Author Topic: Basic Rocket Science Q & A  (Read 271537 times)

Offline mmeijeri

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Re: Basic Rocket Science Q & A
« Reply #180 on: 05/08/2009 11:25 AM »
I would love to get my hands on some of the flight software for modern LVs and spacecraft. I'm sure I could have no end of fun with it.

One interesting experiment would be to have a look at the code and then decide whether you still think launching people with that is a good idea. :)

On a related note, I think the Apollo LM software is out there somewhere on the Internet. There's even an emulator for it.
We will be vic-toooooo-ri-ous!!!

Offline Danny Dot

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Re: Basic Rocket Science Q & A
« Reply #181 on: 05/08/2009 11:35 AM »
What is the desired angular change to the flight path achieved by the pitch maneuver performed after clearing the launch tower?

What are typical pitch angles for current LVs such as Atlas V, Ariane 5 etc.?

Why don't you go ask what the shuttle pitch profile is?  Try the shuttle Q&A thread.

Danny Deger
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Offline AnalogMan

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Re: Basic Rocket Science Q & A
« Reply #182 on: 05/08/2009 12:22 PM »

Footnote: another posting intervention! When I type i-d-i-o-t (without the spaces," it rendered as "inaccurate." (inaccurate?)

Heaven help us if we ever need to talk about a clueless idiot  ;)

Offline mmeijeri

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Re: Basic Rocket Science Q & A
« Reply #183 on: 05/08/2009 12:37 PM »
Heaven help us if we ever need to talk about a clueless idiot  ;)

I was about to ask how on earth you did that, but if you look at the wiki markup it's obvious. :) Well done sir!
We will be vic-toooooo-ri-ous!!!

Offline William Barton

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Re: Basic Rocket Science Q & A
« Reply #184 on: 05/08/2009 12:56 PM »
I would love to get my hands on some of the flight software for modern LVs and spacecraft. I'm sure I could have no end of fun with it.

One interesting experiment would be to have a look at the code and then decide whether you still think launching people with that is a good idea. :)

On a related note, I think the Apollo LM software is out there somewhere on the Internet. There's even an emulator for it.

There's a part of me that doesn't want to see. I spent a part of the 1990s as a "rescue programmer," and I shudder to think...

Offline William Barton

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Re: Basic Rocket Science Q & A
« Reply #185 on: 05/08/2009 12:58 PM »
Heaven help us if we ever need to talk about a clueless idiot  ;)

I was about to ask how on earth you did that, but if you look at the wiki markup it's obvious. :) Well done sir!

Clever! Now the question of why *those* words...

Offline Antares

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Re: Basic Rocket Science Q & A
« Reply #186 on: 05/08/2009 03:20 PM »
What is the desired angular change to the flight path achieved by the pitch maneuver performed after clearing the launch tower?

What are typical pitch angles for current LVs such as Atlas V, Ariane 5 etc.?

No guideline.  It depends on other requirements.  Usually (1) controllability, (2) structural limits, (3) performance (drag) in that order of importance.

Very small compared to airplanes (quantification could involve eye tar so don't ask).  While q is still high, much of an alpha or beta will cause tumble or tearing off the fairing.  Which is why the constraint is often listed in terms of q*alpha or q*beta.
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Offline mdo

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Re: Basic Rocket Science Q & A
« Reply #187 on: 05/08/2009 05:51 PM »
Why don't you go ask what the shuttle pitch profile is?  Try the shuttle Q&A thread.

...because access to emergency landing sites, aerodynamic surfaces and the lack of fairing separation (altitude) make the Shuttle a special case muddying the water when trying to understand the basics as in Basic Rocket Science Q&A.


Offline mdo

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Re: Basic Rocket Science Q & A
« Reply #188 on: 05/08/2009 05:56 PM »
No guideline.  It depends on other requirements.  Usually (1) controllability, (2) structural limits, (3) performance (drag) in that order of importance.

Very small compared to airplanes (quantification could involve eye tar so don't ask).  While q is still high, much of an alpha or beta will cause tumble or tearing off the fairing.  Which is why the constraint is often listed in terms of q*alpha or q*beta.

Thanks Antares.

The Ariane 5 press kit of the upcoming Herschel/Planck launch, for instance,
http://www.arianespace.com/images/launch-kits/launch-kit-pdf-eng/HERSCHEL-PLANCK-GB.pdf
mentions a 10 s pitch rotation. So, no itar issue. My question is not so much about structural limits but about how much the LV is put off the initial 90 deg climb angle? Maybe it rotates to 70 or 80 deg during those 10 s?

My guess for its purpose is to gain just enough vertical speed during the 1st stage burn so that the LV does not drop back below fairing separation altitude. It is an issue when the upper stage thrust/weight ratio is below 1. Since the angle of attack during atmospheric flight is rather limited I am speculating that the initial pitch maneuver is supposed to put the vehicle on a trajectory that limits vertical v to exactly meet this requirement, but again, I'm not sure this is the case.



Offline Jim

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Re: Basic Rocket Science Q & A
« Reply #189 on: 05/08/2009 06:05 PM »

1. mentions a 10 s pitch rotation. ....... My question is not so much about structural limits but about how much the LV is put off the initial 90 deg climb angle? Maybe it rotates to 70 or 80 deg during those 10 s?

2. My guess for its purpose is to gain just enough vertical speed during the 1st stage burn so that the LV does not drop back below fairing separation altitude. It is an issue when the upper stage thrust/weight ratio is below 1. Since the angle of attack during atmospheric flight is rather limited I am speculating that the initial pitch maneuver is supposed to put the vehicle on a trajectory that limits vertical v to exactly meet this requirement, but again, I'm not sure this is the case.


1.  There are other "pitch rotations" in the flight.  This is not the only one.  Also a "pitch rotation" is a rate and not a final position.

2.  Since there are other "pitch rotations", and the vehicle attitude is not fixed, therefore the rest of your post is not applicable. 

The initial pitch rates and attitudes are about structural limitations and aerodynamic loads.   Pitch rates and attitudes to deal with gravity losses and second stage start conditions occur later.
« Last Edit: 05/08/2009 06:10 PM by Jim »

Offline mdo

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Re: Basic Rocket Science Q & A
« Reply #190 on: 05/08/2009 07:40 PM »
1.  There are other "pitch rotations" in the flight.  This is not the only one.  Also a "pitch rotation" is a rate and not a final position.

Let me phrase the question differently then:

Do LVs actively aim for non zero angle of attack at any time during supersonic flight?

If not, then it needs to be tipped off initially, i.e., after clearing the tower and by some angle that I'm interested in. Thereafter, it does a gravity turn. Well, I hope gravity turn is the proper term for aligning thrust and velocity vectors to achieve zero AOA.

Quote
2.  Since there are other "pitch rotations"...

... would mean my assumption is wrong which is LVs strictly stick to a gravity turn during atmospheric flight and after the first few seconds of ascent - UNLESS those other pitch rotations are performed for the very reason of keeping the AOA continuously close to zero?

Offline Danny Dot

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Re: Basic Rocket Science Q & A
« Reply #191 on: 05/08/2009 08:27 PM »
Why don't you go ask what the shuttle pitch profile is?  Try the shuttle Q&A thread.

...because access to emergency landing sites, aerodynamic surfaces and the lack of fairing separation (altitude) make the Shuttle a special case muddying the water when trying to understand the basics as in Basic Rocket Science Q&A.



You are correct the shuttle is a little different, but it still must not fall apart during first stage and fly an optimum pitch profile in second stage.  Someone in the shuttle world can get you a copy of the pitch profile cue card used by the crew.  Try googling "SODB".  You might try the Apollo forum to get a copy of the Apollo cue card.  The is a Saturn V users manual on the web that I think has a copy.

Have you tried the Atlas and Delta User's guides?  I would help find links, but I am on my Treo.

The launch vehicles often don't shoot for zero angle of attack.  But must always keep angle of attack in limits.

Pitch is pretty much contantly changing.  Constant pitch rate gives the fuel optimum profile.

I have never heard the term "pitch manuever" because pitch is always decreasing (with some exceptions).  There is a roll maneuver that puts the LV in the proper roll attitude and it then stay there.

Hope this helps.  Happy hunting.

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Offline Jim

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Re: Basic Rocket Science Q & A
« Reply #192 on: 05/08/2009 08:50 PM »

1.  Do LVs actively aim for non zero angle of attack at any time during supersonic flight?

2.  If not, then it needs to be tipped off initially, i.e., after clearing the tower and by some angle that I'm interested in. Thereafter, it does a gravity turn.

3.  Well, I hope gravity turn is the proper term for aligning thrust and velocity vectors to achieve zero AOA.

4.  ... would mean my assumption is wrong which is LVs strictly stick to a gravity turn during atmospheric flight and after the first few seconds of ascent - UNLESS those other pitch rotations are performed for the very reason of keeping the AOA continuously close to zero?


1.  LV's don't sense AOA or even airspeed.  Everything is based on simulations and balloon data

2.  That "angle" is different for every launch vehicle

3.   That is a benefit of a gravity turn vs the reason for it

4.  they may be for load relief
« Last Edit: 05/08/2009 08:55 PM by Jim »

Offline mdo

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Re: Basic Rocket Science Q & A
« Reply #193 on: 05/08/2009 10:20 PM »
The launch vehicles often don't shoot for zero angle of attack.  But must always keep angle of attack in limits.

Pitch is pretty much contantly changing.  Constant pitch rate gives the fuel optimum profile.

Great! That's exactly the kind of hints I was looking for; will look up the specific references for some sample values. Cheers.

Quote from: Jim
1.  LV's don't sense AOA or even airspeed...

Points taken. It is apparently a bit more sophisticated than anticipated. Thanks.

Offline Danny Dot

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Re: Basic Rocket Science Q & A
« Reply #194 on: 05/08/2009 10:32 PM »
The launch vehicles often don't shoot for zero angle of attack.  But must always keep angle of attack in limits.

Pitch is pretty much contantly changing.  Constant pitch rate gives the fuel optimum profile.

Great! That's exactly the kind of hints I was looking for; will look up the specific references for some sample values. Cheers.

Quote from: Jim
1.  LV's don't sense AOA or even airspeed...

Points taken. It is apparently a bit more sophisticated than anticipated. Thanks.

You are correct.  Ascent profile design is very complicated.

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Offline mdo

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Re: Basic Rocket Science Q & A
« Reply #195 on: 05/09/2009 12:46 PM »
FYI, found some reference material in the meantime. Depicted below are the diagrams for pitch and AOA taken from the Saturn V Flight Manual. It shows that AOA peaks at 4 deg shortly after clearing the tower. Then it stays below 1 deg during atmospheric flight. So, the Saturn V first stage is so kind to behave following my intuition: That is, actively pitch over to some precomputed attitude early on and then just duck the oncoming airflow.

The same with Atlas V; the payload user manual says:
At an altitude of 244 m (800 ft) and time from liftoff greater than 10 seconds, the vehicle begins its initial pitch-over phase. At approximately 2,438 m (8,000 ft), the vehicle enters into a nominal zero-pitch and zero-yaw angle-of-attack phase to minimize aerodynamic loads.

It also mentions an optional alpha-bias angle-of-attack steering mode between 80-120 Kft to account for wind.

Offline Jim

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Re: Basic Rocket Science Q & A
« Reply #196 on: 05/09/2009 01:23 PM »
FYI, found some reference material in the meantime. Depicted below are the diagrams for pitch and AOA taken from the Saturn V Flight Manual. It shows that AOA peaks at 4 deg shortly after clearing the tower. Then it stays below 1 deg during atmospheric flight. So, the Saturn V first stage is so kind to behave following my intuition: That is, actively pitch over to some precomputed attitude early on and then just duck the oncoming airflow.

The same with Atlas V; the payload user manual says:
At an altitude of 244 m (800 ft) and time from liftoff greater than 10 seconds, the vehicle begins its initial pitch-over phase. At approximately 2,438 m (8,000 ft), the vehicle enters into a nominal zero-pitch and zero-yaw angle-of-attack phase to minimize aerodynamic loads.


No, you are still getting it wrong.   Read the top chart, attitude goes from 0 to 60 degree

The pitch rate doesn't go to zero. there still is one (the actual attitude is changing), it is just at a slow rate as to not generate an angle of attack.  This is not "ducking".   The incoming flow is the velocity vector.

A vehicle could fly straight up and hence would not generate an angle of attach.   But because a more efficient trajectory requires the vehicle to go horizontally eventually, the vehicle has to pitch over to turn the velocity vector.  The rate at which it does is a function of structural limitations and the propulsion system.

Notice for Atlas is says "phase".  The phase is short
« Last Edit: 05/09/2009 01:35 PM by Jim »

Offline mdo

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Re: Basic Rocket Science Q & A
« Reply #197 on: 05/09/2009 02:46 PM »
...The pitch rate doesn't go to zero.

Sure, it was never claimed that it's zero. My initial point in reply 178 was about initiating the gravity turn and how much it typically takes to do so. It was not well phrased though because the term pitch over phase was missing from my vocabulary. AOA vs. pitch rate is clarified. Thanks for your patience.

Offline Antares

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Re: Basic Rocket Science Q & A
« Reply #198 on: 05/09/2009 02:48 PM »
Anything you're going to find publicly is also going to be very generic.  In the real world, every single one of these is going to be mission unique.

It's like the cone vs the black line in hurricane forecasting.  You seem to be trying to calculate a black line that's the same for every mission.  In reality, there are just bounds on what the rocket can do (the cone).  How it flies on launch day, the black line withing the cone, is subject to a long list of variables, requirements and optimizations of same.  Where one rocket might be flying a steady zero or non-zero alpha, another one would be pitching.  It's truly unique to each mission.
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Offline clongton

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Re: Basic Rocket Science Q & A
« Reply #199 on: 05/09/2009 07:40 PM »
Anything you're going to find publicly is also going to be very generic.  In the real world, every single one of these is going to be mission unique.

It's like the cone vs the black line in hurricane forecasting.  You seem to be trying to calculate a black line that's the same for every mission.  In reality, there are just bounds on what the rocket can do (the cone).  How it flies on launch day, the black line withing the cone, is subject to a long list of variables, requirements and optimizations of same.  Where one rocket might be flying a steady zero or non-zero alpha, another one would be pitching.  It's truly unique to each mission.

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That is perhaps the very best example I have ever seen for any rocket's actual performance vs. its predicted performance.
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