# Using side thruster on missiles to create an angular acceleration

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Assume a rocket is standing on the ground with mass = 1000 kg, and near the top of rocket we apply a force of 100 N for say 1 sec,now assume that this is sufficient to tip rocket over/start tipping over. If the rocket is mid air and accelerating at 30 m/s^2, then likewise how much force would be required to tilt it pi/180 rad or change its trajectory by some degree, please neglect the change of mass due to consumption of fuel because the value calculated with this assumption is going to be a a value higher than required, so that is not an issue. Will the force required mid air be the same as the force required on the ground with no acceleration ?

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28 minutes ago, sahil saxena said:

Will the force required mid air be the same as the force required on the ground with no acceleration ?

The issue will be not acceleration, but forward airspeed. The body of the rocket acts like a gigantic lifting body. Trying to push it out of alignment is going to produce differential lift which forces it back into the lowest air-resistance orientation relative to the airstream.

Just after liftoff, the forward airspeed is negligible so the required reaction force from the side thruster is essentially the same as it would be at a standstill. In fact, it may be lower, because it does not need to produce tipover (tipover requires the center of mass to be moved beyond the base). But as it accelerates, the forward airspeed increases, and the required force will increase.

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Not enough information.

Assuming equal mass and uniform mass distribution, a point mass has zero rotational inertia and an infinitely long cylinder has infinite rotational inertia.

Also the aerodynamics of a rocket can be stabilising and resist a change in orientation.

Also the rotational inertia and aerodynamics both vary in flight with atmospheric density, fuel depletion and velocity.

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29 minutes ago, sahil saxena said:

Assume a rocket is standing on the ground with mass = 1000 kg, and near the top of rocket we apply a force of 100 N for say 1 sec,now assume that this is sufficient to tip rocket over/start tipping over. If the rocket is mid air and accelerating at 30 m/s^2, then likewise how much force would be required to tilt it pi/180 rad or change its trajectory by some degree, please neglect the change of mass due to consumption of fuel because the value calculated with this assumption is going to be a a value higher than required, so that is not an issue. Will the force required mid air be the same as the force required on the ground with no acceleration ?

Seems like it would depend a lot on the static stability of the grounded rocket (height of CM and width of base) & likewise the aerodynamic stability (CM and center of lift/drag).

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@sahil saxena, an obligatory "Hello and welcome to the forum!"

In this case the old adage about physicists preferring to work with spherical horses in a frictionless vacuum is very applicable. Airspeed will be a significant influence beyond a certain point, but initially the force will be largely the same, which is among the many reasons why you'd want to make the turn early.

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37 minutes ago, DDE said:

@sahil saxena, an obligatory "Hello and welcome to the forum!"

In this case the old adage about physicists preferring to work with spherical horses in a frictionless vacuum is very applicable. Airspeed will be a significant influence beyond a certain point, but initially the force will be largely the same, which is among the many reasons why you'd want to make the turn early.

Thankyou DDE,

May i ask then how do anti ballistic missiles alter their trajectory (in the terminal phase) as and when required, and i believe they can travel at speeds upto 3km/sec.

I was actually thinking of cold gas thrusters on the missiles to create a torque and have a better target lock on the missile target, but now obviously cold gas thrusters are not powerful enough so I was thinking of using any reaction control system. Can you guide me to some resources maybe, that could help me get a better understanding of these concepts, and how would i achieve "maneuvering  a missile at 3km/sec in the terminal phase".

11 hours ago, sevenperforce said:

The issue will be not acceleration, but forward airspeed. The body of the rocket acts like a gigantic lifting body. Trying to push it out of alignment is going to produce differential lift which forces it back into the lowest air-resistance orientation relative to the airstream.

Just after liftoff, the forward airspeed is negligible so the required reaction force from the side thruster is essentially the same as it would be at a standstill. In fact, it may be lower, because it does not need to produce tipover (tipover requires the center of mass to be moved beyond the base). But as it accelerates, the forward airspeed increases, and the required force will increase.

Thankyou sevenperforce,

an anti-ballistic missile for the terminal phase neutralization travels at speeds upto 3 km/sec, and the whole phase lasts only for a  max of 120sec. Pardon me for my silly questions as i do not have a proper hold on the basic concepts of anything, but while it is accelerating, it is already moving "not in the lowest air-resistance orientation" because of the angle of attack, so i would think that the change of orientation, though, would face resistance but because it is accelerating it should hold the changed position.

I am trying to somehow on a small scale replicate the concept of anti-ballistic missile (in the terminal phase) for my major project, i would really appreciate being pointed to some related resources, and maybe something to understand how the existing technology maneuvers mid air to be able to have a direct hit with the target missile.

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Related information:

Missiles are starting to appear on the market with more and more novel methods of increasing lethality.

For example, the MBDA "Aster" series of SAMs, includes models with solid fuelled "divert" thrusters (referred to as "PIF-PAF"), which fire perpendicular to the missile body, acting at the centre of gravity, that can fire in the terminal phase to decrease miss distance and increase pK.

Further details are hard to come by, one presumes much is classified.

20 minutes ago, sahil saxena said:

I am trying to somehow on a small scale replicate the concept of anti-ballistic missile (in the terminal phase) for my major project, i would really appreciate being pointed to some related resources, and maybe something to understand how the existing technology maneuvers mid air to be able to have a direct hit with the target missile.

Luckily much has been declassified about cold war projects (but not everything, mind) so there is a wealth of information here.

And as it happens, your scenario describes a niche for everyonesfavorite terminal defence king: the Sprint missile.

The sprint missile is essentially the reverse of a nuclear reenty vehicle and thus it manoeuvres essentially in the same envelope.

Accelerating at 100G at liftoff, it reaches Mach 10 in about 2-3s.

First stage manoeuvring is done by vectoring the main thrust using fluid injection (rather than using physical vanes, cold gas is injected into the exhaust which causes dynamic effects that deflect the main plume).

Second stage manoeuvring is done with small aerodynamic fins, much with any other missile. At hypersonic speeds, and in the lower atmosphere, aerodynamic lift is extreme. Sprint could manoeuvre at 50-100G laterally and had to be "de-accuratised" because it kept physically hitting the target warheads (it was intended to detonate a neutron bomb - oh yeah, the Sprint missile is the reason neutron bombs were invented, none of that "preserve the infrastructure" junk - in close proximity, nuclear warheads are surprisingly hard to guarantee a kill).

This will be a good read for you, I think:

Sprint missile:

Note how it looks pretty similar to a nuclear RV

Note how small the 2nd stage control fins are

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15 minutes ago, p1t1o said:

Related information:

Missiles are starting to appear on the market with more and more novel methods of increasing lethality.

For example, the MBDA "Aster" series of SAMs, includes models with solid fuelled "divert" thrusters (referred to as "PIF-PAF"), which fire perpendicular to the missile body, acting at the centre of gravity, that can fire in the terminal phase to decrease miss distance and increase pK.

Further details are hard to come by, one presumes much is classified.

Luckily much has been declassified about cold war projects (but not everything, mind) so there is a wealth of information here.

And as it happens, your scenario describes a niche for everyonesfavorite terminal defence king: the Sprint missile.

The sprint missile is essentially the reverse of a nuclear reenty vehicle and thus it manoeuvres essentially in the same envelope.

Accelerating at 100G at liftoff, it reaches Mach 10 in about 2-3s.

First stage manoeuvring is done by vectoring the main thrust using fluid injection (rather than using physical vanes, cold gas is injected into the exhaust which causes dynamic effects that deflect the main plume).

Second stage manoeuvring is done with small aerodynamic fins, much with any other missile. At hypersonic speeds, and in the lower atmosphere, aerodynamic lift is extreme. Sprint could manoeuvre at 50-100G laterally and had to be "de-accuratised" because it kept physically hitting the target warheads (it was intended to detonate a neutron bomb - oh yeah, the Sprint missile is the reason neutron bombs were invented, none of that "preserve the infrastructure" junk - in close proximity, nuclear warheads are surprisingly hard to guarantee a kill).

This will be a good read for you, I think:

Sprint missile:

Note how it looks pretty similar to a nuclear RV

Note how small the 2nd stage control fins are

Thankyou p1t10,

This was very insightful. I understand the dynamics, kinematics and whatnot is extremely complicated up there specially at such speeds. But can this approach (given below) be even remotely be used to know if the concept that i wish to present will work:

Assume the rocket standing stationary on the ground, then applying a force towards the top of the missile, create a torque, and given that we know the moment of inetia of the missile, we can calculate the angular acceleration by dividing the torque by moment of inertia. If i want an angular displacement of say pi/180 rad either side as and when required so that I can stay fixed on the target, we can then calculate the angular velocity and then by using the kinematics equations of motion in an angular aspect, as in

s = u*t + 0.5*a*t^2  can be in angular terms written as theta = w*t + 0.5*alpha*t^2

we can find the time required, where i assume this time to be the time required to bring about an pi/180 radian change in orientation.

So i wanted to know if we just consider stopping the missile mid air, know how much it weighs, recalculate the moment of inertia maybe, and then find the force required to possibly tilt it.

I completely understand how silly and foolish this sounds, but I have no one else to guide me for this project. And f there is a chance that this methodology might work with many more assumptions, this would be quite convenient for me to work it out.

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57 minutes ago, sahil saxena said:

Thankyou DDE,

May i ask then how do anti ballistic missiles alter their trajectory (in the terminal phase) as and when required, and i believe they can travel at speeds upto 3km/sec.

I was actually thinking of cold gas thrusters on the missiles to create a torque and have a better target lock on the missile target, but now obviously cold gas thrusters are not powerful enough so I was thinking of using any reaction control system. Can you guide me to some resources maybe, that could help me get a better understanding of these concepts, and how would i achieve "maneuvering  a missile at 3km/sec in the terminal phase".

They use a combination of thrusters to induce torque or, as they are in the upper atmosphere, they use a 'divert' thruster to move laterally without turning.

This is a terminal section of a US antimissile hovering on divert thrusters.

You can see the smaller orientation engines fire, but only occasionally.

This is achieved using a liquid monopropellant or even bipropellant system (as seen here) or large quantities (hundreds) of small solid-propellant engines. This can be miniaturized and adequately used in the lower atmosphere as well - the Vympel 9M96 has two rows of such engines down the midsection:

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28 minutes ago, DDE said:

They use a combination of thrusters to induce torque or, as they are in the upper atmosphere, they use a 'divert' thruster to move laterally without turning.

This is a terminal section of a US antimissile hovering on divert thrusters.

You can see the smaller orientation engines fire, but only occasionally.

This is achieved using a liquid monopropellant or even bipropellant system (as seen here) or large quantities (hundreds) of small solid-propellant engines. This can be miniaturized and adequately used in the lower atmosphere as well - the Vympel 9M96 has two rows of such engines down the midsection:

Thankyou DDE,

yes i have seen this fantastic video of the multiple kill vehicle. This gives me some confident that i might be able to imply that the concept is possible (obviously it exists, but using maths, physics at my disposal). This is the first time I have turned to forums for help and the experience was overwhelming. Can i also request for some papers or sites mentioning Vympel 9M96 and its miniaturized engines down the mid-section, though I fail to see them in the image provided. That might help me get started with some calculations, instead of assuming everything on my own. Again thanks in advance.

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To all,

would it be possible to give me an idea into how to start calculating the forces acting on a body like this multiple kill vehicle, as in, it maintains a almost constant height, how much force, is required to stay in place, and how that is not being disturbed when side forces are acting on the body and the body has lateral displacement, An idea into maybe how to begin calculating such things maybe on something simpler, then i can climb the way up.

Thanks again.

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3 hours ago, sahil saxena said:

So i wanted to know if we just consider stopping the missile mid air, know how much it weighs, recalculate the moment of inertia maybe, and then find the force required to possibly tilt it.

I completely understand how silly and foolish this sounds, but I have no one else to guide me for this project.

The difficulty is that while tilting a rocket seems conceptually similar to changing its angle in flight, they are fundamentally different problems. A stationary, standing cylinder is "tilted" by a perpendicular force applied above the center of mass. The force must be large enough to rotate the cylinder and lift one edge of the rocket off the ground against the force of gravity, using the opposite edge as a fulcrum. Applying perpendicular force at or below the center of mass, in contrast, will only cause the rocket to slide laterally. The tipping point at which gravity takes over and brings the cylinder smashing to the ground is determined by the point at which the rotation of the cylinder moves the center of mass past the fulcrum point. Gravity is the primary actor being considered here.

Changing the orientation of a cylinder in flight also involves a force applied perpendicularly, but everything else is different. There is no interaction between the cylinder and a stationary surface, so you are no longer pushing against gravity -- in fact, gravity is completely irrelevant to orientation (apart from incidental things like the force of gravity determining the density and pressure of air, etc.). The fulcrum is now the center of mass of the cylinder. Normal force applied at the fulcrum produces lateral displacement (like the diverting thrusters in the examples shown); normal force anywhere else rotates the entire cylinder around the center of mass. The concept of "tilting" the rocket is nonexistent.

5 hours ago, sahil saxena said:

while it is accelerating, it is already moving "not in the lowest air-resistance orientation" because of the angle of attack

During boost phase, a rocket is intended to fly at a zero angle of attack whenever possible. A nonzero angle of attack results in lift-induced drag which slows the rocket, which you don't want. The only time the angle of attack deviates from zero is when the rocket is trying to change direction, either using vectored thrust, guide fins, or RCS. To change direction, you want to use the air stream to "turn" the rocket in flight; you do this by using control surfaces (or vectored thrust or RCS) to produce a nonzero angle of attack, which produces a lift force which turns the rocket.

A rocket needs to keep its center of mass as far forward as possible and its center of pressure as far back as possible in order to remain aerodynamically stable (think of a shuttlecock as the extreme example of mass-forward and pressure-rearward). So it usually make sense to put control surfaces at the back.

What type of project are you doing?

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4 hours ago, sahil saxena said:

Can i also request for some papers or sites mentioning Vympel 9M96 and its miniaturized engines down the mid-section, though I fail to see them in the image provided.

The engines are seen as thin white circles on the blue band.

As to papers, here is

Quote

Mathematical description of motion of a kinetic lateral control interceptor and assessment of its maneuvering potential

The purpose of the study was to consider the motion features of kinetic lateral control interceptors and obtain dynamic equations which describe their motion in a flat rectangular coordinate system by resolving the acting forces with respect to the interceptor velocity vector. We took into consideration the nonuniformity of the interceptor motion and obtained formulas for evaluating indicators of its maneuvering potential. As an example, we assessed the capabilities of the THAAD interceptor missile (USA) in regards to the choice of missed interception.

Alas, it's in Russian.

Edited by DDE
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1 hour ago, DDE said:

The engines are seen as thin white circles on the blue band.

As to papers, here is

Alas, it's in Russian.

Thankyou DDE,

ok now i see them, lets hope the paper gets converted accurately. Thankyou

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3 hours ago, sevenperforce said:

The difficulty is that while tilting a rocket seems conceptually similar to changing its angle in flight, they are fundamentally different problems. A stationary, standing cylinder is "tilted" by a perpendicular force applied above the center of mass. The force must be large enough to rotate the cylinder and lift one edge of the rocket off the ground against the force of gravity, using the opposite edge as a fulcrum. Applying perpendicular force at or below the center of mass, in contrast, will only cause the rocket to slide laterally. The tipping point at which gravity takes over and brings the cylinder smashing to the ground is determined by the point at which the rotation of the cylinder moves the center of mass past the fulcrum point. Gravity is the primary actor being considered here.

Changing the orientation of a cylinder in flight also involves a force applied perpendicularly, but everything else is different. There is no interaction between the cylinder and a stationary surface, so you are no longer pushing against gravity -- in fact, gravity is completely irrelevant to orientation (apart from incidental things like the force of gravity determining the density and pressure of air, etc.). The fulcrum is now the center of mass of the cylinder. Normal force applied at the fulcrum produces lateral displacement (like the diverting thrusters in the examples shown); normal force anywhere else rotates the entire cylinder around the center of mass. The concept of "tilting" the rocket is nonexistent.

During boost phase, a rocket is intended to fly at a zero angle of attack whenever possible. A nonzero angle of attack results in lift-induced drag which slows the rocket, which you don't want. The only time the angle of attack deviates from zero is when the rocket is trying to change direction, either using vectored thrust, guide fins, or RCS. To change direction, you want to use the air stream to "turn" the rocket in flight; you do this by using control surfaces (or vectored thrust or RCS) to produce a nonzero angle of attack, which produces a lift force which turns the rocket.

A rocket needs to keep its center of mass as far forward as possible and its center of pressure as far back as possible in order to remain aerodynamically stable (think of a shuttlecock as the extreme example of mass-forward and pressure-rearward). So it usually make sense to put control surfaces at the back.

What type of project are you doing?

Thankyou sevenperforce,

I totally see how you got the rank physics god, the explanation was damn intuitive, i now have a better perspective.

I think I have heard of something that can perhaps move the Cg, Cp. So is it possible to make a rotation in the cylinder, using rcs or something and then also accordingly change the Cg, Cp so that, the rotation is held on to by making the orientation stable, because we repositioned the Cg, Cp to its stable position ?

Also the cylinder mid air, resists change right, is it because say we are using rcs, because we gave a thrust and then turned the rcs off, maybe if we continuously keep the rcs on, will it even then not hold the changed orientation (obviously thats a waste of fuel and a ineffective way, but is it possible then?)

I am final year undergraduate student of mechanical engineering, I am trying to present a concept of using cold gas thruster (I understand that they have very less specific impulse and a lot of fuel required to get anything done) on a anti-ballistic missile (use case under altitudes of 100 km, by which i mean i will have to counter the atmosphere), as basically a modification to the old proximity fuse triggered anti-ballistic missiles for a better chance of hitting the target missile.

But then spaceX on its reusable first stage booster uses cold gas thruster to re-orient the booster for its descent, how come they got it to work ?

the U.S multiple/single kill vehicle has very similar motion with the divert thrusters and all, can I also ask you, if you can guide me towards how to start calculating these forces. For eg. the kill vehicle is almost always maintaining constant height despite the directional thrusts acting on it, and how much force is required to displace it laterally from one point to other.

Thanks in advance, I just realized I have asked way too many questions under one post, I would be fine with a delayed answer, but if you could answer all these questions i would really be grateful, as the internet has not been so kind to me.

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26 minutes ago, sahil saxena said:

But then spaceX on its reusable first stage booster uses cold gas thruster to re-orient the booster for its descent, how come they got it to work ?

SpaceX uses nitrogen to avoid having toxic chemicals on a reusable vehicle.

They sacrifice efficiency (payload capacity), but can inspect the rocket without a hazmat suit, which is important to the effectiveness of the entire operation.

Anyway... reusability doesn’t seem pertinent to an  anti-ICBM, so why use cold gas?

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13 minutes ago, sahil saxena said:

I think I have heard of something that can perhaps move the Cg, Cp. So is it possible to make a rotation in the cylinder, using rcs or something and then also accordingly change the Cg, Cp so that, the rotation is held on to by making the orientation stable, because we repositioned the Cg, Cp to its stable position ?

If you induce rotation via RCS or control surfaces, the angular momentum will resist changes in pitch. This is how projectiles and certain rockets are spin-stabilized. If the center of pressure or center of mass changes during rotation, it will cause precession of the axis of rotation, like how a gyroscope acts weird. The axis of rotation will still pass through the center of mass.

If you have control surfaces, of course, the atmosphere will damp the rotation rather rapidly.

13 minutes ago, sahil saxena said:

Also the cylinder mid air, resists change right, is it because say we are using rcs, because we gave a thrust and then turned the rcs off, maybe if we continuously keep the rcs on, will it even then not hold the changed orientation (obviously thats a waste of fuel and a ineffective way, but is it possible then?)

Think of it as a feedback chain. You fire the RCS, which produces a perpendicular force displaced from the center of mass, which induces rotation around the center of mass. The rotation changes the angle of attack. The new angle of attack produces induced body lift. Induced lift at the center of mass changes the overall direction while induced lift distant from the center of mass attempts to push the cylinder back to a zero AoA. That push is what pushes back against your RCS. If you keep firing your RCS, the AoA will increase until the resistive force of that induced lift is equal to the thrust of your RCS and it will then hold that AoA, causing a continuous turn.

Think of it like turning the helm of a sailing ship. The harder you turn the wheel, the more the rudder will angle and the more flow of water will push back against the rudder. The ship will turn as long as you keep applying force (the more force, the more the rudder is angled and the faster it turns) then start going straight again once you release the wheel.

13 minutes ago, sahil saxena said:

I am final year undergraduate student of mechanical engineering, I am trying to present a concept of using cold gas thruster (I understand that they have very less specific impulse and a lot of fuel required to get anything done) on a anti-ballistic missile (use case under altitudes of 100 km, by which i mean i will have to counter the atmosphere), as basically a modification to the old proximity fuse triggered anti-ballistic missiles for a better chance of hitting the target missile.

Using RCS for aiming, then?

In atmosphere you can use RCS for translational/diverting thrust but it is more mass-efficient to use guide fins to steer.

13 minutes ago, sahil saxena said:

But then spaceX on its reusable first stage booster uses cold gas thruster to re-orient the booster for its descent, how come they got it to work ?

It's happening in zero gee free fall with no atmosphere. No airstream means no change in the angle of attack because there's nothing to attack. Puffing the RCS begins rotation as before, but without atmosphere the rotation continues at a constant rate until a puff of opposite RCS stops it.

41 minutes ago, sahil saxena said:

the U.S multiple/single kill vehicle has very similar motion with the divert thrusters and all, can I also ask you, if you can guide me towards how to start calculating these forces. For eg. the kill vehicle is almost always maintaining constant height despite the directional thrusts acting on it, and how much force is required to displace it laterally from one point to other.

The kill vehicle is hovering on a thruster so it is not moving. Each of the RCS thrusters used to change position are pointed through the center of mass, so they nudge it in any direction desired without inducing any rotation. Because it's hovering, there's virtually no friction, and so even a very small puff of RCS will produce sufficient force to move it; the question is how quickly you want it to move around. You need more powerful RCS to make quicker movements back and forth.

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47 minutes ago, sahil saxena said:

But then spaceX on its reusable first stage booster uses cold gas thruster to re-orient the booster for its descent, how come they got it to work ?

Adding to the above, the Falcon9 uses grid fins for control during descent. The terminal guidance is performed by vectoring the main engine, this is why F9s with some sort of control failure crash into the ocean, not the landing zone/barge.

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1 hour ago, Nightside said:

Adding to the above, the Falcon9 uses grid fins for control during descent. The terminal guidance is performed by vectoring the main engine, this is why F9s with some sort of control failure crash into the ocean, not the landing zone/barge.

Grid fins are draggier than traditional control surfaces so they are no good for boost but they are suitable for something falling if you don't mind it slowing down. They have high control authority without stall risk and with minimal hydraulic requirements.

And yes, I know the N-1 used them. Still didn't make it a good idea.

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13 hours ago, Nightside said:

SpaceX uses nitrogen to avoid having toxic chemicals on a reusable vehicle.

They sacrifice efficiency (payload capacity), but can inspect the rocket without a hazmat suit, which is important to the effectiveness of the entire operation.

Anyway... reusability doesn’t seem pertinent to an  anti-ICBM, so why use cold gas?

Hi nightside

to be able to maneuver the interceptor to the missile target for a successful hit-to-kill intercept. I also realizes why its not feasible due to its low specific impulse.

13 hours ago, Nightside said:

Adding to the above, the Falcon9 uses grid fins for control during descent. The terminal guidance is performed by vectoring the main engine, this is why F9s with some sort of control failure crash into the ocean, not the landing zone/barge.

yes but the grid fins were not used while in space, or at low altitudes and low speeds because they dont have that much control authority, yes thrust vectoring was the primary source, but also cold gas thrusters during landing for small corrections, for which if thrust vectoring was used, it could be an overkill.

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14 hours ago, sevenperforce said:

Using RCS for aiming, then?

Yes, just for aiming, its also clear to me that what i wanted the rcs to do (aim) is exactly possible, but in space, and not really while under the influence of atmosphere.

14 hours ago, sevenperforce said:

The kill vehicle is hovering on a thruster so it is not moving. Each of the RCS thrusters used to change position are pointed through the center of mass, so they nudge it in any direction desired without inducing any rotation. Because it's hovering, there's virtually no friction, and so even a very small puff of RCS will produce sufficient force to move it; the question is how quickly you want it to move around. You need more powerful RCS to make quicker movements back and forth.

I wonder how they might implement (if not already implemented) it, because it will also face the same problems that you have mentioned about right ? unless they plan on using this for the mid-course phase in space, which would then make sense.

For answering the rest of my questions, thankyou very much. I guess i have cleared most of my doubts.

Edited by sahil saxena
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to ALL,

this was my first time, reaching out for help over the internet, on forums, and I must say this very specific forum was the only fruitful one, or maybe i did not make my question readable on other platforms.

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4 hours ago, sahil saxena said:

Yes, just for aiming, its also clear to me that what i wanted the rcs to do (aim) is exactly possible, but in space, and not really while under the influence of atmosphere.

To be clear, you can certainly do it -- it's just that control surfaces are less massive and more effective.

If your projectile is hypersonic then traditional control surfaces don't work quite as well. You can use grid fins but they are draggy. The preferred guidance method is always going to be vectoring the main engine, either through gimbal or some other design. The Sprint ABM, mentioned upthread by fellow physics god @p1t1o, used inert fluid injection inside the main engine nozzle to vector thrust at 100-gee accelerations. It was very successful.

The one place where an ABM (anti-ballistic missile) might be able to utilize cold-gas thrusters would be in unpowered terminal guidance. If your missile is designed to boost to hypersonic velocities and then coast to impact/detonation, then it's going to need to maneuver. Traditional control surfaces are draggy at hypersonic velocities and require extremely heavy/strong actuators; grid fins are even draggier. So using cold-gas thrusters would actually be a good idea.

This is what SpaceX does with its fairing entry, actually. Its fairings hit the atmosphere at over 2.3 km/s or about Mach 7. They have no control surfaces to maneuver, so they use nitrogen cold-gas to control the orientation of the entire fairing as a lifting body.

If you want to use this for an ABM with a terminal coast phase, you may want to look at base bleed projectiles. A major component of drag on artillery shells is the low-pressure region which forms behind the base of the shell. Base-bleed projectiles add a small, weak rocket/powder cartridge just behind the base which produces a flow of warm gas. By itself it wouldn't produce much thrust, but increasing the pressure in that region reduces base drag and extends range.

An ABM with a terminal coast phase could benefit by using cold-gas thrusters embedded in the tail of the vehicle (to reduce tail drag); it could also use these thrusters differentially to produce torque and guide the warhead to its target. Might be something worth looking into. If you do, you'll want to start by looking at the lift-to-drag ratios of cylinders and cones at hypersonic velocities to get an idea of how much force it would take. Fortunately NASA has studied this in detail.

4 hours ago, sahil saxena said:

I wonder how they might implement (if not already implemented) it, because it will also face the same problems that you have mentioned about right ? unless they plan on using this for the mid-course phase in space, which would then make sense.

They could be used both inside and outside the atmosphere because they fire through the center of mass. Accordingly, they act like the divert thrusters mentioned above. Divert thrusters are actually less efficient than gimbal steering, but they work the same way at any velocity so it's not an issue.

4 hours ago, sahil saxena said:

to ALL,

this was my first time, reaching out for help over the internet, on forums, and I must say this very specific forum was the only fruitful one, or maybe i did not make my question readable on other platforms.

I have noticed that folks here are very helpful. I think KSP is just complicated enough that it prompts people to get better at explaining rocket science, which makes them good at, well, explaining rocket science.

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1 hour ago, sevenperforce said:

The Sprint ABM, mentioned upthread by fellow physics god @p1t1o, used inert fluid injection inside the main engine nozzle to vector thrust at 100-gee accelerations. It was very successful.

:blushes:   aw thanks man, I dont deserve that

For reference, the 2nd stage did use aerodynamic surfaces and it did not have a problem with manoeuvring capacity - however it was exceedingly energy-rich and its mission did not require it to travel far, nor expected to manoeuvre excessively (this was well before manoeuvrable RVs) so drag wasnt really an issue. Thrust vectoring was preferred in the 1st stage - amongst other reasons - due to it needing to make extreme turns immediately upon ejection from the launch tube when it is at a comparatively low speed. Thrust vectoring may have been eschewed in the 2nd stage to save space and weight? Or maybe the conventional surfaces just worked well enough. Unsure.

Fine tuning drag and Isp really was not on the agenda in building this missile! Maximising sheer power output was the name of the game, which they succeeded at to a degree perhaps never seen since.

Which is one of the reasons I love it so much  (and it comes up surprisingly often!)

PS: somebody please fricken fix the spelling of "manoueoueouvre", its impossible!

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15 minutes ago, p1t1o said:

For reference, the 2nd stage did use aerodynamic surfaces and it did not have a problem with manoeuvring capacity - however it was exceedingly energy-rich and its mission did not require it to travel far, nor expected to manoeuvre excessively (this was well before manoeuvrable RVs) so drag wasnt really an issue.

Here's a (sadly low-quality) schematic showing the guide vanes.

If @sahil saxena's application intercepts at higher velocities or under lower air pressure, or if it has a long coast phase, rcs might be better than vanes.

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