I would really like a published quote on RV's heat resistant composites, except on the nose tip.
I'm not proposing destabilizing the RV or changing its flight path. To my knowledge, the trajectory during re-entry is more or less in a straight line. How effective can speeding it up be, in irregular intervals using a thruster along the axis?
hi dear
@The Deterrent
yes,The RVs are indeed entirely covered with heat resistant composites,have discussed this issue with my colleagues number of times.Isnt that the case with pakistani missiles?
Now coming to the main question of firing thrusters along the flight path of RV.Well i am attaching a very simple free body diagram of the RV.
First we need to understand what is our aim?
Our aim from a purely control engineering perspective is to keep the RV in it's intended flight path.That is,we dont want to disturb in direction perpendincular to velocity or in the direction of lift as depicted above in the picture.When we are firing thrusters along the flight path direction in order to fasten the RV up,we are also effectively changing the dynamic pressure Q.Now the force in the perpendicular direction must be balanced,i.e L=Wcos(theta).
Where L=Q*S*Cl(Cl should not be confused with rolling moment coefficient).Now L should always be fixed to Wcos(theta).If we are varying Q(=0.5*rho*v^2) then we need to also vary(in couter direction) Cl to make L equal to Wcos(theta). Now Cl is again approximately equal to itz own first order variation(by taylor's expansion)- d(Cl)/d(alpha)*alpha.
Hence L can be re-written as Q*S*(Clalpha)*alpha------------------where Clalpha is nothing but the slope of the Cl-alpha curve which is fixed.Therefore what is the other parameter left to vary is alpha or angle of attack!
Concluding my explaination,if we want to increase speed we would ALSO HAVE TO CHANGE ANGLE OF ATTACK in order to balance the force in perpendicular direction.
After having explained the reason of changing the angle of attack,now lets delve into how we can change angle of attack of a RV during the final leg of the journey?
We can either vary alpha by externally mounted control surfaces or the flex nozzles inside the RV.At this juncture, We also need to take care of the velocity at which an ICBM RV is travelling which would be in hypersonic range. Control challenges become very difficult because there is inherently a "small lag"-------(mathematically modeled by exp(-s*tau),where s is the laplace variable s=sigma+j*omega and tau is the delay),however small,associated with any actuator(here in our case flex nozzles). Hence even a slight delay(tau) can result into different flight path and our RV will miss itz target!