Development of a Cold Gas Propulsion System for the ... - SSL - MIT

4.1.1 Flight Profiles
Hopper flight profiles were entered into in the MATLAB model in terms of varying vertical and horizontal
thrust. As discussed in section 3.1.3, the vertical thrust requirements throughout a hop of the desired
dimensions were fairly well defined. Because the hopper would constantly lose mass during a hop as it
expended nitrogen propellant, the vertical thrust levels for each phase of the hop were defined in terms
of T/W, with a new vehicle weight calculated at each timestep of the solution based on previous gas
usage. It was assumed that all four vertical thrusters would fire together to provide the total vertical
thrust needed from the CGSE.
There was more variation possible in defining the horizontal thrust for the transit phase of the hop. For
all flight profiles, it was assumed that horizontal thrusters would fire in pairs, to accelerate or decelerate
the hopper in the direction parallel to its Z axis (see Figure 3-2), with no steering or other rotation.
However, the hopper could have short periods of acceleration and deceleration with a longer cruise at
relatively low constant velocity, or longer periods of acceleration and deceleration resulting in a shorter
cruise at higher velocity. In the limit, the hopper would accelerate horizontally for the entire first half of
the transit phase and then decelerate over the second half, with no constant-velocity cruise in between.
Several different horizontal acceleration profiles were modeled in order to judge their effect on CGSE
design requirements.
It should be noted that throughout all phases of the hop, attitude control was known to be necessary
but not modeled. Possible sources of disturbances were identified to include gusts of wind (though it
was planned to operate the TALARIS hopper indoors if possible to mitigate this), misalignments in the
CGSE (i.e. a thruster mounted at a slight angle to its primary direction of force), and torques or lateral
forces caused by the EDF weight relief propulsion system. The magnitude of these disturbances was not
known at the time the model was written, so it was not possible to make an accurate estimate of the
total amount of impulse necessary for attitude control, but it was assumed that it would be much
smaller than the impulse required to perform the major planned vehicle maneuvers. Therefore, flight
profiles were only considered feasible if the model indicated there was surplus gas available in the tanks
at the end of a hop. It was assumed that this additional margin would be used for attitude control during
a real hop.
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