Mod-01 Lec-23 Rocket Nozzles – Thrust Vectoring
Mod-01 Lec-23 Rocket Nozzles – Thrust Vectoring


In the last class we had talked about how the
plug nozzle would have a better performance off design performance let us look at it in
this graph. If we have PC / P on the x-axis and CF on
the y-axis this is a log scale, so if you have a nozzle design for some high-altitude
conditions and we are here a conventional nozzle or a conical nozzle will go like this
whereas a plug nozzle will have a superior performance throughout this is okay, so the
conical nozzle has a much inferior altitude at conditions other than the design conditions
it is performance is not as good as a plug nozzle or a narrow spike nozzle, so next we
move on to the other topic that is thrust vectoring right thrust vectoring is something
that is used by all missiles as well as launch vehicle. If you look at the tactical missiles that
is these are small rockets and if you look at what the level of thrust that they produce
the accelerations that they have initially is of the order of ng so they very quickly
go to a very high velocity, so which is why you do not need any control in the booth space
they usually have something known as a boost phase wherein the thrust is very, very large
and it goes to very high accelerations due to gravity the acceleration achieved are very
high and after which they will have a sustained phase which will essentially keep it at that
particular Mach number okay. So for such rockets or tactical missiles aerodynamics
is a very good aerodynamics offers very good way to control the motion of this as in, if
you want it to wear around to a target and other things you can have fins and you can
make it make it go towards the target very easily, but if you look at launch vehicles
and strategic missiles that is long-range missiles when they take off their accelerations
are very, very small typically of the order of 1point 6 g to 2 g they are not very large. So as they are going up they are going up
very slowly and crosswinds can take them in a direction that is not intended, if you have
strong crosswinds now once we cool you can put off the launch by a few hours you can
say till the weather gets into some kind of reasonable conditions you are not going to
launch it and you will wait for that time to launch it but missiles you cannot afford
to do that because you need to be ready every time yeah even in launch vehicles you would
still want some kind of control in this period of launch where and the velocities are very
small. Up to it the time that it reaches a Mach number
of around 0.5 aerodynamic control will not be very, very effective right, so it is unlike
aircrafts where unlike aircrafts will have a very large surface area right the wingspan
is very large, so it can produce substantial lift here you do not want it to have so much
drag associated with any control surfaces you wanted to be minimal so you will have
very small things, so a under these conditions you would need something known as prospect
ring that is can you change the direction of the thrust itself so as to take the vehicle
in the direction that you want it to go. In addition if you have looked at launch vehicles
let us say PSLV or something like that you will find that it has a main core and on the
side it has small strap points right PSLV has 6 strap-on
motors, now the nozzle of all these boosters are in such a way that their tented away that
is the thrust that it produces is at some angle to the vertical axis right, so if this
is the axis and the thrust produced is not parallel to it but at some angle right so
if all of these thrust vectors meet at some common point on the axis. Then it is perfectly balanced otherwise there
is some imbalance and you need to have a counterforce to overcome this it you need to have a control
force capable of overcoming this otherwise the vehicle could we are in a direction that
is not intended, so for all these things you need this thrust vectoring okay now thrust
vectoring can do all this that is it can allow the vehicle to rule yaw and pitch
in this class we will concentrate on what are the ways in which we can vector this thrust. What are the means that are available and
what is it that is practiced and what can also be done okay, now typically thrust vectoring
there are three modes that are usually followed one is. Jim balling
of trust chamber or nozzle or introducing mechanical control surfaces or second wave fluid injection in the exit
coup we look at each of them in a little more detail in this class now firstly Jim balling the thrust chamber
or the nozzle, if you look at a liquid engine liquid engines typically the propellants are
not stored in the engine or in the chamber where it produces thrust that is the fuel
and oxidizer mix in, but so it is stored in a separate chamber so the engine is as itself
will be very small. So you can look at moving the engine in two
different planes to get the control force that you require okay that is possible in
a liquid engine. That is if the engine were hinged on this
you could move it around by an angle a in either direction and if you have something
known as a universal joint Universal joined us nothing, but if you have two hinges right
if you have two hinges like this using one of them you can move it in one plane using
the other one you can move it in the other plane right, so using both of them you can
move it and two different planes that is what is a universal joint, if you have the entire
engine mounted on a universal joint. Then you can move it in this plane by a and
also in the plane perpendicular to the board by an angle a okay, so that is what a gym
ball or a hinge will do as you can see if the engine is moon who overall engine is itself
moved by an angle then the thrust produced will be the cause of this angle right, so
it will be lesser than the actual thrust so you would not lose some amount of thrust by
vectoring the thrust itself so you can using this move the entire vector the thrust by
plus 0 120 and there will be a small thrust loss. And this is typically used in only liquid
engines that is because as I said earlier, if you
look at solid engines the entire propellant is also inside the motor, so it would be very
difficult to move the entire motor because the weight of the propellant is also there
whereas in a liquid engine the propellant is stored elsewhere and only thing is you
need to have flexible hosing otherwise it will not allow this kind of movement okay. So you need and you need also large actuators
to move this and the critical thing is, if you look at
this the entire thrust of the engine is transferred through this hinge or the gym ball, so this
needs to be very carefully designed because this is a very critical parameter I mean very
critical element because the entire thrust is transferred through this element to the
rest of the vehicle okay so this needs to be carefully designed instead of Jim balling
the entire engine the other option is can we just move the nozzle itself and that is
called as a flexible nozzle. So if you have an actuator and move this nozzle
alone right then you can get the same desired effect of vectoring the thrust only thing
is this joint needs to be flexible here and allow for movement okay this can also give
+ 1 – 1 and again there will be a small thrust loss because you are anyway vectoring
the thrust, so the cause of the thrust is the only one that is available in the axis
along the axis this has also been used in both solids and
liquids rockets here you can use it in solid rockets. Primarily because you do not have to move
the entire motor you are only moving the nozzle but still the actuation force required here
is also quite significant you need the last of this variety is again, if you
have something like a ball and a socket joint instead of this one you can move the nozzle. If you have it mounted on a bearing then you
can move it and therefore get the required trust metric, now in both these methods the
nozzle in some sense is submerged inside the motor and this leads to something known as
submergence loss primarily, if you look at the gases coming out they need to you would
also have propellant stored in this direction, if it is coming in this direction it is a
lot easier otherwise the propeller the gas is coming from burning of the propellant in
this region will have to turn through a large angle. And that will lead to some losses because
the flow will now have to turn through a large angle to get to the nozzle and that is given
by something like it varies between 0.4 to 1.2 % loss in ISP and if you remember the
discussions that we had in the morning that is if this propellant were aluminized propellant
right, then we discussed that the particles noon or us condensed particles do not expand
and they cause loss right this gets coupled with that and depending on the percentage
of aluminum present in the propellant. And the amount that the nozzle is submerged
the loss could be higher I mean this it is within this range it could be higher, if it
has more submerged lesser if it is less submerged the next set of control that we can get is
from mechanical control surfaces here. If you are using mechanical control surfaces
what you will have is something getting into the exit flow and therefore you are going
to obstruct the flow and cause the require prospect jetovator are something like this,
if you have a rocket motor this at its normal position it is something
like this and if you want to have a side force you could change its position to something like this, so this does not protrude
into the flow in its normal position but when you change its position on one side you get
overexpansion and on the other side you get a little bit of enter expansion this causes
a side force okay. So you are going to get a small amount of
thrust vectoring if you are using this you will get something like + 1 – 70 of thrust
victory and compared to the previous ones this is a lot better in that sense that firstly
it does not require a very large actuator actuation power right, because you are only
looking to actuate a small surface and therefore the power required is smaller here these are
used in solid rockets, yeah this small portion is dotted line it is it is in line with the
nozzle when it is not doing anything right. You can then rotate it and on one side it
will it will be flush with the nozzle, so it will act as on one side it will over expand
and on the other side it Lander expand and therefore give you the required side force
then there are something known as jet tabs. Now if we were looking at it from the bottom
it would look like this now you could move this into the flow as per requirement okay
and depending on the amount this up stretch the flow you will get the thrust force proportional
to that okay, so as such this the both these do not obstruct the flow in their normal position,
so they did not add to any loss in their normal operation but when they are used they will
lead to a small loss with this you can get + – 140 of change in the thrust. And whenever this is in use it leads to something
like 1% loss and thrust per degree of deflection and the actuation power here is
also very small because you are only trying to actuate a small tab, so the actuation power
required is smaller here the last in the mechanical control surfaces
is something known as a jet rain. These jet wins are in the exhaust gas flow
so because they are present in the exhaust gas flow itself even without actuation they
are going to lead to some kind of thrust loss okay, so they are going to have you are going
to have some rust loss you can get thrust-vectoring of the order
of + – 90 and the thrust loss will be more when it is actuated okay it will it is there
even without actuation in this case there will be more when it is actuated, so this
needs a pretty good heat-resistant material. Because it is always in the exhaust flow right
what is usually done in the case of jet vanes is you might need this for a small period
of time during takeoff or during some critical operation after which you can throw them away
okay, so these are used in some examples wherein you are launching something from a ship or
launching a missile from a ship or something like that then it has to turn by a large angle,
so you can use this and then throw this event for the rest of the flight this is not useful
okay so it is used for a very small period of time now the last thing that is used actually
in rockets is the liquid engine. Remember in an earlier class we said if you
have an oblique shock the flow separates and it will introduce a side force right in this
method of controlling or thrust-vectoring we actually make use of that and we inject
a liquid at some known points in the divergent portion that is let us say if we inject the liquid
at this portion because of the liquid coming out here the an oblique shock develops up
upstream of this liquid injection point and because of this you will get a side force
right because in this portion it actually acts as a nozzle being cut off and after some
length and therefore it get gets you the required side force okay. SI TVC is used in ESL ways stage one motor
for respective control in this case because of the liquid injection there is a small amount
of thrust augmentation that is happening the liquid that is used is strontium perchlorate,
now the reason for using such a liquid is if you look at the momentum of the jet that
is coming through if you are injecting this debt this jet needs to have a higher momentum
than the jet, so ?J VJ2 square must be comparable to ?g Vg2 now if you notice here the density
of the gas is very small and but the velocities are very large and that is squared right. What else here the liquid velocities are not
going to be so large, so you need to make it up with a higher density liquid which is
right this strontium perchlorate is used people have also thought about using a fraction of
the exhaust gases themselves taking a certain bleed from the combustion chamber and using
it, but the trouble with that is you have to have leak proof valves that operate not
only at high temperature but also at high pressure which is not very easy otherwise
if there is a leak there will be always a side force. So you do not want that and therefore this
kind of taking a certain bleed from the combustion chamber has not been pursued, so therefore
you have to have a large tank containing this stronger perchlorate primarily because if
it needs to be injected at some velocity the pressure here of the jet must be greater than
the pressure here, so this tank needs to be pressurized okay and then the liquid has to
be expelled under pressure so that makes it a little more bulky but as I said. If you want to use gas injection then hot
gas handling is not very easy in liquid engines in some of the liquid engines
you usually have something known as a gas generator which is used to run the turbine
which in turn runs the pumps, this gas generator also has an exhaust which can be used in auxiliary
nozzles provide the kind of thrust-vectoring that we want, so you will have small motors
for that. If this is the main engine you could have
small engines that can be moved you could have these small thrust chambers which you
can control and move, it move the entire thrust seamer that will give you the required sight
force that is required, but in this case the thrust vectoring that is possible is very
small not very large so the only other thing that people have kind of thought of using
but not yet looked at this in this case instead of using a liquid it is a one has a liquid
rocket motor a small liquid rocket motor and inject the exhaust gasses here that could
also give you the required side force. Primarily because you will have a very high
jet velocity okay and if you want to switch off and switch on this it is not very difficult,
because you are only going to operate the liquids right switching on and switching off
a liquid rocket motor is not a problem but this is something that has not been pursued
and probably could be pursued sometime later there is also another method wherein if you
have for rocket motors and the exhaust coming out through canted nozzles right something
like this. Let us say you had four rocket motors liquid
rocket motors that are providing you the thrust then you could have a situation wherein, if
you look at it from the bottom let us say these are the exhaust ports of these four
motors you could increase the thrust and decrease the thrust in two of them to get your required
control force that is let us say, if you want to pitch up and pitch downright so you could
actually increase the thrust of this to pitch up and reduce the thrust in this so that it
will this will cause a this will make the pitch which if you are looking into the board
this will cause pitch up pitch down okay. If you are looking into the board this will
cause the vehicle to pitch down and similarly you can have this is for pitch or yaw increase
the thrust in these two or the other two to get the required I emotion and you could also
have rule using two of them in this fashion okay, if
you increase the thrust in these two then you can have this is only possible, if you
have a liquid rocket motor and if you have four of them if you have this is canted nozzle
so the thrust vectors will not be parallel I mean perpendicular to the board it is at
an angle. So you will have that role till now we have
looked at this rocket propulsion course in this fashion that is we have not looked at
what is the thing that is there in the thrust chamber right we have looked at nozzle we
have tried to derive equations to get the specific impulse of the nozzle, if you not
bothered ourselves about what is the kind of engine that is there liquid solid or hybrid
now let us look at in the next class we will start looking at the different kinds of engines
that is solid liquid and hybrid motors okay thank you.

2 thoughts on “Mod-01 Lec-23 Rocket Nozzles – Thrust Vectoring”

  1. ashish anand Jha says:

    I am an electronics graduate but learning rocketry from these lectures to pacify my thirst of knowledge

  2. Laxman Chhetri says:

    worst lecture ever

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