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Q: I have a young son who would love to watch a bottle fly into the air, and wouldn't mind getting soaked, and the idea of doing it at night with glow attached, probably would thrill him. However, after a few launches, he would want to make it better.

Could you post a pdf file with the correct shape of fins made from say a cereal box (would be strong enough wouldn't it?) that he could transfer and cut out? I also assume that some sort of glue would be useful as well, suggestions?

Would a pointy end added to the top of the bottle (like a nose cone) add to the height of flight?

A: The Hydroflite launcher uses an automatic release. (I tried for years to invent a simple, reliable, cheap, and easy release mechanism without success.) The method that works best is my o-ring piece being machined to very tight tolerances. The bottle is held in place by friction between the bottle and o-ring. When pressure is applied, the launch force overcomes the friction force and you get liftoff. By tight tolerances of the o-ring grooves, launch occurs between 65 and 85 psi. Of course friction coefficients play a big role. As you make repeated launches, the o-ring friction seems to go up. I sometimes have to add a bit of “lubrication” to the o-rings by wiping the side of my nose or forehead with my fingers and rubbing the o-rings. This reduces the launch pressure back down. Different bottle brands can also make a bit of difference. Wal-Mart bottles are too small and too flimsy. Most other brands seem to work. It can be fun for kids to experiment with different brands (flavors) to see which goes at a higher pressure.

Q: If compressed air exhausts through a slit directly into ambient air at sea level (no cone or flanges) in the choked flow regime (choke plane across the slit) what is the relationship between upstream absolute pressure and the velocity of the air exhausting from the slit? Is the velocity proportional to mass flow?

The mass flow rate is a function of pressure because density is a function of pressure. The velocity and flow area don’t change but density does, so mass flow rate goes up with higher pressure. Mass flow rate=PAg/C* where P= pressure, A= throat area, g=gravitational constant and C*= gas characteristic gas velocity. Hence, mass flow rate is directly proportional to pressure.

About Hydroflite	Ask a Rocket Scientist ask@hydroflite.net If you happen to have a question about rocket science or what a rocket scientist does, email me the question and I will post the answer here. September 27, 2004 Q: I have a young son who would love to watch a bottle fly into the air, and wouldn't mind getting soaked, and the idea of doing it at night with glow attached, probably would thrill him. However, after a few launches, he would want to make it better. Could you post a pdf file with the correct shape of fins made from say a cereal box (would be strong enough wouldn't it?) that he could transfer and cut out? I also assume that some sort of glue would be useful as well, suggestions? Would a pointy end added to the top of the bottle (like a nose cone) add to the height of flight? A: There is no real critical shape for the fins. The hardest part is trying to match the bottle profile. Different manufacturers have slightly different shapes but I will try to give it a good approximation. Cereal box is a good start for fins but it might not be rigid enough. Balsa wood, or foam board would be stiffer. I found one web site that used CD’s cut in half for fins. The possibilities are endless. The adhesive is a bit of an art. The bottle stretches during pressurization so the adhesive must be flexible. Hot glue, Goop, and duct tape are all good candidates. A nose cone will add a little altitude but not too much. Here is a PDF file with a fin profile. September 26, 2004 Q: How is the Hydroflite rocket pop bottle held in place while it is pressurized prior to launch? By hand? A:The rocket is held in place by the friction of the o-rings. I have done many experiments to determine the exact depth of the o-ring groove that will hold the rocket back until launch pressure is reached. When the launch pressure is reached, the rocket automatically releases. This adds a bit of excitement to the launch because you are never quite sure when it will blast off. It is NOT recommended that your hands (or face or any other part of your body) are anywhere near the rocket once pressurization has begun. It really hurts to get hit by the bottle and could also cause severe injury. March 9, 2006 Q: Can I control to launch at certain psi with your launcher? A: The launch pressure is a function of the o-ring friction with the bottle. The o-ring compression is set by the depth of the o-ring groove. The average launch pressure is between 65 and 85 psi. If you want a lower launch pressure, lubricate the o-rings a bit by wiping them with your fingers. (There is a little oil on them.) If you want a higher launch pressure, wash the o-rings with hot water. There is a bit of variability anyway with the different brands of soda pop. This “automatic” release technique does not allow a specific pressure to be selected. March 12, 2006 Q: I am very interested in your launchers, but I cannot see HOW the rocket is launched. Can you please explain the launch procedures to me? A: The Hydroflite launcher uses an automatic release. (I tried for years to invent a simple, reliable, cheap, and easy release mechanism without success.) The method that works best is my o-ring piece being machined to very tight tolerances. The bottle is held in place by friction between the bottle and o-ring. When pressure is applied, the launch force overcomes the friction force and you get liftoff. By tight tolerances of the o-ring grooves, launch occurs between 65 and 85 psi. Of course friction coefficients play a big role. As you make repeated launches, the o-ring friction seems to go up. I sometimes have to add a bit of “lubrication” to the o-rings by wiping the side of my nose or forehead with my fingers and rubbing the o-rings. This reduces the launch pressure back down. Different bottle brands can also make a bit of difference. Wal-Mart bottles are too small and too flimsy. Most other brands seem to work. It can be fun for kids to experiment with different brands (flavors) to see which goes at a higher pressure. Long story short, you pump until it launches! August 21, 2006 Q: If compressed air exhausts through a slit directly into ambient air at sea level (no cone or flanges) in the choked flow regime (choke plane across the slit) what is the relationship between upstream absolute pressure and the velocity of the air exhausting from the slit? Is the velocity proportional to mass flow? A: The answer to your first question is not intuitively obvious but the answer is there is no relationship between upstream pressure and velocity at the throat. There are only three variables and none of them are pressure. The equation for sonic velocity is V=sqrt(kRT) where k= ratio of specific heats (sometimes called gamma), R= specific gas constant, (R=R’/M where R’ is the universal gas constant and M is molecular weight) and T is the absolute temperature. For air, k and R are constants, so the only variable governing sonic velocity is temperature. The mass flow rate is a function of pressure because density is a function of pressure. The velocity and flow area don’t change but density does, so mass flow rate goes up with higher pressure. Mass flow rate=PAg/C* where P= pressure, A= throat area, g=gravitational constant and C*= gas characteristic gas velocity. Hence, mass flow rate is directly proportional to pressure.
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