Importance of Fuel-Regression Rate
The rate that the unit mass of solid fuel gasifies per unit time is determined by: the heat needed for the fuel to rise to vaporization temperature; the vaporization heat (latent heat) of the fuel material; and the amount of heat per unit time provided from outside. The local fuel regression rate is obtained by dividing the mass of the fuel gas generated from unit surface area per unit time by the density of the solid fuel.
According to the above ratio, it is evident that the fuel regression rate increases as the heat provided from outside increases. Since the heat from outside is transferred by the convection and/or radiation from the flame, the closer the flame is to the fuel surface, the greater the amount of heat transferred per unit time per unit area. In the case of solid or liquid propellants, the distance between the flame and the propellant is thought to be several tens µm. Meanwhile, in the case of boundary-layer combustion such as the hybrid rocket, the distance is believed to be several mm. Therefore, we cannot deny that the amount of heat propagated to the fuel becomes small.
Small fuel regression rate has various effects on the rocket design. One problem is that, when using a simple cylindrically shaped fuel as shown in Fig. 1, the aspect ratio of the fuel is inversely proportional to the fuel regression rate. Specifically, as the speed becomes small, the aspect ratio increases, i.e., the rocket becomes slender. In a slender rocket, the volume-packing ratio of fuel decreases and rocket efficiency also suffers in terms of structural weight. As an example, assuming the scale of S-520 sounding rocket using solid propellant, if it were to be replaced by a hybrid rocket with a fuel regression rate 1mm/s, the aspect ratio of the fuel is calculated to be around 40 when its initial fuel inner diameter is 30cm. Compared to the aspect ratio of about 10 with the S-520, this value is too large.
To solve this problem, there is a technique called multi-ports whereby the surface area is increased by boring many holes in the fuel. It was employed on the hybrid rocket SpaceShipOne that twice successfully reached an altitude of 100 km carrying a man. It must be pointed out, however, that it is prone to leave combustion residues and its design is difficult. To solve the problem fundamentally, it is essential to raise the fuel regression rate.
Improvement of Fuel Regression rate and Combustion Efficiency
Fig. 3 is the graph plotting data of fuel regression rates obtained in Japan and overseas in the past against the mass flux (mass flowing per unit area per unit time) of the oxidizer. To improve the fuel regression rate, HRrWG is taking two approaches. The first is to swirl the oxidizer flow so as to bring the flame closer to the fuel surface using centrifugal force. With the flame closer to the fuel surface, we can raise the speed up to three to four times compared to axial-flow injection. As shown in Fig. 4, the flame is formed near the surface in the oxidizer swirling flow-type combustion chamber. When using liquid oxygen as oxidizer, we can obtain the swirling flow effect by gasifying the liquid oxygen upstream in the injector. To this end, HRrWG is proceeding with R&D on a regenerative cooling-type nozzle to evaporate the liquid oxygen.