An inflatable shroud allows a propulsion system that is stored in fixed length and diameter cylinders or silos to occupy all of the available packaging length. When inflated the shroud provides the desired aerodynamic shape to reduce drag and improve performance. The inflatable shroud may be used on any aerodynamic structure that would benefit from an improved aerodynamic shape.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
This invention relates to an improvement in aerodynamic structures. More 
particularly, the invention relates to an inflatable shroud that may be 
used as an aerodynamic structure to improve the aerodynamic 
characteristics of the forward section or nose cone thereof. The invention 
is especially applicable to length constrained propulsion systems. 
2. Description of the Prior Art 
Some propulsion systems, such as those launched from submarines, must be 
highly packageable, have low weight and provide high performance. Such 
systems are packaged or stored in fixed length and diameter cylinders or 
silos. To maximize performance within the available cylinder or silo 
volume, the majority of this volume is occupied with propulsion 
components. An aerospike is packaged within the third stage motor and is 
deployed after launch to reduce drag and improve aerodynamic performance. 
The aerospike takes up a great deal of the available storage space and 
adds undesirably to the weight of the system. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide apparatus including an inflatable 
shroud that will allow a propulsion system to occupy substantially all of 
the available packaging or storage space with the shroud, when inflated, 
providing the desired aerodynamic shape for reducing drag and improving 
performance. 
In accordance with the invention, this is accomplished by folding the 
shroud, either ductile metal or suitably coated fabric, flush against the 
forward end of the propulsion system, and inflating the shroud to the 
desired shape by means of a gas generator system once the propulsion 
system is deployed into the operational mode thereof. The shape of the 
inflated shroud is selected to minimize the drag of the propulsion system 
to the maximum extent possible. The skin of the shroud is air tight, 
flexible enough to be collapsed and folded around the forward end or nose 
cone of the propulsion system, and has material properties capable of 
withstanding the aerodynamic heating and the heat produced by the gas 
generator. Internal structure, such as cords or straps, may be provided, 
as necessary, to obtain the rigidity necessary for the shroud to maintain 
the desired shape thereof during the operational mode of the propulsion 
system.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
There is illustrated in the fragmentary view of FIG. 1 the forward portion 
of a submarine launched aerodynamic structure or propulsion system 10 that 
is stored in a fixed length and diameter cylinder 1 that is appropriately 
positioned in the submarine. The propulsion system 10 includes a boost 
propulsion stage 12 and associated exhaust nozzle (not shown), a main 
propulsion stage 14 and associated nozzle 16, and a nose cone 18. 
Contained within the nose cone 18 is an upper propulsion stage 20, payload 
elements 22 and 24, and a solid propellant gas generator 26. The nose cone 
18, as illustrated, is blunt, being rounded and shortened, and just long 
enough to accommodate the upper stage 20, the payload elements 22 and 24, 
and the gas generator 26. A nose cone 18 having such shape has poor 
aerodynamic characteristics. 
The gas generator 26 may be a conventional solid propellant generator 
similar to those used for inflating automobile gas bags, and disclosed, 
for example, in U.S. Pat. No. 4,296,084 to G. V. Adams and F. E. 
Schneiter. 
Attached to the nose cone 18 in an air tight sealed manner, at or near the 
base thereof, indicated as attach point 28, is an inflatable shroud 30. A 
protective shipping plastic cover 32 for shroud 30 desirably may be 
provided. Shroud 30, in stowed position, as shown in FIG. 1, is folded 
flush against the forward surface of the nose cone 18. Once the propulsion 
system is placed in operation, the gas generator 26 is, activated by 
means, not shown, to produce a rapid generation of gas that flows into the 
shroud 30 for deploying the latter to the inflated shape thereof, as 
illustrated in FIG. 2. To that end the outlet of the gas generator 26 is 
suitably connected to the interior space between nose cone 18 and shroud 
30. The gas generator 26 may be suitably mounted to the interior forward 
surface of the nose cone 18, as shown. 
The shape of the shroud 30, when inflated, is selected to minimize the drag 
of the propulsion system as much as possible. The material of the skin of 
the shroud 30 may be ductile metal or coated fabric or mesh, but it should 
be air tight, be flexible enough to be collapsed and folded flush against 
the nose cone 18, have sufficient strength to withstand the dynamic 
pressures of flight, and have material properties capable of withstanding 
the aerodynamic heating and the heat produced by the gas generator. 
A metallic coating deposited on a plastic sheet such as aluminum coated 
Mylar is an example of a ductile metal that may be employed as the skin of 
the shroud 30, the metal preferably forming the outer surface thereof. 
Mylar is a trademark of E. I. duPont de Nemours & Co. of Wilmington, Del. 
An example of coated fabric that may be employed for the shroud 30 is 
neoprene coated rib stock nylon sheet, the seams of which are sewed and 
sealed with silicone rubbers, similar to the material of inflatable bags 
used for conventional automobile gas bags. 
Pressure levels necessary to withstand the dynamic pressures of flight, 
approximately 25 to 35 psia for typical ballistic missile trajectories is 
well within the capabilities of the material used in inflatable gas bag 
type systems. Additional pressure or internal structure, cords or straps 
may be provided, if desired, to obtain the rigidity necessary for the 
shroud 30 to maintain the selected and desired shape during the 
operational mode of the rocket. 
A specific application for the inflatable shroud 30 according to the 
invention is to replace the aerospike currently being used on certain 
submarine-launched propulsion systems which are stored in the submarine in 
approximately positioned fixed length and diameter cylinders. In such 
application, the aerodynamic shape of the shroud 30 is selected to provide 
the same or less drag than the aerospike that is being replaced. An 
advantage of the inflatable shroud is the ability thereof to provide the 
same or better aerodynamic performance with less added inert weight while 
occupying substantially less space, that is, shorter length when in the 
stowed position. Since the shroud 30 can be stowed flush against the nose 
cone, the only volume required inside the nose cone is a small space to 
accommodate the gas generator 26 that is needed to inflate the shroud 30. 
The current system employed for separating the nose cone from the forward 
portion 10 of the propulsion system may also be used in this application 
of the invention. Since the gas generator occupies less volume and length 
than the aerospike, the third or upper propulsion stage 20 may, if 
desired, be made longer. Without the aerospike bucket in the upper 
propulsion stage motor 20, the propellant weight may be increased and the 
ballistic characteristics of the upper propulsion stage improved, both of 
which would improve the system performance. 
Thus, there has been provided, according to the invention, a flexible 
inflatable shroud 30 that allows the propulsion system including boost, 
main propulsion stage and upper propulsion stage to occupy substantially 
all of the available storage space in a submarine storage cylinder or 
other silo with the shroud, when inflated, providing the desired 
aerodynamic shape for reducing drag and improving performance. As those 
skilled in the art will understand, the inflatable shroud, according to 
the invention, is not limited in its application to submarine launched 
propulsion systems but may be employed to minimize the weight and stowage 
space required for aerodynamic shaping for many propulsion applications. 
The inflatable shroud of the present invention may be used on any 
aerodynamic structure that would benefit from an improved aerodynamic 
shape, and is especially applicable to length constrained propulsion 
systems.