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

The flight profile begins with maximum vertical thrust of approximately 120 N, or about 30 N produced
by each of four vertical thrusters, as was calculated in section 3.1.3. This period of upward acceleration
lasts for about 1.4 s, after which the vertical CGSE thrust drops to a much lower level with less than 30 N
produced by the four vertical thrusters together. During this second half of the ascent, the hopper is still
moving upwards due to its vertical momentum, but the total T/W is less than 1 even with the 5/6
weight relief provided by the EDFs. This causes the vertical rise of the hopper to decelerate until it
reaches its operational altitude of 2 m with zero velocity approximately 2.8 s into the flight.
After the ascent, the horizontal transit phase of the hop begins. Throughout this phase, the CGSE
vertical thrusters and EDFs together maintain a constant T/W of 1, but because the mass of the vehicle
continually decreases as propellant is expended, the thrust level can be seen to decrease with time in
the plot. Meanwhile, the horizontal thrusters fire to translate the hopper. In this particular flight profile,
the hopper is first given a constant horizontal acceleration of 1.33 m/s 2 for 4 s. This acceleration is
provided by two of the four horizontal thrusters; as with the vertical thrusters, they must decrease their
thrust over the firing period in order to maintain constant acceleration as the hopper loses mass.
At about 6.8 s into the flight, the horizontal thrusters shut off, and the constant-velocity cruise begins.
At this point, the hopper has translated 10.67 m from its starting point, and it has a horizontal velocity of
5.33 m/s. The hopper cruises for 1.625 s, in which time it covers 8.66 m of additional horizontal distance
(assuming drag is negligible).
At the end of the horizontal transit phase, the hopper again fires a pair of horizontal thrusters for 4 s.
However, this time it fires the pair opposite to the pair that initially fired, so the hopper decelerates
horizontally. (Note that Figure 4-3 plots thrust magnitude, not direction.) The horizontal transit phase
ends about 12.4 s into the flight, with the hopper again hovering at 2 m altitude with zero velocity, but
now located 30 m from its starting point.
The final phase of the flight profile is the descent. It is essentially the mirror image of the ascent, with
the vertical thrust first decreasing so that the hopper begins to drop but then increasing again to
decelerate the hopper’s fall so that it lands with zero velocity. As illustrated in Figure 4-3, the total flight
time for this profile is just over 15 s.
The MATLAB model tracked the remaining nitrogen propellant mass as well as its thermodynamic state
throughout this hop, and it also computed the component