Rick,
I'm sorry to have to disagree with so much of what you say, but my work
indicates that things are not as dire as you seem to think.
Yes, you need a core large enough to produce liftoff thrust; that
doesn't mean a *giant* core--in fact for the rockets I designed, the
core was only about an inch long and maybe 3/4" in diameter (IIRC).
BATES grains need far larger cores and waste far more volume.
Yes, the nozzle can't be too small or you cato; however, you can make
the casing stronger (and lighter) than most amateurs seem to have built.
Mine were of uni-carbon fiber, with an MEOP of 1200-1500 psi and were
far lighter than aluminum casings could be because I wasn't fighting
minimum gage issues. This allows the throat to be optimized between the
core-burning phase and the end-burning phase.
"Usable thrust" depends on how fast you think you need to fly. If you
only need to fly 200-300 mph, you need far less thrust than if you need
to be going Mach 1.5. My finding was that if you build light enough you
can afford to fly really slowly (relatively speaking) to get to the
target altitude.
Your comment that you can increase the diameter and achieve the same
thing as by increasing the burn rate is true only from a thrust point of
view. If you take a system view and consider the effect of increased
diameter on drag, you will find that, at some velocity, you've lost more
in drag than you gained in increased thrust. Hence, increased burn rate,
for some designs, for some target altitudes, is of interest.
If you take "altitude per unit mass of propellant" as a figure of
merit, you will find amateur rockets are very, very poor compared to
something like the Atlantic Research sounding rockets. I believe that
indicates that there is a great deal of room for improvement in the
amateur area. I wouldn't write off end-burners just yet.
--Steve
On 09/06/2017 02:33 PM, Rick Maschek (Redacted sender rickmaschek for
DMARC) wrote:
Using a core with an end burner has many draw backs for 'launching' a rocket, here are a few:
1) the core needs to be long enough to lift off the rocket (at least a 3-1 thrust to weight ratio).
2) the nozzle throat needs to be large enough to prevent a CATO with the higher core burning pressure.
3) that larger size throat does not produce much thrust during the low pressure end burning portion of the motor burn.
and any nozzle throat erosion increases that effect.
4) the extra weight of the end burning propellant (just how long is it to be?) means the core needs to be longer to lift off the rocket
the end burning portion of the motor turns out to be more smoke tracking than usable thrust.
I've experimented with pouring multiple times on top of previously cast sugar propellant with success. I've also made sugar grains with a > 2.5 cm/second burn rate, something that could cause more thrust with an endburner but you begin to realize simply using a larger diameter motor has the same effect. Here is an example of a 'RaD' AP motor we made a few years ago with different propellants in the same motor to reach different burn characteristics, in this case, not so much burn rates but different colors:
RaD multicolor rocket propellant static test <https://www.youtube.com/watch?v=oH6xE_Ej0QY&t=7s>
RaD multicolor rocket propellant static test
First test of the 'RaD' multicolor APCP solid propellant Bates grains. This version produces Red-to-Whit...
<https://www.youtube.com/watch?v=oH6xE_Ej0QY&t=7s>
On Wednesday, September 6, 2017 12:58 PM, Richard Nakka <richard.rocketry@xxxxxxxxx> wrote:
I've never attempted to make a sugar propellant end burner but I have often given it thought. Here are some of my thoughts:
1) A conical propellant surface, rather than a flat surface, would greatly increase burn area at no expense to the burn time.
2) PVC casing may be the ideal choice. I would expect ablation of the casing surface would keep it reasonable cool, and bearing in mind at low pressure, a thin wall is all that's needed so you can afford to ablate away much of the wall. Also, PVC is very lightweight.
3) Brown iron oxide may be the key to getting sufficiently fast burn rate to produce useful thrust.
http://www.nakka-rocketry.net/oxidex.html
Richard
On Wed, Sep 6, 2017 at 8:14 AM, Nikolai Nielsen <nielsen.nikolai86@xxxxxxxxx <mailto:nielsen.nikolai86@xxxxxxxxx>> wrote:
has anyone dabbled with end burner motors? i know that it would
cripple the thrust that the quantity of fuel could produce but it
seems like it should work.
Nikolai Nielsen
