Re: Design, build, test a small liquid rocket engine.

From: William Gabler (b_gabler_at_worldnet.att.net)
Date: 01/24/05 

To: sci-space-tech@moderators.isc.org
Date: Mon, 24 Jan 2005 01:33:06 GMT

I checked the equation in step 13 and it seems ok. You
are right that the answer is wrong.

In step 13, from the equation for cooling water speed, I
get A = .0004139957 sq ft. The eq he gives for D2 is ok, but
remember that D1 = 1.2 inches from a previous step, and
everything here uses feet for length, so D1 =.10 ft. Then
D2 = sqr(4A/pi + D1^2) = sqr(.010527)
but the book/web site has sq1(.0151).
Given that this was done in sliderule times, it looks
like he read 1051 on the sliderule, and placed the
decimal correctly, but dropped a 0 to get .0151
(reading 1051 or what the calculator gives of 1053
is as good as it gets sometimes).

So, let's check: I get D2 =sqr(.010527) = .126012 ft (or 1.231 in).

A= (pi/4)(D2^2 - D1^2) = pi/4(.126012^2 - .100^2) = .00461737 ft^2
Then go back to Vw = Wv/(rho A) = .775 lb/s/(62.4 lb/ft^3 .000461737 ft^2)
so Vw = 26.9 ft/s versus 30 ft/s design requirement.
The consequence is left as an exercise for the reader.

    --- Bill

"jak785" <jak785@hotmail.com> wrote in message
news:1106356894.166803.216650@z14g2000cwz.googlegroups.com...
> Has anyone here ever tried building a small liquid rocket engine?
> Preferably from this website: http://www.risacher.org/rocket/
> I am building one from that website, but I am making it 50lb-thrust
> rocket instead of 20. I know how to use all the euqations, but I want
> some imput from other people that built one before from this website or
> from their own calculations. Also on this link:
> http://www.risacher.org/rocket/example.html on step #13 the equation
> to get D2 is messed up or they made the awnser wrong, and I was
> wondering if anyone knew what is wrong with that equation. I would
> greatly appreciate any help.
>
> -Jason
>

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