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Using side thruster on missiles to create an angular acceleration


sahil saxena
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26 minutes ago, sevenperforce said:

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

wfMEvbDQqbi_98C-CkY_dQ-zMvUCSKaUrjwfMCSB

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.

Yes, in a more rarified envelope, for sure. Sprint was optimised for low altitude.

There is a slightly higher quality version of the image (amongst much more) in the document I linked in my first post.

Edited by p1t1o
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18 hours ago, sevenperforce said:

The one place where an ABM (anti-ballistic missile) might be able to utilize cold-gas thrusters would be in unpowered terminal guidance.

sounds interesting, will try to look into it.:)

 

18 hours ago, sevenperforce said:

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.

Thankyou (excited to read one more paper, of which most i wont understand.)!

 

18 hours ago, sevenperforce said:

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.

So is it pure KSP, or the folks here have some previous experience in this area. To be honest, to know that my doubts have an answer is relieving, but I still am having a tough time, making a progress. Where do I start studying these topics, can i be pointed to resources that might help me ?

Thanks.

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I love visuals so here is the Sprint ABM in all its glory:

 

Specifications:

Height: 27 ft

Max diameter: 4.5 ft

Weight: 7,600 lb First stage: 650,000 lb thrust for 1.8 sec.

Second stage: 150,000 lb thrust for 2 sec.

Acceleration: 1st stage - 130 G, 2nd stage - 90 G,

Maneuvers - 60 G Max velocity: 10,200 ft/s

Warhead: W66 thermonuclear warhead (a low kiloton range neutron bomb)

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19 hours 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.

Meanwhile in the Soviet Union:

2018_ukraine_strategic_rocket_forces_20.

The did go with airfoils on the smaller S-300V series:

9M82+9M83-Missile-Layout-AC.png

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@DDE Oh yes, Im no stranger to the giant/gladiator, the coolest of the S300 series :D

 

I didnt recognise the missile at the top - google says thats the famous "Gazelle" - that might be the first time I have seen an actual picture of one. If so, how does it guide in the coast phase? One presumes it has a similar profile to the Sprint - high impulse acceleration with coast. Is it an endo-atmospheric ABM?

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49 minutes ago, p1t1o said:

@DDE Oh yes, Im no stranger to the giant/gladiator, the coolest of the S300 series :D

I didnt recognise the missile at the top - google says thats the famous "Gazelle" - that might be the first time I have seen an actual picture of one. If so, how does it guide in the coast phase? One presumes it has a similar profile to the Sprint - high impulse acceleration with coast. Is it an endo-atmospheric ABM?

Yes, it's Sprint's long-lost Muscovite cousin. Not much coast phase to be had there.

And the Gladiator/Giant is alive and well. It appears that S-500 is just an upgraded S-400 on a new chassis and with S-300V's ABMs as well as its own anti-air missiles.

cMDDZ.jpg

The chassis manufacturer proved to be insufficiently tight-lipped.

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Now on the other hand an standard missile managed to intercept and hit an decaying satellite, its an reason we are moving into hypersonic weapons and fights. Now the obvious issue is how do you target trough the plasma? 
You are aiming an moving target who know you launched. modern IR satellites plots 105 mm guns and fighter jets on afterburner. 
And who idiot came up with the Standard Missile name? You have the Gun system gun who was just an improved 155 mm towed gun. 
If anything just use codes like M32, and G145 and let the use make them legends. 
However you had M72 who was an rather weak single use anti tank rpg, and M71 who was white camouflage pants in Norway, getting lots of M71 on the range would be amusing. 

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13 hours ago, magnemoe said:

Now on the other hand an standard missile managed to intercept and hit an decaying satellite

Hardly a surprise. This is its upper stage.

i?id=a7f577dedb4ee4efedeab3fddafab6d5-l&
SM-3 is the missile defence part of Standard while SM-6 is the anti-air missile and is being pushed as an impromptu anti-ship weapon as well.

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On 4/3/2020 at 5:13 AM, sahil saxena said:

sounds interesting, will try to look into it.:)

Thankyou (excited to read one more paper, of which most i wont understand.)!

So is it pure KSP, or the folks here have some previous experience in this area. To be honest, to know that my doubts have an answer is relieving, but I still am having a tough time, making a progress. Where do I start studying these topics, can i be pointed to resources that might help me ?

Thanks.

One thing to look into would be whether you could use cold-gas thrusters to produce flow separation around the base of the projectile during terminal flight, altering drag effects and thus producing differential torque. Might be possible with far less thrust than traditional RCS.

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13 hours ago, sevenperforce said:

One thing to look into would be whether you could use cold-gas thrusters to produce flow separation around the base of the projectile during terminal flight, altering drag effects and thus producing differential torque. Might be possible with far less thrust than traditional RCS.

Thank you sevenperforce,

recently understood flow separation , and yes this seems like a possibility.

On 4/1/2020 at 3:27 AM, sevenperforce said:

The body of the rocket acts like a gigantic lifting body.

sevenperforce,

Referring to something you said earlier, I have been trying to understand, how much lift will a missile of certain dimension produce, is there a formula or some literature to figure that out, that way I would know how much more force is required to resist the resisting force. I have been trying to understand airfoil, but I am not able to relate the two.

Thanks in advance.

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6 hours ago, sahil saxena said:

Referring to something you said earlier, I have been trying to understand, how much lift will a missile of certain dimension produce, is there a formula or some literature to figure that out, that way I would know how much more force is required to resist the resisting force. I have been trying to understand airfoil, but I am not able to relate the two.

Thanks in advance.

It's going to depend weakly on overall size and much more directly on fineness ratio.

What you're looking for is the L/D, or Lift-to-Drag ratio. L/D is usually measured in a wind tunnel and should be known for most shapes, including cones, cylinders, half-spheres, and so forth. L/D will be a function of airspeed and angle of attack (AoA).

At an AoA of 0 degrees, the L/D of a cylindrical symmetric body will be zero. As it tilts relative to the airstream, its L/D will change. If you know the relationship between L/D and AoA, you can rearrange the equation to determine how much deflection you need to produce a desired amount of lifting force and from there determine how much thrust you need to use to deflect.

These values depend on too many variables to be calculated de novo and so you need analytic solutions based on real-world data.

A starting hypothesis for your purposes might be "aerodynamic control surfaces do not work well at hypersonic speeds during coast, but cold-gas-thruster-induced flow separation can be used to produce more lift-based diversion than divert thrusters of commensurate mass during the following flight regimes."

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4 hours ago, sevenperforce said:

What you're looking for is the L/D, or Lift-to-Drag ratio. L/D is usually measured in a wind tunnel and should be known for most shapes, including cones, cylinders, half-spheres, and so forth. L/D will be a function of airspeed and angle of attack (AoA).

Thank you sevenperforce,

I will start understanding it.

4 hours ago, sevenperforce said:

aerodynamic control surfaces do not work well at hypersonic speeds during coast

for an ABM, after the first stage has given the required velocity through accelerating it, it then still produces a thrust, after the separation of first stage, I can only assume that that is for some form of maneuverability through thrust vectoring and essentially for MAINTAINING the achieved velocity from the first stage. So that very small thrust to keep it going, can that phase be called COASTING, because I don't know if an ABM after the first stage should coast without any propulsion at all.

Thanks in advance.

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Just now, sahil saxena said:

for an ABM, after the first stage has given the required velocity through accelerating it, it then still produces a thrust, after the separation of first stage, I can only assume that that is for some form of maneuverability through thrust vectoring and essentially for MAINTAINING the achieved velocity from the first stage. So that very small thrust to keep it going, can that phase be called COASTING, because I don't know if an ABM after the first stage should coast without any propulsion at all.

Depends on the application. The Sprint Missile had virtually no time between second-stage burnout and warhead trigger. Other interceptors did have coast periods. Residual-thrust coast is not a likely scenario, however.

The second stage of Sprint was a full stage.

I would look at modern interceptors and investigate which ones tend to have coast periods in the atmosphere.

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10 hours ago, sevenperforce said:

The Sprint Missile had virtually no time between second-stage burnout and warhead trigger

i see.

 

10 hours ago, sevenperforce said:

I would look at modern interceptors and investigate which ones tend to have coast periods in the atmosphere.

yes thankyou, i would like to know how they deal with the loss of velocity during the period of coast.

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On 4/1/2020 at 7:55 PM, sevenperforce said:

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

Hi,

was going through some material, and it was stated with the help of a graph for one of the rockets, that the CP of the rocket moves ahead of the CG drastically as it breaks the sound barrier and then continues to stay above the CG, and if this might happen for every rocket/missile in general, then the rocket is inherently unstable at higher speeds, so now i need to be more careful in applying the side thrust because it might easily over-shoot and then spin erratically, which means that i don't need to worry about any resistive corrective force which generates from the CP being behind the CG. Am i correct in assuming these points ?

This video along with the animation, talks about a rocket and the graph indicating the CP moving ahead of CG.

Thanks in advance.

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6 hours ago, sahil saxena said:

Hi,

was going through some material, and it was stated with the help of a graph for one of the rockets, that the CP of the rocket moves ahead of the CG drastically as it breaks the sound barrier and then continues to stay above the CG, and if this might happen for every rocket/missile in general, then the rocket is inherently unstable at higher speeds, so now i need to be more careful in applying the side thrust because it might easily over-shoot and then spin erratically, which means that i don't need to worry about any resistive corrective force which generates from the CP being behind the CG. Am i correct in assuming these points ?

This video along with the animation, talks about a rocket and the graph indicating the CP moving ahead of CG.

Thanks in advance.

The center of mass does move forward as propellant burns, which is one reason why you need active stabilization during boost. I am not certain of this, but I suspect divert thrusters are used either during coast, when the center of mass (CoM) is fixed and unchanging, or during a boost period where airspeed is so extreme that any torque is rapidly damped by the airflow. Even if the divert thruster doesn't point exactly through the CoM, it will apply less torque if it is close to the CoM, and at supersonic speeds any change to the AoA will quickly be scrubbed out.

Here's an article that looks at plume interaction with airflow for kinetic kill vehicles -- according to their paper, a proper understanding and utilization of the plume interaction and associated aerodynamic effects can increase the divert capability by 100% without any added propellant.

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16 hours ago, sevenperforce said:

during a boost period where airspeed is so extreme that any torque is rapidly damped by the airflow.

torque applied to the rocket by any means would slightly change the AoA which would generate a lift in the opposite direction to the direction of applied torque, and that is how the extreme airflow rapidly dampens any torque, and lift generated in the opposite direction to the direction of the torque is only possible when the CoP is behind the CG, right ?

But the above mentioned video says otherwise, I would kindly request you to view the video maybe once more, it clearly shows through the graph, that the CoP was always ahead of CG.

16 hours ago, sevenperforce said:

center of mass

i hope center of mass and CG can be used interchangeably.

 

16 hours ago, sevenperforce said:

i shouldn't be discussing these things here, but i do not have a subscription to ARC journals, and hence cannot view the paper, throughout my search I have seen some very interesting abstracts on their journals.

Thanks sevenperforce

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6 hours ago, sahil saxena said:

lift generated in the opposite direction to the direction of the torque is only possible when the CoP is behind the CG, right ?

The lift is generated regardless; whether the lift results in corrective torque is dependent on the center of pressure.

6 hours ago, sahil saxena said:

But the above mentioned video says otherwise, I would kindly request you to view the video maybe once more, it clearly shows through the graph, that the CoP was always ahead of CG.

That graph uses the center of mass for the first stage (S-I) only, because they were analyzing how the center of mass changes in the first stage. The Saturn V first stage would not have been stable on its own if the rest of the rocket was simply an empty fairing. Fortunately it was not, and so the center of mass was always well forward of the center of pressure when the Saturn V was in flight.

6 hours ago, sahil saxena said:

i shouldn't be discussing these things here, but i do not have a subscription to ARC journals, and hence cannot view the paper, throughout my search I have seen some very interesting abstracts on their journals.

You may be able to find it on Google Scholar or elsewhere. 

With the global pandemic, libraries are all fubar, but if you find a couple of papers you really would like to see then PM me and I will see if I can get them through my university on digital interlibrary loan.  

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20 minutes ago, sevenperforce said:

The lift is generated regardless; whether the lift results in corrective torque is dependent on the center of pressure.

yes yes.

 

20 minutes ago, sevenperforce said:

That graph uses the center of mass for the first stage (S-I) only, because they were analyzing how the center of mass changes in the first stage

my bad, i missed that detail and assumed everything accordingly.

 

21 minutes ago, sevenperforce said:

PM me and I will see if I can get them through my university

thankyou for the support, i really appreciate it. I will.

 

 

On 4/1/2020 at 3:27 AM, sevenperforce said:

The body of the rocket acts like a gigantic lifting body.

i am going to try to do something with the shape of the missile taken as an airfoil, the shape of which would resemble the sideview of a sharpened pencil. Even if i can get some approximate calculations done, that would be useful to begin with. Does this sound do-able ? calculate lift etc..

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43 minutes ago, sahil saxena said:

i am going to try to do something with the shape of the missile taken as an airfoil, the shape of which would resemble the sideview of a sharpened pencil. Even if i can get some approximate calculations done, that would be useful to begin with. Does this sound do-able ? calculate lift etc..

Yes, definitely. As I have posted upthread, there is a lot of data NASA has collected on L/D ratios as a function of Mach number and fineness ratio for cone-tipped cylinders. Start there.

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14 minutes ago, sevenperforce said:

Yes, definitely. As I have posted upthread, there is a lot of data NASA has collected on L/D ratios as a function of Mach number and fineness ratio for cone-tipped cylinders. Start there.

oh, i hadn't realized that that was what it meant, amazing, thanks.

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  • 2 weeks later...
On 4/9/2020 at 7:01 PM, sevenperforce said:

As I have posted upthread, there is a lot of data NASA has collected on L/D ratios as a function of Mach number and fineness ratio for cone-tipped cylinders.

Hi sevenperforce,

i had gone through the document, though very informative but it doesnt contain the values of Cm (moment) vs AoA graphs. Which i what i require i believe to understand how much, resistive torque/moment will I have to counter to get a change in orientation.

Are there any softwares with which i could get hold of some counter moment generated values for a given stability margin.

Also in the post-boost phase vehicle (without any boosters), say i use the rcs to change the orientation, I would require to trim to that orientation (equate all forces in that orientation to be able to hold that orientation), now as they say this will change the CoP, probably forward with increase in AoA, so how do i get CoP back for a sufficient stability margin. And why do we care about the CoP when the actual aerodynamic forces act on the aerodynamic center (a.c)

 

Thanks in advance

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