The word ballute is a conjunction of 'balloon' and 'parachute'. As the name implies, ballutes are balloon-shaped, however, they perform the same function as parachutes: deceleration or stabilization of a vehicle in an atmosphere. Ballutes have the advantage that they offer great performance at high Mach numbers,  where most other parachutes fail.  

Generally speaking, there are two types of ballutes: self-inflating and air-inflated ballutes. The former uses an on-board gas generation system to inflate the ballute. The later creates a high-pressure region inside the ballute by allowing the high-velocity air to enter via inlets. Air-inflated ballutes have the disadvantage that they rely on the varying flight conditions for inflation, reducing their reliability. However self-inflating ballutes tend to be heavier as these have to carry a pressurization system on board. Both types make use of a so called 'burble fence', that increases the drag of the ballute by triggering flow separation and increasing the low-pressure wake's size behind the ballute. This burble fence looks like a doughnut-shaped ring that encircles the ballute. 

The geometry of a ballute is not strictly limited to the widely used teardrop shape. It is possible to use a toroidal or doughnut-shaped ballute. This concept has however not yet been flight-tested but could offer solutions for the EDL of large scale planetary missions. 

History of the ballute

In the early '60s, a crew ejection system was developed for the Gemini project to recover the pilots safely in case of a launch vehicle malfunction. In case that the pilots would eject above 2300 m altitude, the pilots require a stabilizing parachute to prevent them from tumbling too rapidly. The ballute was chosen to fulfil this role. Ballutes had already been developed before Gemini by the Goodyear Aerospace Coorporation to serve as bomb decelerators at high velocities.   

Between 1964 and 1965, Goodyear performed a test program to develop a ballute recovery system for the Arcas rocket launched payloads. The final design featured a ballute with an inlet at the attachment point and a hexagonal burble. 

Goodyear performed an extensive test program in 1966, the Aerodynamic Deployable Decelerator Performance Evaluation Program (ADDPEP), during which a wide variety of ballute configurations were tested. Even some very exotic conditions and systems. For example, a steel fabric welded ballute or an alcohol-water filled ballute at Mach 10 that inflates due to the vaporisation of the alcohol by aerothermal heating. 

The X-23 PRIME test vehicle used a ballute as a high velocity drogue parachute during all three of its flights between 1966 and 1967.  

Inflight test of a ballute

Render of  various ballutes from space and parachute test missions.

In 1969, during the Planetary Entry Parachute Program (PEPP), a large 5.5 meter ballute was tested at high altitude at supersonic conditions. Unfortunately, the ballute failed to inflate correctly, as can be seen in the footage to the left. 

In the 21st century, the ballute has seen an increase in popularity for high altitude/high speed applications. In 2014-2015 4.4 meter ballutes were tested during NASA's Low Density Supersonic Decelerator (LDSD) program as a drogue chute.  In recent years, the ballute has also been selected as a high altitude stabilization and decelerator for reusability. For example by Armadillo Aerospace's SARGE rocket, Rocket Lab's Electron rocket, Exospace's STIG rocket and even for Copenhagen Suborbital's manned Spica capsule.