Optimal ballistic coefficient control for Mars aerocapture
Taking advantage of the aerodynamic force to capture the vehicle from hyperbolic orbit to planetary orbit is a preferable approach because of the lower fuel consumption. This paper aims to research the Mars aerocapture process using the ballistic coefficient as control variable, and the inflatable deceleration device is used to change the ballistic coefficient. Firstly, the corridor is described in detail considering the different ballistic coefficients. Besides, the optimal aerocapture trajectories are analyzed, which show that they have bang-bang control structures. Additionally, the optimal results are showed under the different control structures, and the corresponding performances are presented. Finally, the optimal constant control problem is analyzed through the statistical results with the different initial conditions, and the change rules of the peak heating rate and the ballistic coefficient are given.
Minimum-fuel aeroassisted coplanar orbit transfer using lift-modulation
Minimum-fuel trajectories and lift controls are computed for aeroassisted coplanar transfer from high orbit to low orbit. The optimal aeroassisted transfer requires less fuel than the all-propulsive Hohmann transfer for a wide range of high orbit to low orbit transfers. The optimal control program for the atmospheric portion of the transfer is to fly at maximum positive L/D initially to recover from the downward plunge, and then, to fly at negative L/D to level off the flight, such that the vehicle skips out of the atmosphere with a flight path angle near zero degrees. To avoid excessive heating rates, the vehicle flies initially at high angle of attack in order to slow down higher in the atmosphere, allowing recovery from the downward plunge, which occurs subsequently using the maximum positive L/D, to take place at a lower atmospheric density, or equivalently, at a higher altitude.
Effects of reservoir volume on performance of pulse tube cooler.
A pulse tube cooler has the advantages of long-life and low-vibration over conventional cryocoolers such as G-M and Stirling coolers because of the absence of moving parts at low temperature. On the other hand, the combination of a reservoir and orifice is indispensable to optimize the performance of pulse tube coolers. In order to make the pulse tube cooler compact for practical applications, the volume of reservoir should be minimized. This paper analyzes the effects of the reservoir volume on the thermodynamic performance of various components in a simple orifice and a double-inlet pulse tube cooler by combining a linearized model with a thermodynamic analysis. Expressions of entropy production for those components are presented. The results show that the reservoir volume has a significant influence on the entropy production in the various components when the reservoir to pulse tube volume ratio is smaller than about 5. The ratio is important to determine the minimum reservoir volume for a pulse tube cooler. Optimum settings for a double-inlet pulse tube cooler are also discussed.