RE: max alititude: boattails and minimum diameter rockets...

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ogivemeahome
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Re: RE: max alititude: boattails and minimum diameter rocket

Postby ogivemeahome » Tue Jan 17, 2012 10:58 pm

It's my understanding that on small projectiles at subsonic speeds, *pressure drag accounts for very little if any of the total drag. (*proportional to frontal area). This assumes that the projectile is 'pointy' as opposed to blunt.
Skin friction accounts for a large proportion of the total drag. To compare 24mm vs 27mm, the increase in area will be around 12%. If the surface is very smooth and the projectile is relatively short, then 12% more of a small value is only slightly less small.
There is a principle referred to as the 'area rule'. It translates to increasing the cross-sectional area as gradually as possible (the nose-cone) and just as importantly, decreasing the cross-sectional area as gradually as possible (the purpose of the boat-tail). I can't quantify any of this but it sounds reasonable. How many dolphins or seals do you see with a big rear end?
Assuming a larger diameter to accomodate a boat-tail, the increase in weight of the airframe may outweigh (pun) the benefit (over a short distance).
I like boat-tails simply because they look good and it seems more of a challenge to build that way.
A smooth sea never made a skilled mariner. (English Proverb)

rocket_troy
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Re: RE: max alititude: boattails and minimum diameter rocket

Postby rocket_troy » Wed Jan 18, 2012 7:19 am

PK wrote:
rocket_troy wrote:The answer to this more specific question is a simple "NO"


Nah, not so simple. It depends on whether the aft diameter of the boat tail is < the motor diameter. Eg a boat tail motor retainer.
If it isn't then the simple NO applies. If it is then it's marginally less simple.
PK

That's true.

TP
ERG #02


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