Optimization of rocket system - realization of both high performance and low cost
The next solid rocket will implement several excellent features suitable for a successor of M-V rocket and the next-generation launcher. First, I would like to mention rocket optimization. While optimization on the M-V rocket centered on performance, on the next solid-rocket system we will optimize the entire vehicle from the two aspects of performance and cost. Of course, this is not so easy. Performance and cost are conflicting requirements and, therefore, fulfilling both simultaneously is generally impossible. The real potential of system engineering, however, is to satisfy two competing requirements at the same time. You can find similar cases everywhere.
When you drive a car, for example, there are two important properties, directional stability and controllability (ease of turning). These are basically contradictory properties (i.e., when directional stability is good, turning the car becomes difficult). If we think about these properties from the standpoint of velocity, however, we notice that the velocity range where directional stability is required is different from that where controllability is required. Stability in direction is required at relatively high speeds. Controllability, i.e., ease of turning the car, is required at relatively low speeds. A car with good directional stability makes you feel comfortable on the highway. Apart from Formula 1 drivers, nobody boasts of their turning technique at high speed. Thus, if we design a car with the condition that control properties are functions of velocity - i.e., directional stability is preferred at high speed while ease of turning is preferred at low speed, we can satisfy these two requirements simultaneously. A similar approach was adopted in rocket control. Two contradictory properties, stability and responsiveness, were realized simultaneously (refer to the article “Control of Rocket” in ISAS News, April 1997, for more detail).
In the next solid rocket, we plan to introduce a concept of “capacity sensitivity” (magnitude of influence on satellite mass that can be put into orbit) to achieve both performance and cost requirements. More specifically, we will reduce the cost drastically for low capacity sensitive part while improving performance intensively for high capacity sensitive part. For instance, we plan to divert the low-cost SRB-A to the first stage motor (first-stage rocket), which has low sensitivity to launch capability but expensive due to high consumption of propellant. Meanwhile, for upper-stage motors (second- and third-stage motors) that have high capacity sensitivity but are relatively low cost, we intend to have a new design based on M-V experience to realize higher performance than M-V. We are now considering this improvement from various perspectives, for example, reducing the weight of motor cases and improving propellant filling efficiency.
Designed as a sub-booster, the SRB-A has a thrust that is too small to use as a first-stage motor. It also has a small propellant-loading capacity. These weaknesses sacrifice the entire rocket’s capability. To compensate for the low performance SRB-A, we will optimize power allocation of the second and third stages in the next solid rocket. In this way, we can maximize the satellite launch capability by using the low-power SRB-A in the first stage. This technique is called “optimal staging,” an important concept in rocketry.
Next, I would like to talk about the performance of the next solid rocket. Fig. 2 shows changes in the mass ratio of the third-stage motor (the second-stage motor in the next solid rocket), which is most sensitive to orbit-injection capability, and the payload ratio of rockets. The motor’s mass ratio means the ratio of propellant mass to motor mass and this is an indication of motor efficiency. The rocket’s payload ratio is a measure of satellite mass to overall rocket mass and shows the rocket’s transportation efficiency. In each case, the bigger the ratio is, the higher the performance is. As you can see, the next solid rocket will have an excellent upper-motor mass ratio exceeding that of the M-V rocket and, in addition, attain a payload ratio as high as that of the M-V. I should emphasize that the M-V’s payload ratio was recognized as the world’s best.