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would making lifting powder packing it in ur engine tube and then letting it dry produce more or less thrust than black powder? Also everyody says dont use pvc for engine tubes and this baffles me because ive always use it and ive never had a problem infact ive found it to work better i always seem to split cardboard could sombody explain to me why i shouldn't use pvc? And third what is a good way of getting two or more engines to fire at the same time as i have a 7.5 foot rocket that i don't feel like packing an engine the size of a bottle for again.

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ive tried the kno3 rocket but never with a catalyst il try that and also on a different forum treb said that srb fuel was thermite and a catalyst and ive also heard its similar to flash powder with sulphur and pvc in it does anybody know which one it is and also would thermite and sulphur lit with magnesium ribbon be a good rocket engine?

not thermite, but some of the ingredients are there.
http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/srb.html
"ammonium perchlorate (oxidizer, 69.6 percent by weight), aluminum (fuel, 16 percent), iron oxide (a catalyst, 0.4 percent), a polymer (a binder that holds the mixture together, 12.04 percent), and an epoxy curing agent (1.96 percent)"

thermite is iron oxide and aluminum at high temp producing iron and aluminum oxide. flash powder is potasium perchlorate and aluminum (sulfur makes it more sensitive, good for an igniter, not a solid fuel). srb would have ammonium perchlorate and aluminum burning, the pvc/polymer just holds everything in place.

burning thermite is a liquid, due to temprature and products (iron and aluminum oxide), i'm not sure that sulfur is able to affect that chemistry in a way that make a good fuel i.e. lots of gas.

my plugs are adhering fine i sand the inside to give it more grip and im not getting the bentonite wet if u pack it it becomes solid like which is great because it expands to make a tighter seal and there is no dry time but what i was trying to say is that 50% my engines work but when they do blow out they tend to fail at the plug which is why i dont worry about shrapnel the other thing about cardboard is i allways seem to get burn thourghs but with pvc i dont experiance that problem and i can never roll my tubes right i have to use old used estes tubes mine always burn though and the always split which is another prolem overiden by the pvc can u think of anywhere i can get a well rolled small estes D size diameter tube that i could cut into varying lengths and the other day i was discussing with my friend is to light a rather tall engine at the top (of course ud need a different chute deployment system) but he said the reason you would do this is because as the engine burns from bottom up u lose thrust at the end he said u would want to gradually increase thrust but i think that either way it would not make a difference because u would either lose thrust at the begging or the end of the flight still producing the same amount of overall thrust.

for tubes of a specific size, it's make 'em or buy 'em (try a fireworks website). to change the rate of burn, look into seperately loaded grains and retarded grain propellent. these are ways of changing the burning surface area to change when you get peak power. try looking up 'rc candy' or '5 cent sugar rocket' where the fuel is sugar and potasium nitrate? and sometimes a catalyst such as sulfur or rust (red iron oxide) in small amounts.

i think jt's sounds most logical although if u make a good engine with black powder it usually burns a total of about 3 seconds and right after it fires its not even hot so i would assum that photo's reason is why most people say this as far as te shrapenal my engines never blow out like a grenade but they either burn out the top and light the rocket on fire or they blow the bentonite plug out the bottom and but all of them are logical

sounds like your plugs aren't adhering to the pvc. bentonite shrinks when drying , pvc expands with heat and you don't mention any sort of glue so there may be a crack that develops and exhaust is burning out. hard to tell without a visual post-mortem.

use 2 igniters in series? depends on the powder, chemical makeup, grain size, physical size of chemicals, packing pressure, ect. strength to weight ratio, plus heat softening and more likely to burn than cardboard.

not to mention that when something goes wrong, PVC will easily create razor sharp shrapnel. Metal tubes are less prone to do so, and cardboard is safe.

PVC + Heat = deadly gases.... mainly the "C" in PVC -- which is chloride (chlorine).

Just a guess :P