Space Rapid Transit – Landing Gear Design

Hello everyone, I am an MIT extern here at Princeton Satellite http://dailyhealthymale.com Systems through MIT’s Externship Program. Over the past three weeks, I have been able to play a part in and help out with a number of assignments. The most recent assignment is what I will be detailing in this post.

One of the projects PSS is working on is Space Rapid Transit, a two-stage-to-orbit launch vehicle with horizontal takeoff (think space vehicle that can “launch” like an airplane).  I was given the task of designing the nose landing gear, and in particular figuring out what type of linear electric actuator should be used to handle the load of retracting the landing gear.  Here is a preliminary design drawing I sketched to conceptualize the task.

prelimdesign

In order to find a solution, I first needed to make a few design assumptions.  The first assumption was that the landing gear would retract toward the nose (which is a reasonable assumption because it allows more space behind the landing gear).  Next, I chose to model the retraction under the assumption that the vehicle is undergoing a 2-g turn.  I then selected the strut and tire sizes and found the maximum speed and altitude at which operation of the landing gear is allowed, using the specifications of the Airbus A320 because of its similar takeoff mass.  I now had enough information to approximate the force on the linear actuator.  For this I made a simplified sketch, drawing the side and top view of the landing gear as it undergoes retraction.

LandingGear

Using the side view in the diagram above, I simplified the landing gear retraction into a torque balance problem, where all torques were evaluated about the fixed pivot.  I found the time it takes to retract the landing gear to be around 10 seconds and estimated a full sweep angle of the landing gear (from fully extended to fully retracted) to be 90 degrees.  Assuming constant angular acceleration, I was able to calculate this angular acceleration using the time and angle noted above.  I then calculated the distance of the center of mass of the wheel and strut configuration from the pivot as well as the moment of inertia.  After this I computed the drag force and gravitational force (from the 2-g turn) on the strut and wheels and computed how much torque each force would apply about the pivot.  Since the angular acceleration was so small that the resultant torque was negligible, the problem became a balance of the torque applied by the actuator with the torques resulting from air flow and the 2-g turn.

With this new found torque required from the actuator, I searched for linear electric actuators that could supply the force and stroke length.  The stroke length was approximated as the distance of the applied actuator force from the pivot.  As a result, I selected a Size 5 Moog Standard Linear Electric Actuator because it fit the design requirements.