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Rocket acceleration (from tonight's show)

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PDR
482406.  Sat Jan 17, 2009 6:32 pm Reply with quote

Confucius wrote:
Reading your reply we're actually only nit-picking over whether pure control surfaces deflecting air downwards are the same as wings. I would not describe a symmetrical (in relation to it's curvature above and below it's longitudinal axis) surface passing through air/liquid in the same direction as its longitudinal axis as a true wing. It will not provide lift unless angled or given a different profile above the axis to that below. I would not describe the surfaces on a missile as lifting surfaces.


Except that, as you yourself have said, missiles *don't* fly with the velocity vector parallel to the body axis - they fly with the whole vehicle at a positive angle of attack, so all the surfaces develop lift.

PDR

 
Confucius
482513.  Sun Jan 18, 2009 4:50 am Reply with quote

PDR wrote:
Confucius wrote:
Reading your reply we're actually only nit-picking over whether pure control surfaces deflecting air downwards are the same as wings. I would not describe a symmetrical (in relation to it's curvature above and below it's longitudinal axis) surface passing through air/liquid in the same direction as its longitudinal axis as a true wing. It will not provide lift unless angled or given a different profile above the axis to that below. I would not describe the surfaces on a missile as lifting surfaces.


Except that, as you yourself have said, missiles *don't* fly with the velocity vector parallel to the body axis - they fly with the whole vehicle at a positive angle of attack, so all the surfaces develop lift.

PDR


PDR, *that is exactly what I've been saying, yes*. What a waste of time the last few posts have been!

The fact also remains that a cylindrical missile has no surfaces with which to develop lift. Surfaces are not *required* by a missile/rocket to fly, even if they help it to do so. A missile can 'fly' purely by using vectored thrust. Of course I appreciate that this can only be achieved during the burn phase, to suggest otherwise would be stupid.

 
PDR
482520.  Sun Jan 18, 2009 5:03 am Reply with quote

Now I'm really confused! This conversation started from my assertion that A-A, A-G and S-S missiles were air vehicles (with wings) rather than ballistic vehicles:

PDR wrote:
Most air-air, air-ground, surface-air and surface-surface "missiles" are actually "aeroplanes" in that they have lifting surfaces in the right places to carry the weight of the missile, so they're aerodynamic vehicles rather than ballistic ones.


You disputed this assertion by suggesting that these wings were somehow not lifting surfaces:

Confucius wrote:

I'm being picky, but I'm not trying to pick tour post apart, but most missiles have symmetrical flying surfaces. They are for steering and stability, not lift. As I said a missile in level flight will have a (very slight) nose up attitude so some of the thrust is counteracting the weight. An alternative to this would be thrust vectoring combined with control surface deflection. Some of the latest short range AAMs have no control surfaces at all, and rely purely on thrust vectoring - much like many ICBMs.

Cruise missiles are really the only type to rely, or use, lift provided by wings.


So all of this discussion clearly relates to flat-trajectory, non-ballistic vehicles. Now you say:

Quote:

The fact also remains that a cylindrical missile has no surfaces with which to develop lift. Surfaces are not *required* by a missile/rocket to fly, even if they help it to do so. A missile can 'fly' purely by using vectored thrust. Of course I appreciate that this can only be achieved during the burn phase, to suggest otherwise would be stupid.


Which is an interesting (and contentious in some circles) comment on purely cylindrical missiles, but these are not relevent to a discussion on NON BALLISTIC missiles which have PLANAR SURFACES!

PDR

 
Confucius
482530.  Sun Jan 18, 2009 5:29 am Reply with quote

My main dispute is over the nomenclature of small movable surfaces on missile bodies. Perhaps to you as an aero engineer this qualifies them as wings? To me as an aviator they are not wings but control surfaces. The fact that they must deflect to provide lift to me means they are there to provide directional changes, be it up, down, left or right. Since a natural ballistic path is down they wil, in the main, be used to steer the missile away from the ground, if you will. To me a wing should provide lift without the need for it to be deflected.

A patriot SAM, for example, has four surfaces at the rear of the missile. Clearly they are well behind the C of G so do not provide lift as I would describe it, they provide stability and steer the missile so that the thrust vector steers the missile to the target. To me they are not wings, they are stabilisers or control surfaces. Are they wings to an aero engineer? Perhaps so, to an aviator they are not.

Like I said, this is all semantics!

 
Davini994
482584.  Sun Jan 18, 2009 6:43 am Reply with quote

Do ICBMs fly slightly nose up to generate lift from the body? Or is it to direct the thrust?

Confucius wrote:
[For ICBMs]The thrust overcomes the drag, each acting, when not vectoring, through the fore/aft axis of the missile. Gravity acts vertically downwards. The only force counteracting this is the thrust provided by the main motor, therefore in extremis level flight can only be achieved with a slight nose up attitude.


Confucius wrote:
A missile can 'fly' purely by using vectored thrust. Of course I appreciate that this can only be achieved during the burn phase, to suggest otherwise would be stupid.

Flight is a very interesting topic but one I'm still struggling to grasp.

 
thegrandwazoo
482778.  Sun Jan 18, 2009 10:37 am Reply with quote

Davini994 wrote:
Flight is a very interesting topic but one I'm still struggling to grasp.

Perhaps you should ask a penguin!

 
Davini994
482788.  Sun Jan 18, 2009 10:55 am Reply with quote

I did try, but he was too busy showing off.

 
PDR
482850.  Sun Jan 18, 2009 11:51 am Reply with quote

Confucius wrote:
My main dispute is over the nomenclature of small movable surfaces on missile bodies. Perhaps to you as an aero engineer this qualifies them as wings? To me as an aviator they are not wings but control surfaces. The fact that they must deflect to provide lift to me means they are there to provide directional changes, be it up, down, left or right. Since a natural ballistic path is down they wil, in the main, be used to steer the missile away from the ground, if you will. To me a wing should provide lift without the need for it to be deflected.


All wings are mounted at an angle of attack to develop lift - It's rather inherent in the process. To me (as an aeronautical engineer, er...and a pilot actually) if something develpps lift forces it is reaaspnable to treat it as a wing. That's whay the wings, tailplanes, foreplanes, fins and even blade antennas of aircraft are all treated in the same way. The point is that the weight of the vehicle is carried primarily by the "wings", not by the vertical componet of thrust. The lift derives from either the fins, or the body of the missile itself (look up the characteristics of ultra-low-aspect-ratio aeropdynamics - it's a field called "slebder body theory" which gets many people quite excited until they realise that it's nothing to do with women like victoria beckham). It's not semantics - it's a misaprehension!

Quote:

A patriot SAM, for example, has four surfaces at the rear of the missile. Clearly they are well behind the C of G so do not provide lift as I would describe it, they provide stability and steer the missile so that the thrust vector steers the missile to the target. To me they are not wings, they are stabilisers or control surfaces. Are they wings to an aero engineer? Perhaps so, to an aviator they are not.


Patriot is an interesting example, partly because it has a fixed (not vectorable) rocket nozzle and partly because it's one where enough of the numbers are in the public domain (for the PAC-1 varient, anyway)that we can actually do something useful with them. Patriot has a cruise speed of mach 3 (call it 1000m/sec) and a range of 70km. If its weight was carried by the component of thrust then (as we agree earlier) it would have to be accelerating allthe way. This would mean that the acceleration would have to be:

a=v^2/2s

= 1000*1000/2*70000

= 7ish m/sec^2 or around 0.7g

Now to achieve horizontal flight at this sort of acceleration its attitude would have to be such that the vertical component of thrust wa 1g, while the horizontal component was 0.7g. In other words its normal flight attitude (relationship between pitch heading and velocity vector) would be pitched up at:

arctan(1/0.7)= 55 degrees

Now whilst we're agreed that missiles fly with a small angle of attack, I think you'll agree that to suggest this AoA is 55degrees would be "controversial". In the case of the Patriot (which does actually achieve its cruise speed quite soon after launch, at which point the acceleration drops to zero) it's lift comes mostly from the missile body which, al thse speeds, can generate quite useful amounts of lift at small angles of attack (well under 5 degrees).

One of the things which upsets a lot of people when it comes to understanding missile design is the way that the tail surfaces are often as large or larger than any mid-mounted wings. The reason for this is that the design is a compromise. The missile is inteded primarily to cruise at well-supersonic speeds, but it must also be stable at subsonic speeds after launch. To achieve this the relationship between CG and neutral point is configured for subsonic stability (assuming an aerodynamic centre at 25% MAC). But once the missile is supersonic the AC shifts to 50 MAC and this configuration is seriously nose-heavy. To reduce the trim drag to acceptable levels the tail surfaces are made well oversized so that they can provide the required trim input with relatively small AoA. Modern supersonic fighters address the same problem by configuring the CG for supersonic flight and then dealing with the consequent subsonic instability using synthetic auto-stabilisation from a fly-by-wire control system.

The smaller missiles tend to use forplanes and tailplanes with no mid-planes because the rocket is a much bigger proprrtion of the all-up weight and so the CG moves a lot more as the fuel burns off. the "extreme canard" layout is much more CG-tollerent than the "wing+tail" layout.

PDR

 
PDR
482854.  Sun Jan 18, 2009 11:53 am Reply with quote

Davini994 wrote:
Flight is a very interesting topic but one I'm still struggling to grasp.


I have much the same problem with women.

PDR

 
Confucius
483121.  Sun Jan 18, 2009 4:29 pm Reply with quote

PDR wrote:
Davini994 wrote:
Flight is a very interesting topic but one I'm still struggling to grasp.


I have much the same problem with women.

PDR


Give them money and praise. Sorted.


Hooray, one up on PDR!

 
Flash
483157.  Sun Jan 18, 2009 5:22 pm Reply with quote

Classic stuff, gentlemen. That's all perfectly clear and I can't imagine why we got it wrong on the show. Or right, or whatever it was.

 
Confucius
483191.  Sun Jan 18, 2009 6:37 pm Reply with quote

Flash wrote:
Classic stuff, gentlemen. That's all perfectly clear and I can't imagine why we got it wrong on the show. Or right, or whatever it was.


I don't think it matters any more!

T'was always a discourse better suited to a pub rather than a forum.

 
Posital
483204.  Sun Jan 18, 2009 7:05 pm Reply with quote

Did you spill my pint?

 
gruff5
483206.  Sun Jan 18, 2009 7:07 pm Reply with quote

Do we have the proverbial rocket scientist on the forum to settle this question? ;-)

 
PDR
483990.  Mon Jan 19, 2009 12:40 pm Reply with quote

I believe he's on leave, visiting his family back in Proverbia.

PDR

 

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