Lets say, hypothetically ofcourse, that I put some gunpowder at the bottom of a tube and put something above it. If I lit this gunpowder, could I expect this item to fly out of the tube or just blow up in my face?
*Note, I’m not planning on anything, its just something I’ve considered, incase I ever need to build something like what is described above or a rocket
Assuming your tube was durable enough to vector the ensuing blast upwards, I can imagine that you would be able to launch said item. Don’t fireworks use something very similar to gunpower?
Gunpowder doesn’t have to explode; it can just burn very rapidly producing volumes of hot gases; yes, it can be used as a rocket propellant.
The question is a rather surprising one - I thought it was common knowledge that the first rockets, invented in China, used gunpowder as the propellant.
That’s not to say rocket manufacture is a trivial thing for the amateur; it’s most likely that you’d produce something that either blows up almost immediately, or goes fwoosh but hardly moves.
But actually, the OP is talking about a cannon (a tube with gunpowder and something else in it on top) - yes, it’s most likely to blow up; despite your lame disclaimer, it actually sounds like you’re planning to try this; please don’t.
Gunpowder means a mixture of charcoal, potassium nitrate and sufur as opposed to modern nitrocellulose based propellants. Let me assure you that gunpowder will explode if ignited. It does not have to be constrained and even a loose pile will explode. There are ways to moderate the the burn rate but unless you know what you are doing you will be making not a rocket but a bomb. If this happens you will have a ringing in your ears but this isn’t your primary concern as you will be nowhere near them. Humor aside this isn’t something to experiment with even if you think you know what you’re doing. There is a saying in the gunpowder business that you need to have two powder mills, one to produce powder and one to replace the first one when, not if, it blows up.
A loose pile of gunpowder will not explode, but will burn rapidly. If it is contained, the flame from the first part that is ignited is forced into the space between granules, causing the rest to ingite much more rapidly. The same will happen if it is constrained enough to be forced out of the nozzle of a rocket engine (I know, ‘motor’). For it to work as a propellant, it would need to be packed very tightly. Probably best mixed as a slurry, and dried solid. It might be a bit too slow to be a propellant though.
As for the ‘don’t experiment with this stuff’ thing, I spent my childhood making my own gunpowder. It is not bad stuff at all. In fact of the common explosives, it is one of the lowest.
Yeeep, yeep yep, them were the days 
Back in the days when my cousins and I were kids and 35mm film cans were metal with screw-on caps, we would poke 3 small holes in the bottom, evenly spaced around the edge and one hole in the middle of an empty can and fill it up with match heads broken off wooden matchsticks and screw on the lid.
Using three of the sticks to prop up the can, and one whole match in the middle hole as the fuse, we used the back patio at grandma’s house for a launch pad and would light the fuse and stand back. After several seconds the matchheads inside the can would light up and with a satisfying “whoosh” and lots of smoke the can would rocket up into the air. I don’t remember how high those cans actually soared, maybe 20 feet, but it was an impressive height to eight-year-olds.
If we wanted to make a bomb instead of a rocket, all we had to do was compress the matchheads with tissue paper and pack the cap as well so the contents were under pressure. Then when the matches lit, instead of shooting into the air, the can would just explode on the launch pad, blowing the cap off and throwing smoking matchheads and tissue everywhere.
No one ever got hurt and we would keep it up til we ran out of matches. The cans were good for many launches. It was great fun.
UK Dopers may remember Tim Hunkin’s “The Secret Life Of Machines” popular science series from about 10 years ago.
In “The Secret Life Of The Internal Combustion Engine”, he did this very experiment. A teaspoonful of gunpowder shot the projectile (a beer can) about 6" out of the top of the tube. With a teaspoonful of gasoline, it landed about 200’ away. 
Gunpowers, including those used in cannons, are all propellants and are technically not explosives and so would would just fine as rocket fuel.
So will kitchen match heads. We used to make small rockets 3 in. or so in length and 3/8 in dia., fill them with match heads and shoot them off. The went 50 to 75 ft. like gangbusters.
I see that Thin Ice also knows about match heads. How did I miss that the first run through the thread?
As a counterpoint to that, along with my best friend at the time, I used to dabble with explosives; we tried gunpowder, but never got it to do more than burn rapidly, but we experimented with another explosive recipe popular with terrorists of the day (which I will not divulge here); we made various devices, starting with flares, then explosives and, eventually, very successful rockets (at least the ones that didn’t explode violently were very successful).
My friend decided we needed a cannon (not unlike the device the OP is describing), so he cut a piece of heavy steel tubing from a chair, welded a steel billet in one end to close it off, and drilled a fuse hole.
This we packed with a few tablespoons of explosive mix, some cotton wadding and a marble or ball bearing. My friend wanted to test it handheld, mounted on a wooden gunstock he had specially made, but we decided to test fire it on a tree stump.
Cuit a long story short, it made a deafening bang and caused a fair bit of damage to the tree stump; when we eventually found the cannon, the thick steel of the tube had been ripped wide open and now resembled something like a large metal holly leaf. If he had been holding the device when it was fired, he would certainly have been killed.
Yep, I made gunpowder all the time as a kid. Found the basic recipe, then mixed up different batches with different proportions of ingredients. Plus I would unwrap firecrackers, etc to get that gunpowder. No problems…
…until one day I decided to unwrap 3 E-size model rocket engines…
…And put all of the chunky powder in a pan…
…and took a wee-tiny piece out and held it with a pair of pliers…
…and tried to light it…
…and failed…
…the first three times…
…and it finally did spark…
…in fact, it threw sparks a few feet in all directions…
…one of which was into said pan…
Can you say WHOOSH reaaaaalllly loud? Cuz that was the sound of that wee-tiny piece sparking that small pile of engine rocket powder. Blew me onto my ass a few feet away and burned off my eyebrows, forehead hair and gave me 2nd degree burns on my face and neck. Luckily I was wearing leather gloves and safety goggles (I wasn’t TOTALLY stoopid, just slightly).
I recommend flash paper, flash cotton, flash powder and other magician’s supply incendiary devices. Easier to control, less danger and all the showmanship of more powerful explosives.
-Tcat
Properly constructed, of course it would work: In essence, the OP describes the basic construction of a modern bullet. A tube with a projectile at one end and gunpowder inside. Rapidly expanding gases force the projectile out of the tube at high speed. Just turn on CNN - bullets work pretty reliably all the time.
It’s been a while since I’ve needed to know this, but propellants are explosives. However, they are classified as low order explosives, as opposed to high order explosives.
If I recall correctly, a low order explosive deflagrates; a high order explosive detonates. The difference between deflagration and detonation is the speed at which the flame front travels. If the flame front travels at less than the sound of speed (in the explosive medium) then you have deflagration. If the flame front travels faster than the sound of speed in the explosive medium then you have detonation. This is why a high order explosive creates a powerful shock wave.
Of course, when you talk about gunpowder, you need to distinguish between black powder and smokeless powder. Smokeless powder is a low order explosive. I’m not sure if black powder is high or low.
There was a mythbusters episode where they build a bunch of rockets out of gunpowder (they were trying to test some Chinese legend about a guy who flew into outer space on a throne) using a traditional Chinese recipe for the stuff. Their only concession was to use a small ring in order to simulate a rocket nozzle. The thing was pretty weak (and unstable as hell…it just flipped over after rising about a foot off the ground). I think there was another episode where they made a rocket motor out of gunpowder too but I can’t recall what they were testing in that one.
The OP though sounds more like s/he is simply building a cannon.
-XT
Pardon me, but is not combustion another word for explosion? If the explosion is slower than the speed of sound, it is a low-order explosive, faster explosives are high explosives.
What the OP is describing is a cannon (or a mortar more exactly) as the projective will only be powered while in the launch tube and will be forced up at a high angle, recoil being absorbed by the base of the tube. A rocket motor would keep pushing the projective after launch and is ‘recoilless*.’
The exact composition of the gunpowder is critical in this sort of application. You want high push with low shock more like 'Black Powder" or ‘Blasting Powder’ than ‘Gun Powder.’ Same ingredients, with different compositions and integration.
*I know, that is why it is in quotes.
The sound of speed, for those of you who are wondering, is “Whooosh”.
Sailboat
You’re right. I wasn’t careful enough with my terminology. I suppose it’s possible to confine gunpowder in a case strong enough for the pressure to increase enough to get detonation. The usual thing is for the case to rupture and the pressure to be released before the burning rate is high enough to detonate the charge.
The way the title of the OP was phrased, I though he was talking about bottle rockets, which are really nothing more than an open-ended firecracker. I’ve experimented with these a million times, trying to get a successful multi-stage to launch without flipping over at apogee and rocketing after the engineer! 