot to visualize the area he should be or stay for recovery or deploying
Figure 5 shows an overview of those systems and there relations to each other.
The drogue has two proposes. It is supposed to keep the latching hole stable in the air so that the UAV can home into it by using the IR light source inside the latching hole (Figure 6). The second purpose is to guide the pin into the latching hole, where the pin is latched. The IR light source is either supplied by electric wires in the rope or by a small battery which gets inductive recharged when it is in storage position.
The probe is designed to house all additional systems for the UAV. An IR camera behind an optical notch filter window is located on the front of the probe (Figure 7). Behind that camera is the control system with the data link installed. These are surrounded by the latching mechanism. The whole probe is supposed to slide into the latching hole of the drogue. The probe’s power is supplied by the UAV and gets information such as position and speed from the autopilot. This information is used together with the information of the mother airplane via data link to line up the UAV short behind the mother airplane. The precision IR based homing system is used for the final phase. In both phases the control system sends correction data to the UAV’s autopilot.
The pod is equipped with the winch, a data link, refuel system and the control system for it. The rear of the pod contains the storage bay for the UAV. The back of the storage bay is shaped in a way that the drogue and the the UAV guides easy into the storage bay. In storage position (Figure 10) the UAV is held only by the locking mechanism in the drogue and the storage bay shape. The Figure 8 to 10 shows the pulling of the UAV into the storage position.
Mother Airplane Pilot Display
This display gives the pilot of the mother airplane the information regarding where the airplane has to be to be able to recover the UAV in a certain time period. The different time periods can be displayed with different colors. The stand off areas for the mother airplane can also be integrated so the pilot is able to determine where they are and where they should be with just one look.
Distributed Control of the UAVs
The UAVs can be either operated as independent vehicles with different tasks allocated by the operator or as a small swarm. In the first implementation multiple operators will control the UAVs and coordinate their mission.
In a further development stage a coordinated swarm control may be beneficial depending on the mission. In my doctoral thesis (Unmanned Aerial Vehicle Swarming with Distributed Nonlinear Model-Predictive Control) one possible approach of swarm coordination and control is described.
To control a swarm of Unmanned Aerial Vehicles (UAVs) efficiently, every UAV has to have a higher level of autonomy than just a single UAV. The operator does not want and can not give every single member of a swarm detailed commands. The autonomy has to ensure that the operator can give high level commands and every member of the swarm is collaborative and reliable fulfilling its desired aim.
Applications and Use cases
This concept, of launching and recovering, can be applied for every UAV which can be equipped with the probe and its flight envelope is a sufficient match to the mother airplane. This includes fast propeller driven UAVs in pusher and twin engine configurations as well as all turbine driven UAVs. The UAV’s maximum speed needs to be sufficient above the minimum speed for safe operations of the mother airplane. In some cases the UAV wing and tail structure (which are exposed in storage) may need to be modified to ensure the survivability in storage position for the complete speed envelope of the mother airplane.
The use of a vertical take off and land (VTOL) UAV like a modified Aerovel Flexrotor UAV with higher maximum speed would allow to deploy precisely and recover small payloads to and from everywhere. This will offer new kinds of operations and missions.
Advantages / Disadvantages
- Based on existing technologies like In Flight Refuelling and glider towing.
- Mainly use of commercial of the shelf components possible.
- Coupling of the UAV can be done far behind the mother airplane, which ensures a safety buffer.
- Drogue can be designed in a way that the reaction on turbulences are very similar to those of the UAV. This will allow a coupling also under imperfect conditions.
- Fast recovery, refuelling / recharging and redeploy possible.
- Adaptable for many existing UAV / UAS.
- Modular design.
- Only one UAV can be recovered per pod if they are mounted on the wing and not inside of the fuselage such as in Figure 4.