Sonic Dreams

[moocrew]

Alright so I've been watching this stuff about SpaceShipOne on the discovery channel, just saw a thing about Chuck Yeager on the history channel, and it got me all excited...and thinking...

Alright Mach 1 is about 700 mph...right?
Whats the speed of sound?/How fast do you have to move to break the sound barrier?
Will anything that breaks the sound barrier make a sonic boom?.....even a small rocket?

My goal here is to make a VERY small roc that will break the sound barrier very close to the ground.
So a highpower cluster is probably in order?

Any help here is great. I need some info on this before i go do anything too stupid.

thanks -matt

 

[phaar]

Well, one thing you should know is that there is not a "sonic boom" when a rocket is launch vertical. It has to be going horizontal to hear much. Although, if the rocket is large enough you will hear a pop, so I have been told. I have also heard that you must be directly under the path of the rocket. Basically, flying supersonic is cool just to say you did it.
Second, as you go up, the air gets thinner, making it easier to fly supersonic due to less air resistance. This is why Chuck Yeager had to fly it so high. Therefore, the higher you fly, the better chance you will have to break Mach I when the engine is at its peak thrust. If you make a small rocket, you will want to make sure that it is glassed etc. etc... If you do not, it will just crush as it hits the sound barrier. But, if it is too small, you will not hear ANYTHING, not to mention it will probably not come back.

 

[r1dermon]

a common misconception is that the sound barrier is like...700mph. thats only at sea level, where the air pressure is 14.4psi. way up high, the sound barrier can be as low as 620mph. the sonic boom is an effect that is not well understood. we know that it happens because of the speed that is attained, and that a cyclonic cloud is formed from air pressure. the big thing is that your body tube HAS to be strong at the top, where the nose cone is, unless you have a really small diameter rocket. because once it breaks the sound barrier, it starts fighting with itself, and its pretty unpredictable at that point. it can start going in a spiral etc..your nose cone should be strong, but the top of the body tube, where the nose cone fits into, i'd recommend glassing about 1cm on the outside of it. for safetys sake. also, at those speeds, you'll need a lot of glass on the fins. and small fins. a lot of motors can reach supersonic, as long as they burn long enough. and your rocket is light enough. but, at sea level, its going to be tough, to get a rocket to hit supersonic within 100' up is quite difficult. unless you stuff an H motor inside a 2 ounce rocket. lol. rockets are so small however, that they dont produce much of a boom. in the event that you can actually hear one over the motor itself, it wouldnt be very loud, as you are directly under the rocket and all the sonic forces are being pushed aside by a wedge(the cone). going supersonic isnt really a noticeable thing, other than you being able to see the rocket get out of sight EXTREMELY fast. i'd consult a physicist on this one!

 

[edwardw]

Actually, the speed of sound at sea level is 761.2 miles per hour, or 340.29 meters per second.

Also, the pressure at sea level is 14.696 psia (absolute pressure).

The speed of sound can drop below 620 mph, but it only does it at VERY high altitudes. From 280,000 to 290,000 feet the speed of sound is 613.2 mph.

The lowest it goes for most rocket flights is from 40,000 to 65,000 feet and there it is a constant speed of 660.1 miles per hour.

Here is a chart of Mach Number and altitude

Mach Table

Just getting the right data out there.

Edward

 

[bobkrech]

Very good reference Edward. Everyone needs to read it.

If you go to Edward's reference site https://www.aerospaceweb.org/question/atmosphere/q0126.shtml

you will see that the speed of sound depends only on the local temperature of the air and the composition of the air (through the ratio of specific heats which for sea level altitudes means slight differences in the humidity levels).

From the above reference.

"...the speed of sound is not a single value, but changes with altitude. To be more precise, the speed of sound (a) can be directly calculated based on the air temperature (T), and temperature is a function of altitude. A procedure for calculating the temperature, as well as the density and pressure, using the standard atmosphere model was described in a previous question (learn more). Towards the end of that explanation, an equation for calculating the speed of sound based on temperature is also introduced. This equation is based on the more general form

a= sqrt( g R T )

where

a = speed of sound [ft/s or m/s]
g = specific heat ratio, which is usually equal to 1.4 (for dry air)
R = specific gas constant, which equals 1716 ft-lb/slug/°R in English units and 287 J/kg/K in Metric units
T = atmospheric temperature in degrees Rankine (°R) in English units and degrees Kelvin (K) in Metric units
Once the speed of sound is known, the Mach number can be easily computed by dividing the airspeed of the vehicle by the speed of sound.

... The Mach number (M) is simply the ratio of the vehicle's velocity (V) divided by the speed of sound at that altitude (a).

M = V / a. "

Now to correct some misconceptions presented in the thread.

r1dermon

There is no "sound barrier" and the generation of sonic booms is well understood.

When a rocket exeeds the speed of sound, it is the the interaction of the surrounding air that changes. Instead of simply compressing the air in front of it, the rocket generates a shock wave. The subsonic air compression interaction in front of a rocket is themodynamically reversible where as the supersonic air compression is not.

phaar

A sonic boom is always generated anytime an object goes supersonic. To hear it you have to be in the right place.

A small object shocks a relatively small volume of air, and the shock wave get weaker and slower as it travels away from the rocket. At some point, the wave slows down to the speed of sound and becomes a simple sound wave. All sound waves travel at the local speed of sound as calculated from the equation above. The intensity of any sound wave decreases as 1/D² (distance squared) from the source and at some point is too weak to be heard.

You do not need to glass a rocket that will exceed Mach 1. Again there is nothing magical about it. As it always the case, the total load on your rocket equals the instantaneous thrust of the motor. Sustained Mach flight places certain thermal constraints on the vehicle skin becase of the loss of strength due to aerodynamic heating, but this not an issue for model and most high power rockets.

There is no cyclonic cloud formed due to a shock wave. I think you probably referring to the spectacular video of the Navy jet traveling a Mach 1 near a carrier at sea level and have observed the formation of a conical condensation cloud caused by the expansion and cooling of the air after the shock wave has passed.

Bob Krech

 

[r1dermon]

ok, i took physics in HS, i know for a FACT, that there is always a cyclonic cloud when the sound barrier is exceeded. and thats what i was referring to that was not well understood. there are many theories, including the one you stated, as to the rapid cooling and condensation of the air that is being depressurized around the object that is exceeding the speed of sound. it works like a BB. do you have a BB gun? well, when you go to buy a BB gun, more often than not, you will ask for the fastest one. 1250fps...etc...now, 1250fps is faster than sound at sea level. take that BB gun, assuming its .177 caliber, and shoot it at a target 50ft away with a flat head pellet. now take a 1000fps pellet gun and shoot the same BB at the same target from the same distance. guess what? that 1000fps BB gun will be more accurate. because the pellet is fighting with the air pressure around it to go faster. not only that, but initially there is a good amount of turbulance created, which creates many fluctuations in air pressure, which puts great force on your rocket, which is why i recommended glassing. also, how would you know if no glassing is needed? do you know the specs on the rocket? all you know is that its a tube with a cone and fins that is going to break mach. you dont know if they are balsa fins, paper fins...etc...if they are steel fins then they obviously wouldnt need glassing, but if they are cardboard...would YOU recommend glassing?
if i can find the website that i am referring back to then i will be overjoyed to post it for your enjoyment.

 

[daveyfire]

Originally posted by r1dermon
ok, i took physics in HS, i know for a FACT, that there is always a cyclonic cloud when the sound barrier is exceeded. and thats what i was referring to that was not well understood. there are many theories, including the one you stated, as to the rapid cooling and condensation of the air that is being depressurized around the object that is exceeding the speed of sound. it works like a BB. do you have a BB gun? well, when you go to buy a BB gun, more often than not, you will ask for the fastest one. 1250fps...etc...now, 1250fps is faster than sound at sea level. take that BB gun, assuming its .177 caliber, and shoot it at a target 50ft away with a flat head pellet. now take a 1000fps pellet gun and shoot the same BB at the same target from the same distance. guess what? that 1000fps BB gun will be more accurate. because the pellet is fighting with the air pressure around it to go faster. not only that, but initially there is a good amount of turbulance created, which creates many fluctuations in air pressure, which puts great force on your rocket, which is why i recommended glassing. also, how would you know if no glassing is needed? do you know the specs on the rocket? all you know is that its a tube with a cone and fins that is going to break mach. you dont know if they are balsa fins, paper fins...etc...if they are steel fins then they obviously wouldnt need glassing, but if they are cardboard...would YOU recommend glassing?
if i can find the website that i am referring back to then i will be overjoyed to post it for your enjoyment.

Yeesh man. Breathe a little

Perhaps the "cyclonic" cloud you are referring to is instead a "conical" shape. This is simply the pressure wave moving out and away from the object as it breaks the speed of sound. When the pressure wave passes over you, you hear the sonic "boom", (which is just a pressure spike). That's why it rattles the windows on your house. For objects with multiple protrusions going really really fast, you will often hear a double sonic boom as both parts of the vehicle create their own cone. When a rocket breaks the speed of sound high up, you won't hear a sonic boom -- the shock wave has already passed over you. Frank Kosdon used to launch minimum diameter O10,000s... THOSE you could hear a sonic boom from

Think about it as a wake behind a boat. As the boat pushes the water out of the way in front of it, it has to go somewhere, so it pushes out and around the boat in a conical pattern. A corollary to this is that a rocket that has passed through the speed of sound often has less turbulence acting on it (and as such less drag) than a rocket dancing in the transonic region. As you approach sonic, all the pressure waves gather up in front of the rocket. This obviously causes lots of problems drag- and strength-wise. You can see a definite increase in C_d vs. velocity plots from accelerometer data as Mach 1 approaches, and a LARGE drop-off as the rocket exceeds the speed of sound. This is what causes the sonic boom -- all these pressure waves all of a sudden get out of the way of the rocket and it's outrunning its own sound. As long as you pass through this turbulent zone quickly and smoothly, almost *anything* can fly Mach. It's here that most fins are shredded and tubes are crumpled.

You don't necessarily need glass on a well-built and aligned model, either. I've sent unglassed cardboard rockets through Mach before with no troubles at all. As a matter of fact, glass often adds unnecessary weight to the vehicle, often restricting its top speed. The key words here are "properly aligned". As long as the rocket remains pointing at 0 degrees AOA (or close to it) going through the sound barrier and is rigid along its entire length, you won't have troubles. Make sure that your couplers are at least (more is better) 1 caliber into each airframe. I'd make 'em 1.5 calibers to insure exact alignment.

Just a little practical experience talking. HTH!

 

[HeadHunter]

I belong to the AEROPAC club on the west coast and thought you might get a kick out of this vid. It shows fin flutter at mach... with some MPEG4 engine artistry thrown in. The flutter excceded frequency that the camera could record. But needless to say you need to build'em tough to join that club, mistakes quite often end up rainning down over a large area

 

[r1dermon]

right, but davey, on models that CAN break mach, but are cheap enough to afford, often times, there are no fin slots to attach to. therefore its glue alone thats holding that fin on...takes the LOC aura, i just bought it offline for 22 dollars, it has no slots for fins, are you suggesting that it will be fine with wood glue on a G80? this thing weighs like...12 ounces with a motor. in this case, glassing would probably be a good idea...would you agree?

 

[daveyfire]

Originally posted by r1dermon
right, but davey, on models that CAN break mach, but are cheap enough to afford, often times, there are no fin slots to attach to.

It works.

No joke!

You just gotta design the rocket correctly -- fins with relatively small span, long(er) root chord, swept, and asymmetrical in profile will all help keep the fins on. A LOC Aura won't survive going through Mach without glass, but a LOC Lil' Nuke will. Seen it done too.

Flutter is your main enemy when reaching high velocities. Usually when a rocket shreds going through Mach, it's because the fins were fluttering so fast that they just tore the rocket apart. If you build the fins with material of a proper thickness, designed correctly, and attached well, you will have no problems going through Mach. It's not some mystical "impossible-to-reach" speed.

Attached is a photo of my Laser/LOC 2.1 taking off on a J570. Mach 1.2ish and a little over 12,000 feet. This was with surface-mounted fins. No problems, except it separated at apogee and came in flat with a slight zipper.

 

[r1dermon]

thats what im talking about...fin flutter. im not worried about a LOC tube, that thing is as rigid as they come. the thing that worries me and makes me thing...'glass' is the fact that there is only a bit of glue holding my fins on that will be experiencing tremendous lateral forces as the rocket rotates. thats why, before i would fly an aura on anything G, im glassing.

 

[daveyfire]

Originally posted by r1dermon
thats what im talking about...fin flutter. im not worried about a LOC tube, that thing is as rigid as they come. the thing that worries me and makes me thing...'glass' is the fact that there is only a bit of glue holding my fins on that will be experiencing tremendous lateral forces as the rocket rotates. thats why, before i would fly an aura on anything G, im glassing.

Go for it... doesn't need it, but you're welcome to add excess weight. If you're flying an H180, that's another story. But for G80s and even G125s, the rocket is going slow enough that it's not that big of a deal.

Epoxy is reeeeeeeeeeally strong when used properly!

 

[r1dermon]

right...too strong in a sense. so strong that it will NOT give any leeway. so when the fin moves, it crimps the body tube just as much as it moves. thats why glassing gives it a better base. not only that, but i've read testimonials on auras and 3 out of 4 people using a G-80 had lost a fin. its better safe than sorry, i'd rather add weight to an already superb performer and be able to fly it 4-5 times in a day, than to fly it once and have to call it a day.

 

[moocrew]

all good points. Other than I would probably scratch build mine and be using a type of plastic tube i have...its in credibly ridgid...i can't even break it in half when i stand on it. and at only 6oz for a 3ft section its not TOO heavy but packs some weight. and a 2" diameter is more than enough to pack a G in there and some other boosters of choice. I may or may not do TTW with the fins...its not really any added work to do so....looks like I'm just going to have to give this thing a few jabs and see what happens...thanks for your info! ill be sure to post pics..IF these babies even make it into the air

-Matt

 

[GL-P]

A good idea is to use a real light core material for your fins.Replace your plywood fins with a lightweight material then carbon fibre the fins tip to tip. You get a lighter weight but get ultra strong fins. Suggested cores:

Basswood
balsa
foamcore