A.3.2.3 Thrust Vector Control1

A.3.2.3 Thrust Vector Control

From a dynamics and controls perspective, thrust vector control (TVC) is a relatively simple process. We are only concerned with altering the effect of the thrust vector on the launch vehicle to ensure stability and steering control. Thus, from a dynamics and control perspective, there is very little difference between a gimbaled nozzle and a liquid injection (LITVC) scheme. If both systems satisfy the requirements for thrust vector control, factors other than control issues will influence our decision on which system to use. In particular, gimbals are more suitable for liquid propelled engines and LITVC is more suitable for solid and hybrid propellant engines.

As we discuss in the section on launch vehicle equations of motion (A.3.2.2), we decompose the thrust vector direction into two angles, δ and κ. The variable δ is the angle that the thrust vector makes with the launch vehicle axis of symmetry and is constrained to be between 0o and 5o. The limits on δ were originally selected from historical data, and we later confirmed them from the data on LITVC. The angle κ is a measure of the thrust vector’s rotation about the axis of symmetry and is constrained to be within ±1800 from the body axis. The simulator actively controls δ and κ, and records the needed spin torque, to keep the launch vehicle stable and as close to the nominal trajectory as possible. Unfortunately, as we can see in Eqs. 3.2.1.27 to 3.2.1.28, even the simplified EOMs have δ and κ coupled, complicating the control scheme.

Baski et. al. discuss a control method for a small, gimbaled liquid propelled launch vehicle.1 Gimbaled systems typically control the orientation with respect to the launch vehicle body of both the nozzle and the combustion chamber. Such methods are acceptable for liquid propellants but changing the orientation of the chamber for solid or hybrid motors would require high powered motors and a significantly increased stage diameter. To control roll with a gimbaled TVC method, we will have to include a secondary system dedicated only to roll, such as transversely mounted nozzles.

We recognize that there is another method for gimbaled TVC which has more application to solid and hybrid motors: instead of gimbaling the combustion chamber and nozzle as a unit, we gimbal only the nozzle. We could include a flex seal between the combustion chamber and nozzle to control the flow of exhaust into the nozzle. However, flex seals lead to efficiency losses, are difficult to build, and must be highly heat resistant, necessitating expensive materials. Current flex seal designs also have high failure rates, another strong reason not to use flex seals.

In contrast, we recognize that LITVC systems are much simpler to construct, require only light-weight valves, and can in fact provide additional thrust while in use.2,3 We still need to assure that injection TVC methods will give side forces adequate to satisfy the thrust vectoring requirements. One performance parameter of injection TVC systems is the ratio of side force to axial thrust, as in Eq. A.3.2.3.1

/ A.3.2.3.1

where Λ is a unit-less ratio, Fs is the side force and T is the axial thrust. Using the constraint that δmax=5o, we calculate that Λ must be at least 0.08715. Referencing Ho and Blackwell, we see that historical data confirms that injection TVC methods can provide the needed side force.2,3

LITVC also provides us a simple way to control roll. Using a scheme presented by Zeierman and Manheimer-Timnat, Fig. A.3.2.3.1 shows a cross-sectional schematic of a nozzle that allows for spin control using the primary TVC system.4

Figure A.3.2.3.1: Schematic of thruster nozzle for ITVC roll control, based on diagram by Zeierman and Manheimer-Timnat

(Author: Jeffrey Stuart)

To control pitch we use either ports A and B or D and E in conjunction. Similarly, we use either port C or F to control yaw. Finally, we use either ports A and D or B and E to control the roll of the launch vehicle. Unfortunately, time constraints on the project did not allow us to implement this control method into our simulator.

Based upon our research and analysis, we decide that we should use LITVC for our launch vehicles, which use either a hybrid or solid propellant for all stages. In addition to satisfying the basic TVC requirements, the LITVC approach also provides a simple way to control the roll of our launch vehicles. We see that LITVC will be cheaper to construct, be less massive, and provide thrust enhancement over gimbaled systems, particularly for our hybrid and solid engines. Further work that we suggest is inclusion of roll effects from the LITVC system into the developed controller.

References

  1. Baksi, S., et. al., “Low Cost Thrust Vector Control System Development for Small Launch Vehicles,” AIAA Paper 03-5246, July 2003.
  2. Ko, H., and Yoon, W.S., “Performance Analysis of Secondary Gas Injection into a Conical Rocket Nozzle,” Journal of Propulsion and Power, Vol. 18, No. 3, pp. 585-591, May-June 2002.
  3. Broadwell, J.E., “Analysis of the Fluid Mechanics of Secondary Injection for Thrust Vector Control,” AIAA Journal, Vol. 1, No. 5, pp. 1067-1075, May 1963.
  4. Zeierman, I., and Manheimer-Timnat, Y., “Full Control of Solid Propellant Rockets by Secondary Injection”, Journal of Spacecraft, Vol. 10, No. 3, pp. 161-162, March 1973.

Author: Jeffrey Stuart