A Method to Compute Flameout Limits of Scramjet-Powered Hypersonic Vehicles

Abstract

As a hypersonic vehicle travels upward along an ascent trajectory, the static pressure (p 3 ) in the scramjet combustor will decrease, which can lead to engine flameout. At low pressures the chemical reactions between the fuel and air become excessively slow. However, during the ascent the flight Mach number is increasing; this increases the stagnation temperature and the static temperature (T 3 ) at the combustor entrance so it tends to prevent flameout. To investigate this tradeoff, a general method to understand how the flameout limit varies during ascent was developed. The method consists of two parts; first the static temperature and pressure at the entrance to the combustor (T 3 , p 3 ) are computed as a function of the vehicle altitude; this is done using a reduced-order propulsion model called MASIV. In the second part the values of (T 3 , p 3 ) are inserted into an empirical relation for the critical Damkohler number in order to determine if flameout occurs or not at each altitude. The empirical flameout relation is based on previous ground-based measurements made at AFRL and elsewhere.

Combustion Efficiencies and Flameout Limits Computed for a Hypersonic Vehicle During Ascent

“…Previously, the advantages of ROMs have been pointed out by U.S. Air Force Research Laboratory (AFRL) researchers Bolender and Doman [26], O'Neill and Lewis [27], O'Brien et al [28], Bowcutt [29] at Boeing, Dalle et al [1][2][3], Torrez et al [4,5], Mbagwu and Driscoll [6], Lamorte et al [7], Mbagwu and Driscoll [8,9], and Chavez and Schmidt [30]. The Bolender-Doman AFRL ROM was developed in 2006 [26] to simulate the flight dynamics, poles, and zeros of a hypersonic vehicle.…”

“…To improve on the Bolender-Doman AFRL analysis, a secondgeneration model, called MASIV (from "Michigan-AFRL scramjet in vehicle") was developed during a joint collaboration between the University of Michigan and AFRL [1][2][3][4][5][6][7][8][9], some details of which are summarized in Sec. III.…”

“…The MASIV combustor submodel was described in detail in [1][2][3][4][5][6][7][8][9], and so only a summary is provided here. The code includes finite rate chemistry, real gas properties, a three-dimensional jet mixing model, a separated boundary-layer model, and gas dissociation.…”

“…T HERE are several interesting tradeoffs that occur when a hypersonic waverider ascends along a trajectory of constant dynamic pressure (q 1∕2ρ ∞ U 2 ∞ ). Figure 1 shows the geometry of the MAX-1 waverider that the authors have analyzed [1][2][3][4][5][6][7][8][9] using their reduced-order model called the University of Michigan-U.S. Air Force Research Laboratory scramjet in vehicle (MASIV). A constant-q trajectory might be selected from one of the three solid curves in Fig.…”

“…A desired trajectory is one that avoids crossing these two limits. Computations of these operability limits have not been reported previously; to do so, our reduced-order model [1][2][3][4][5][6][7][8][9] had to be improved using the approach described in Sec. IV.…”