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Rocket acceleration (from tonight's show)

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PDR
482076.  Sat Jan 17, 2009 11:25 am Reply with quote

djgordy wrote:


Penguins don't fly, they swim.



Rubbish, they fly. They clearly said it on telly so it must be true. QED

:0)

PDR

 
djgordy
482084.  Sat Jan 17, 2009 11:28 am Reply with quote

soup wrote:
According to Stephen (spot the reference to a "higher authority") flying and swimming are the same, it's just the mediums that are different.


Would that be the same Stephen who said that the Earth has a moon called Cruithne?

 
Davini994
482113.  Sat Jan 17, 2009 11:36 am Reply with quote

Quote:
No, I'm being much simpler - if you put an accelerometer on the missile what would it read? This wouldn't vary with the direction of flight.


Quote:
The effects of gravity and acceleration are indistinguishable, following Einstein's equivalence principle. As a consequence, the output of an accelerometer has an offset due to local gravity. This means that, perhaps counter-intuitively, an accelerometer at rest on the earth's surface will actually indicate 1 g along the vertical axis. To obtain the acceleration due to motion alone, this offset must be subtracted.


I understand what you mean now then. I think the non aviation engineer would think in terms of acceleration relative to a point on the ground, would they not? By which definition going up would mean less acceleration.

 
PDR
482175.  Sat Jan 17, 2009 11:53 am Reply with quote

Davini994 wrote:
I understand what you mean now then. I think the non aviation engineer would think in terms of acceleration relative to a point on the ground, would they not?


Probably, but the aeronautical engineers would just jeer, heckle and throw things at them.

:0)

I guess it's down to the viewpoint. Mine is vehicle-centric - my interest is in the consequences for the vehicle. The world beneth it is just an annoying irrelevance which just complicates things and distracts attention from important matters like analysing penguin flight.

PDR

 
PDR
482183.  Sat Jan 17, 2009 11:58 am Reply with quote

djgordy wrote:
soup wrote:
According to Stephen (spot the reference to a "higher authority") flying and swimming are the same, it's just the mediums that are different.


Would that be the same Stephen who said that the Earth has a moon called Cruithne?


BURN THE BLASPHEMER!

PDR

 
Confucius
482188.  Sat Jan 17, 2009 12:01 pm Reply with quote

Quote:
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.


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.

Oh, I've seen a few videos of rockets accelerating back to Earth. Although obviously these devices were experiencing 'deferred success'.

 
Davini994
482198.  Sat Jan 17, 2009 12:10 pm Reply with quote

PCP wrote:
Probably, but the aeronautical engineers would just jeer, heckle and throw things at them.

Yes, but they always do if they haven't been fed their bananas.

 
PDR
482218.  Sat Jan 17, 2009 12:38 pm Reply with quote

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.


Symetrical (and even flat-plate) surfaces still develop lift when presented to the airflow at an angle of attack. Most aircraft have symetrical sectioned fins and tailplanes, but these develop lift to provide the stabilising forces. Aerobartic aeroplanes like the Extra230/300/330 family, the Stephens Acro, the Edge etc all have symetrical sections on the wings as well to provide identical handling upright and inverted.

Once a missile has achieved cruse speed it is physically impossible to carry its weight with a component of a force which acts at a distance from the centre of gravity (draw it up for yourself - it's a simple resolution of forces exercise) - so the aerodynamic surfaces do it. If you look at the AIM9 (sidewider) you'll see it has tail surfaces and canards distributed either side of the CG. The AIM120 (AMRAAM) has mid-mounted "wings" and tail surfaces etc.

PDR

 
gruff5
482220.  Sat Jan 17, 2009 12:43 pm Reply with quote

PDR wrote:
The assertion was made that rockets achieve maximum acceleration when flying horizontally because the weight of the rocket isn't sitting on the motor. I'm not sure this is actually true.

I didn't see the show, so don't know the exact context in which the question was framed.

QI programme does sometimes get things mixed up*

I think in any typical NASA earth-orbit rocket ascent, the maximum acceleration will occur towards the top of the parabolic curve, when the rocket is flying almost horizontally.

This is partly because the rocket is not pushing in the opposite direction to the gravitational pull of the Earth. But the main cause will be because the take-off mass of the rocket was 90% accounted for by the fuel contained (liquid or solid) and that this will be almost all gone by the end of the ascent. Thus enabling the same thrust to accelerate the rocket shell much faster without having to accelerate that rocket-load of fuel as well.

*yes, I know that is blasphemy, please don't ban me, moderators! he he ;-)

 
Posital
482222.  Sat Jan 17, 2009 12:57 pm Reply with quote

PDR wrote:
The "downwards" option is ruled out because no rockets (and very few missiles) fly in a downwards direction with the rocket burning.


It's not clear that it's ruled out if you check the wording of the question. The answer may demand a hypothetical solution.

Also are we saying that this has never been tried?

 
Confucius
482248.  Sat Jan 17, 2009 1:52 pm Reply with quote

PDR wrote:
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.


Symetrical (and even flat-plate) surfaces still develop lift when presented to the airflow at an angle of attack. Most aircraft have symetrical sectioned fins and tailplanes, but these develop lift to provide the stabilising forces. Aerobartic aeroplanes like the Extra230/300/330 family, the Stephens Acro, the Edge etc all have symetrical sections on the wings as well to provide identical handling upright and inverted.

Once a missile has achieved cruse speed it is physically impossible to carry its weight with a component of a force which acts at a distance from the centre of gravity (draw it up for yourself - it's a simple resolution of forces exercise) - so the aerodynamic surfaces do it. If you look at the AIM9 (sidewider) you'll see it has tail surfaces and canards distributed either side of the CG. The AIM120 (AMRAAM) has mid-mounted "wings" and tail surfaces etc.

PDR


OK. A missile with surfaces will use these to provide lift by deflecting air downwards, but it is incorrect to call these wings in a conventional sense. The rear surfaces are generally fixed, the forwards ones are moveable. The rear surfaces provide stability not lift, the forward ones merely direct air to provide a force either to maintain direction against gravity, or to manoeuvre as required to the target. If a missile uses the aft surfaces for maneouvre and has fixed forward surfaces, level flight will be achieved by the rear fins pointing downward as the c of g is ahead of them, thus the nose will point up during level flight.

ICBMs do not use fins to manoeuvre, using thrust vectoring instead.

http://uk.youtube.com/watch?v=fl37UZvFsz0

Note how the motor lights, and then the missile tilits towards the target. With gravity acting downwards the path of the missile is always going to be slightly 'downwards' in relation to the thrust vector, there are no control surfaces.

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. In a sense the missile 'thinks' it is trying to climb slowly, gravity is pulling it down at the same rate. The nose of the missile will always point 'upwards' of the actual trajectory.

The effect is much more pronounced during a cruise missile launch from a ship, I'm looking for a video this evening.


Last edited by Confucius on Sat Jan 17, 2009 2:31 pm; edited 3 times in total

 
AndyMcH
482266.  Sat Jan 17, 2009 2:06 pm Reply with quote

Quote:
ICBMs do not use fins to manoeuvre, using thrust vectoring instead.


Me too....

 
Confucius
482283.  Sat Jan 17, 2009 2:35 pm Reply with quote

http://uk.youtube.com/watch?v=8Eevs2IL7y8

The first few seconds are an excellent example of what I'm trying to describe.

The Tomahawk does not deploy its wings until the boost phase is complete.

 
PDR
482360.  Sat Jan 17, 2009 5:11 pm Reply with quote

Confucius wrote:

OK. A missile with surfaces will use these to provide lift by deflecting air downwards, but it is incorrect to call these wings in a conventional sense. The rear surfaces are generally fixed, the forwards ones are moveable. The rear surfaces provide stability not lift, the forward ones merely direct air to provide a force either to maintain direction against gravity, or to manoeuvre as required to the target. If a missile uses the aft surfaces for maneouvre and has fixed forward surfaces, level flight will be achieved by the rear fins pointing downward as the c of g is ahead of them, thus the nose will point up during level flight.


Oh gawd, I didn't want to have to go deep into the detail on this, but as one who used to do this stuff for a living I couldn't disagree more!

Firstly the tomahawk launch videos. These show the boost-phase only, while the missile is accelerating and this is the one time when you *can* support the missile on the thrust, because the net acceleration vector (the resultant of forward acceleration and gravity) passes through the nozzle. It's actually more complicated than that, because the missile gets its initial impetus from being supported by the rails in the launch tube and (depending on which specific tomahawk varient it is) sometimes from an ejectioon charge in the launch canister as well. For the initial part of the flight the missile follows a completely ballistic trajectory during which it climbs steeply. Then ass the speed increases it is able to develop some lift from the missile body and so pitches over towards the horizontal. During this period the turbojet engine starts and runs up to speed. After a short time the rocket boosters are exhausted, and the turbojet engine takes over, with the wings being extended just after the rockets shut down. But the Tomahawk is a red herring!

The statement "A missile with surfaces will use these to provide lift by deflecting air downwards, but it is incorrect to call these wings in a conventional sense" is frankly bizzare! In what sense WOULD you call them "wings"? Place a planar surface in a fluid flow at any angle of attack and it will develop lift & drag - it cannot avoid doing so. You have stated yourself that the missile flies with an angle of attack, and that the "fins" are nailed on parallel to the long axis of the missile so they too will be at an angle of attack. They will therefore develop lift. Now take a stab at the surface area of these surfaces, pick an airspeed and a nice low lift coefficient and do the maths. The lift developed is easily enough to support the missile.

Again, it's more complicated than that because most missiles are quickly supersonic, and usually cruise above Mach 3. At these speeds the conventional (I hesitate to say "Bernoulian") lift mechanism doesn't work. This is why the missiles mostly fly with the "fins" in X orientation rather than in + orientation. The anhederal lower surfaces act as a tunnel to generate "Shock Lift" (as later used on aircraft like the B-70 Valkyrie).

Now look at the classical air-to-air missiles like the AIM-9 Sidewinder, the AIM54 Phoenix and the AIM 120 AMRAAM - all of these are equipped with fixed rocket nozzles, with only the latest AIM-9X varient getting a vectorable nozzle for improved dogfighting manoeuverability. So clearly the fixed rocket motor cannot be vectored to support the missile.

But the final point is probably the most definitive. Even the air-to-air missiles, with their generally "flat" flightpath, only have rocket thrust for perhaps 60% of their flight-time. Short-range "dogfight" missiles have a maximum motor burn of perhaps 5 seconds, but can be used effectively with times-to-target of 20 seconds or more. The rest of the time they're on the glide. If the sole means of opposing gravity were indeed a thrust component from the motor they would plummet like penguins as soon as the motor stopped, but they don't.

PDR

 
Confucius
482385.  Sat Jan 17, 2009 6:11 pm Reply with quote

Ok, you are an aeronautical engineer. I did Aero Eng at uni and have been an aviator in the RAF for 20 years.

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. I think we're disagreeing on semantics!

I am aware of the theory behind everything else you have written, apart from: how can symmetrical, as on a AAM, X 'wings' provide shock lift? The symmetry means that the force up down left and right will also be symmetrical surely?

Also, a cylindrical object cannot develop lift unless it's path is something other than completely along its own axis, thus its trajectory must, during acceleration, be at a lower angle to the horizontal than its axis. A ballistic missile continues to accelerate until cut-off, during this time its trajectory again must be lower, at any given moment, than the angle of its axis against the same datum as the trajectory, at no time does a ballistic missile reach a cruising speed.

 

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