Author Topic: Basic Rocket Science Q & A  (Read 272216 times)

Offline Integrator

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Re: Basic Rocket Science Q & A
« Reply #420 on: 01/25/2010 02:08 AM »
I understood on Saturn V the fins were to add stability in case of an abort.
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Offline Antares

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Re: Basic Rocket Science Q & A
« Reply #421 on: 01/25/2010 03:00 AM »
Yep, positive static stability is a GOOD thing!  Gotta
keep CG in front of the AC, or bad things will happen...

Not with a digital FCS.  B-2s fly just fine.... usually.
If I like something on NSF, it's probably because I know it to be accurate.  Every once in a while, it's just something I agree with.  Facts generally receive the former.

Offline Jorge

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Re: Basic Rocket Science Q & A
« Reply #422 on: 01/25/2010 05:58 AM »
I understood on Saturn V the fins were to add stability in case of an abort.

That is my understanding as well. They did not add much stability to a nominal ascent, but in a first-stage loss of control case, they would delay the onset of high rates enough to allow the LES to pull the CM away.
JRF

Offline kyle_baron

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Re: Basic Rocket Science Q & A
« Reply #423 on: 01/25/2010 01:36 PM »
Fins could also be placed at the base of the rocket, unless you can think of a reason why they shouldn't be.

No need, only extra weight and drag.

This is what is used.  The scoops are on right and left of the base of the stage

I congradulate Jim for comming up with a simple, yet practical solution for the base heating problem, concerning the RS-68 engine!  Well done.
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Offline Jim

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Re: Basic Rocket Science Q & A
« Reply #424 on: 01/25/2010 01:48 PM »

I congradulate Jim for comming up with a simple, yet practical solution for the base heating problem, concerning the RS-68 engine!  Well done.

That isn't a solution.  The base heating problem is radiant heat from SRB.  The scoops are for base recirculation issues.

Offline Downix

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Re: Basic Rocket Science Q & A
« Reply #425 on: 01/25/2010 01:53 PM »

I congradulate Jim for comming up with a simple, yet practical solution for the base heating problem, concerning the RS-68 engine!  Well done.

That isn't a solution.  The base heating problem is radiant heat from SRB.  The scoops are for base recirculation issues.
If he'd read the thread he claimed was solved, he'd know this too.
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Offline kyle_baron

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Re: Basic Rocket Science Q & A
« Reply #426 on: 01/25/2010 03:53 PM »

I congradulate Jim for comming up with a simple, yet practical solution for the base heating problem, concerning the RS-68 engine!  Well done.

That isn't a solution.  The base heating problem is radiant heat from SRB.  The scoops are for base recirculation issues.

It may not be the total solution, but it certainly is a partial solution in keeping the base heat from rising any higher than it otherwise would.  Maybe in combination with heat dissipating fins on the RS-68 nozzles, to keep the ablative from disintigrating faster.  As you stated, the SRB heat radiation problem isn't likely to be solved, short of extending the perimeter of the base to cover the RS-68 nozzles, that are nearest to the SRB exhaust.
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Offline Jim

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Re: Basic Rocket Science Q & A
« Reply #427 on: 01/25/2010 04:00 PM »
Maybe in combination with heat dissipating fins on the RS-68 nozzles,

Not feasible.  The fins wouldn't dissipate heat, they would absorb it from the SRB's.  Plus the construction of the nozzle (composite overlap) is not conducive to fins

Offline Jim

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Re: Basic Rocket Science Q & A
« Reply #428 on: 01/25/2010 04:00 PM »
This problem is not going to be solved on this forum.  It is too complex

Offline kyle_baron

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Re: Basic Rocket Science Q & A
« Reply #429 on: 01/28/2010 02:21 PM »
  As you stated, the SRB heat radiation problem isn't likely to be solved, short of extending the perimeter of the base to cover the RS-68 nozzles, that are nearest to the SRB exhaust.

What is this "Boat Tail" that I read about in other threads?  Is it similar to an inverted, tapered, interstage at the base of the rocket?
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Offline kyle_baron

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Re: Basic Rocket Science Q & A
« Reply #430 on: 01/28/2010 07:51 PM »
On page 3, I found a picture of a Boat Tail on a Model Rocket:

http://www.2020vertical.com/nar_edu_cd_dev/lessons/apogee/Reports/Rocket_parts.pdf

Boattail
Boattail is a drag reducing part on the back of the rocket.
It helps direct airflow around the base of the rocket. In effect,
it keeps the flow smooth, which reduces the aerodynamic drag
and allows the rocket to fly higher into the air.


Hmmmm.... Combine a Boat Tail with air scoops, and what do you have?  A possible solution to base heating, for the RS-68 engine?
« Last Edit: 01/28/2010 08:04 PM by kyle_baron »
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Offline MarsInMyLifetime

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Re: Basic Rocket Science Q & A
« Reply #431 on: 01/28/2010 10:55 PM »
That model rocketry magazine also describes shock cords, which don't apply to SRBs, so you have to watch how you extrapolate some information you come across.  ;)

I come back to what Jim said: the problem is radiant heat, and the problem exists beyond the atmosphere where scoops and boattails no longer matter. Not to discourage your thinking, but Jim mentioned radiant heat specifically because redirected air is a solution to the wrong problem.

Model rocketry does have some good things to offer on subsonic problems of smaller rockets, though. This NAR test report is a nice layman's introduction to the subject of base drag and boattails:
http://www.interactiveinstruments.com/pdfs/28.pdf
Don

Offline kyle_baron

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Re: Basic Rocket Science Q & A
« Reply #432 on: 01/29/2010 03:20 PM »
That model rocketry magazine also describes shock cords, which don't apply to SRBs, so you have to watch how you extrapolate some information you come across.  ;)

Understood.  I needed a picture and description of a boat tail on a rocket, and this was the best I could find.

I come back to what Jim said: the problem is radiant heat, and the problem exists beyond the atmosphere where scoops and boattails no longer matter.

The air traveling thru the scoops must be traveling at sonic speeds with in the boat tail.  And could be directed downward to the RS-68 nozzles by short ducts.  I disagree about the problem existing beyond the atmosphere, because we're talking about the 1st stage only.  And the SRB's fall away after 2 min. 

 
Not to discourage your thinking, but Jim mentioned radiant heat specifically because redirected air is a solution to the wrong problem.

The RS-68 nozzles are completly covered, and are protected directly from the radiant heat of the SRB's by a (carbon fiber) boat tail.  Unless you're saying that the radiant heat (from the SRB) is comming directly thru the RS-68 plume.
« Last Edit: 01/29/2010 03:27 PM by kyle_baron »
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Offline Jim

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Re: Basic Rocket Science Q & A
« Reply #433 on: 01/29/2010 03:30 PM »
The air traveling thru the scoops must be traveling at sonic speeds with in the boat tail.  And could be directed downward to the RS-68 nozzles by short ducts.  I disagree about the problem existing beyond the atmosphere, because we're talking about the 1st stage only. 


First stage does leave the sensible atmosphere

Offline kyle_baron

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Re: Basic Rocket Science Q & A
« Reply #434 on: 01/29/2010 03:41 PM »
The air traveling thru the scoops must be traveling at sonic speeds with in the boat tail.  And could be directed downward to the RS-68 nozzles by short ducts.  I disagree about the problem existing beyond the atmosphere, because we're talking about the 1st stage only. 


First stage does leave the sensible atmosphere

Does a sensible atmosphere exist, before the SRB's fall away?  I agree that there is a "point of diminishing returns" for the air scoops, as the rocket gains altitude.
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Offline Downix

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Re: Basic Rocket Science Q & A
« Reply #435 on: 01/29/2010 03:51 PM »
On page 3, I found a picture of a Boat Tail on a Model Rocket:

http://www.2020vertical.com/nar_edu_cd_dev/lessons/apogee/Reports/Rocket_parts.pdf

Boattail
Boattail is a drag reducing part on the back of the rocket.
It helps direct airflow around the base of the rocket. In effect,
it keeps the flow smooth, which reduces the aerodynamic drag
and allows the rocket to fly higher into the air.


Hmmmm.... Combine a Boat Tail with air scoops, and what do you have?  A possible solution to base heating, for the RS-68 engine?

You do realize this is why the DIRECT proposals using the RS-68 had a boattail, yes?

The issue with Ares V having one is due to the sheer # of engines.  DIRECT had, at most, 3 engines on an 8.4m core.  A 10m core would not add enough width, and the engines would burn out before they had used that much fuel, and would be unable to lift that size anyways.

Review DIRECT 2.0 if you want to see how they worked on solving these issues.
chuck - Toilet paper has no real value? Try living with 5 other adults for 6 months in a can with no toilet paper. Man oh man. Toilet paper would be worth it's weight in gold!

Offline Jim

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Re: Basic Rocket Science Q & A
« Reply #436 on: 01/29/2010 03:52 PM »
The air traveling thru the scoops must be traveling at sonic speeds with in the boat tail.  And could be directed downward to the RS-68 nozzles by short ducts.  I disagree about the problem existing beyond the atmosphere, because we're talking about the 1st stage only. 


First stage does leave the sensible atmosphere

Does a sensible atmosphere exist, before the SRB's fall away?  I agree that there is a "point of diminishing returns" for the air scoops, as the rocket gains altitude.

it is gone maybe 30 seconds before that

Offline MarsInMyLifetime

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Re: Basic Rocket Science Q & A
« Reply #437 on: 01/29/2010 05:29 PM »
Kyle, I'll note that scoops have their place--they make great sense for cruise missiles, which fly at one speed at one height, so the design variables are well controlled for that case. The SRB phase has the complexity of speeds ranging from 0 to Mach 4 and air pressures from sea level to 25 nautical miles, so the conditions are extremely dynamic during the period where their radiant heating is the concern.
Don

Offline kyle_baron

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Re: Basic Rocket Science Q & A
« Reply #438 on: 01/29/2010 08:21 PM »



You do realize this is why the DIRECT proposals using the RS-68 had a boattail, yes?

IIRC, Direct had a smaller boattail that didn't completly cover the nozzles of the RS-68.  I guess the reason is, that the engines were lined up perpendicular to the SRB's and were a good distance away.


The issue with Ares V having one is due to the sheer # of engines.  DIRECT had, at most, 3 engines on an 8.4m core.  A 10m core would not add enough width, and the engines would burn out before they had used that much fuel, and would be unable to lift that size anyways.

Any version of Direct that Nasa may adopt for HLV, will most likely have a streched tank, and one or more engines than Direct (in a square pattern)
« Last Edit: 01/29/2010 08:23 PM by kyle_baron »
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Offline edkyle99

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Re: Basic Rocket Science Q & A
« Reply #439 on: 01/30/2010 12:44 AM »
I'm guessing for stability, but are there other reasons? 
What would be the result in air pressure, if an SRB were placed between 2 fins?

Early missiles like V-2 and Corporal and Redstone had fins in part because the fins had movable surfaces that were part of the missile control system (along with movable vanes that extended into the exhaust).  The rocket nozzles didn't move in these rockets. 

Later missiles, like Jupiter (the real Jupiter) and Atlas and Titan (and later Thors) did not have any fins.  (Early R&D Thors had small fins, but on one flight some of the fins were ripped off of the missile, and the missile kept flying stable, so Douglas deleted them from the design.)

Saturn I Block I did not have fins.  They were added to the Block II Saturn for two reasons.  First, to provide stability for a projected winged Air Force payload.  Second, to provide stability for a few seconds in case of engine shutdown and activation of an Apollo launch escape system (Saturn I Block II was slated for several manned Apollo flights when it was designed).  Fins appeared on Saturn IB and Saturn V for the same reasons. 

 - Ed Kyle