The largest supersonic parachute ever developed is one of the test articles on the Supersonic Flight Dynamics Test (SFDT) vehicle of the Low Density Supersonic Decelerator (LDSD) project. The typical method for deploying a supersonic parachute from an entry vehicle, by firing it from a mortar, is not viable for this application due to its noncentral location on the vehicle and the associated high reaction force. Instead, the parachute is pulled off the vehicle using the Parachute Deployment Device (PDD). The PDD uses a ballute, a smaller, balloon-like, soft-good drag body that maintains positive internal pressure by ingesting air at supersonic speeds through a set of ram-air inlets. The PDD, being significantly smaller than the supersonic parachute, is deployed using a mortar.
There is the possibility that the ballute may not properly inflate, or inflate too slowly, under the influence of air ingestion alone due to misalignment of the inlets. A slow inflation increases the risk that the supersonic flow causes rapid flagging or whipping of the ballute fabric. Under these conditions, the ballute could tear itself apart prior to full inflation. In order to accelerate and increase the likelihood of proper inflation, an additional gas-generating device (inflation aid, or IA) is used. Traditionally, gasgenerating devices (such as those found in automobile airbags) utilize the rapid combustion of pyrotechnic materials to produce hot gases. This combustion is typically initiated using an electric source. However, both electric initiation and high enthalpy gas were undesirable for the PDD application, thus a solution was developed that involves mechanical initiation and methanol vaporization as the source for lower enthalpy gas generation.
The core of the IA device consists of a housing, which contains a pair of redundant firing mechanisms and combustion chambers; a reservoir filled with an aqueous methanol solution; and a rupture disk. The front (venting end) of the IA is inserted into the mouth of the ballute and threads into an attachment collar that forms the mechanical connection of the ballute. The riser from the SFDT vehicle connects to a pin on the rear of the IA. An aerodynamic shroud shields the exposed part of the device from the oncoming supersonic airflow. Two redundant trigger lanyards run alongside the ballute, and are inserted into the firing mechanism.
A small pyrotechnic initiator is mechanically discharged when the lanyards are pulled out of the firing mechanism. The lanyards are connected to the ballute, and are actuated as the ballute is stripped out of its deployment bag. The initiation only occurs once the ballute has sufficiently emerged from the bag to allow for proper inflation. The combustion of the pyrotechnic material pressurizes a reservoir that contains an aqueous methanol solution, causing it to burst and release the mixture into the ballute.
The firing mechanism of the IA contains components similar to those found in many modern firearms. The design of the IA is easily scalable. By varying the amount of methanol, the fraction of water in solution, and the temperature of the device (and ballute), both the rate of pressurization and the amount of gas may be adjusted. The methanol evaporation process is also self-regulating (as opposed to automotive airbag gas generators utilizing high enthalpy propellants). As the pressure in the vented cavity increases to the vapor pressure of the methanol at a given temperature, the evaporation stops, reducing the risk of hardware damage from over-pressurization. In addition, the methanol absorbs energy to vaporize and expand, effectively lowering the surface temperature of the ballute slightly during inflation, which is a beneficial feature given that the ballute operates in a relatively hot aerothermodynamic environment.
The unique mechanical activation design of this device, and its all-metal construction, makes it less susceptible to accidental activation due to electrostatic discharge (ESD) or electromagnetic radiation. Activation via tension in the trigger lanyard ensures that the device is only discharged once the ballute reaches the correct bag-strip condition, and not during the high-acceleration environment experienced during mortar fire or line-stretch events. The rigid nature of the IA housing allows it to be packed within the PDD bag without risk of rupture during packing, handling, or when mortar-fired. The IA also serves as the structural link between the riser bridle and the ballute, thus reducing the overall PDD system mass.