AIM-9/2 Sidewinder!
1/2 SemiScale H Powered AIM-9 Sidewinder High Powered Rocket Project

Design

Aerodynamics

The aerodynamic stability of the Sidewinder design was analyzed with VCP 1.64. Each of the major aerodynamic components (nose cone and fins) were specified according to the scale data and their estimated weights entered into the model. The Extended Barrowman mode was used for analysis because it supported the use of an elliptical nose cone that was closer to the Estes nose cone intended for the rocket. Next the non-aerodynamic components or mass items were added (body tube, payload, parachute, motor mount, and motor) to complete the mass model for the rocket. A number of different mass items were used for the different engines that could power this rocket. These were easily swapped in and out by changing their Associated property from Sustainer to None. However, the H123W was the heaviest motor the rocket was designed for and therefor the most restrictive in terms of center of gravity calculations. The resulting analysis for the H123W motor is shown below.

The results of the initial analysis showed that the rocket was unstable with the H123W motor without a significant payload. This was due in part to the forward fins, which move the center of pressure forward, and to the weight of the motor, motor mount, and other components used in the rear of the rocket. With a payload mass of approximately 9 ounces, the rocket attains the required one caliber static margin with the H123W motor. With lighter motors, e.g. G64W, the static margin increases to just over two calibers - a safe range for unassisted flight. This means that, when guidance or other payloads are not used, additional nose ballast will be necessary to maintain a stable flight.

As part of this analysis the weight of the finished rocket, minus motor, was estimated at about 35 ounces (2.2 LB / 1 Kg).

Performance

A modified version of Model Rocket Simulation via Excel was used for flight performance analysis. This program is an incremental rocket altitude modeling program, similar in mathematical basis to RASP, but built on an Excel spreadsheet. The Excel base makes this program easier to change and more flexible than RASP/wRASP which is built on C and the Windows API. The enhancements that were made included support for an additional motor burn type profile, and the addition of marking lines on the graph for motor burnout and ejection charge firing. The additional motor burn type profile allowed for the specification of the initial thrust level with a regressive thrust reduction across the burn time. This provides for a more accurate representation of most motor types.

The results of the simulation runs for Aerotech F52-5T, G64-7W, H73-10J, and H123-10W engines are shown in the following images. The F52-5T and G64-7W configurations are for an Aerotech RMS-29/40-120 casing and the H73-10J and H123-10W for the RMS-38/240 casing. Each image includes the key input parameters for the model as well as the predicted time vs altitude graphs. Note that the green line on the graph represents motor burnout and the blue line represents ejection charge firing.

Sidewinder Altitude Performance on an Aerotech F52-5T Motor

Sidewinder Altitude Performance on an Aerotech G64-7W Motor

Sidewinder Altitude Performance on an Aerotech H73-10J Motor

Sidewinder Altitude Performance on an Aerotech H123-10W Motor

Structure

The structure of the Sidewinder is described in terms of major sections, the fuselage, the motor section, the payload section. The fuselage forms the outer structural component of each section and is designed to allow the major components on the inside of each section to be removed. The motor section contains the motor and rear fin mounting structures. The payload section contains the payload support and front fin mounting (and control) structures. Each of these section is described in detail below. The design for the Sidewinder layout and the pictures included below were done using DesignCAD 3D Version 8.

Fuselage

The overall design of the rocket is shown in the picture at the right. The body design follows the basic lines and scale dimensions of the Sidewinder missile using a sectioned tube fuselage, a conical nose cone, and front and rear fins.

To meet the repairable/upgradable requirement, the fuselage actually consists of three components that form two different sections. The payload section of the fuselage consists of the nose cone and fuselage tube up to approximately the middle of the front fins and an additional length of fuselage tube (the remainder of the forward green area in the picture). This section is composed of two parts in order that the payload, including the front fins, may be easily removed and replaced. It is held together by the payload frame that is further discussed below. The aft section of fuselage tubing also contains a payload section aft bulkhead, shock chord mount, and tube coupling for attachment to the motor section.

The motor section of the fuselage consists of a fuselage tube containing rear fin mounting flanges, a motor section retaining ring, and a motor section forward bulkhead. The rear fin mounting flanges are permanently attached to the fuselage tube. The fins are attached to the fuselage tube by means of bolts that go through the mounting flanges and the fins. This provides the effect of Through-The-Wall (TTW) fin mounting but allows the fins to be removed and repaired/upgraded. The motor section retaining ring provides reinforcement for the rear of the fuselage and supports the securing of the motor section assembly by L-brackets and bolts. The motor section forward bulkhead provides a brace against forward travel of the motor section and for load transfer to the fuselage.

Motor Section

The motor section is based around a 38mm motor tube which houses the motor, either reloadable or single use. The motor tube is centered inside the body tube through the use of 3 centering rings: one in the front which rests against the forward bulkhead and one on each end of the rear fin section. Also attached to the motor tube are 8 rear fin mounting flanges - 2 flanges for each fin. The rear fins are attached to the fin mounting flanges by means of bolts allowing the fins to be replaced. The motor section assembly is completed by the use of 4 L-brackets attached to the back centering ring. These are fastened to the body tube by means of 4 bolts that go through the L-brackets and the fuselage tube to prevent rearward travel of the motor section during ejection charge firing.

Not obvious from the picture, is the use of a buffered ejection gas system similar to that found in Aerotech rockets. The forward end of the motor tube contains a mesh that will cool the ejection charge gases. The forward end of the motor tube is plugged but holes are made in the sides of the forward section to allow gases to escape to effect the ejection operation. Recovery is performed with the aid of 2 36" parachutes attached to the payload section and the motor section.

The motor section, with rear fins attached, is meant to be inserted into the motor section of the fuselage assembled as shown. In order to achieve this, the rear fin slots in the fuselage extent to the end of the body tube and the motor section retaining ring is a removable component. This is necessary to allow clearance for the bolting of the rear fins to the motor section, which could not be done once the motor section was inside the fuselage. To assemble the complete motor section, the rear fins are first attached to the motor tube flanges using bolts. Then the motor section with rear fins is inserted into the rear of the fuselage and the fins bolted to the rear fin mounting flanges on the fuselage. Finally, the motor section retaining ring is placed over the end of the fuselage and the bolted to the motor section via the L-brackets on the end.

In the motor section design, the bolting of the fins to the fuselage tube and the motor tube flange, the forward bulkhead in the fuselage, and the L-brackets/retaining ring work together to provide the load bearing elements. Attachment of the fins to both the fuselage tube and the motor section provides the equivalent of through-the-wall construction and transfers all fin loads to the fuselage and motor tubes. This attachment, combined with the forward bulkhead transfers thrust forces to the fuselage tube through the fins and the forward bulkhead. The fin attachment also combined with the rear L-brackets transfers reverse thrust forces (i.e. ejection charge firing) to the fuselage in a similar manner.

Payload Section

The payload section's primary function is to support payloads and the front fin assembly. It is consists of a frame formed by a series of bulkheads connected by a stringers. The removable front fins are mounted on axles that are supported by bearings on one of the bulkheads. The payload frame is attached to the rear section of the payload portion of the fuselage by four bolts through the fuselage tube and payload section frame. It is secured from rearward movement by a rear payload bulkhead. The frame is attached to the nose section (fuselage tube forward of the front fin attachment and nose cone) by two bolts through the fuselage tube and payload section frame. Additional cross braces and mounting brackets are attached to the payload frame as needed by the specific payload.

The payload section is assembled by first attaching the front fins to the front fin axles mounted in the payload frame. The payload frame is then attached to the rear section of the payload fuselage section via bolts. Finally, the front section of the payload fuselage section is attached to the payload frame via bolts.

There are two major loading forces exerted on the payload section. During boost, the downward force due to acceleration, puts a significant load on the payload section proportional to the weight of the payload (approximately 1lb maximum). This load is born by the payload frame and the payload fuselage tube through its attachment to the payload frame and the rear payload bulkhead. During parachute deployment, the payload section must withstand forces in the opposite direction. These are born through the shock chord mount through the fuselage tube to the payload frame through its attachment screws. Aerodynamic loading forces resulting from the front fins also act on the payload section through the front fin axles and axle bearings to the mounting bulkhead. These forces are assumed to be minor compared to other forces acting on the payload section and will be supported by the mounting bulkhead and the payload frame.

Guidance Section (future)

Two types of inertial guidance are being investigated for future incorporation into the Sidewinder. The first is a strap-down guidance system based on R/C heliocopter components. The second is a mechanical system based on gyroscopic precession. In both cases, the purpose of the guidance system is to keep the rocket flying in the direction of original launching to counteract heading changes resulting from wind. The system may also provide the basis for a backup parachute deployment system by using it to detect excessive attitude changes, such as those that occur during apogee.

The two guidance systems being investigated are intended for pitch and yaw control only. The original sidewinders utilized a novel roll control mechanism call rollerons. This is also being investigated as a future enhancement to the guidance of the Sidewinder.

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