Shock Waves on Apogee Aspire

[RocketRoll]

Just trying to figure something out...

An Apogee Aspire with a G80-7 engine supposedly goes just over Mach 1, if modified to withstand the high drag forces at those speeds.

My question is, will the shocks formed on the nose and at the fins be normal (bow) shocks or oblique shocks?

 

[aerostadt]

Technically, if the nose cone shape is blunt and the model is near Mach 1.0, there can actually be a detached bow shock ahead of the nose cone. In principle if the Mach is 1.0, than there will be a normal shock. If the Mach number is greater than one and the nose cone has a definite point, then the shock will be considered to be attached to the nose cone tip (or very near) and the shock will be oblique. The greater the Mach number is above 1.0, the more oblique will be the shock.

 

[RocketRoll]

Agreed. The problem for me is in determining if the Aspire's nose cone is "blunt" or not. My maximum velocity is projected at ~Mach 1.1, where the maximum allowable deflection angle is ~2 degrees and the shock wave angle is around 70 - 90 degrees.

Here's a picture of my nose cone:

 

[ksaves2]

Have fun finding it and no you won't hear the "sonic boom". I had one last 6 flight of which 3 were the F10-8 with that 8 second burn. That is really cool. Don't spend that much time with the paint as it has a tendency to get beat up with streamer recovery anyways. It's a given if you don't get a visual on descent, you're going to lose it unless it lands on you. If you use a metallized reflective streamer, run some scotch tape on the edges and fanfold the streamer. The tape though it adds weight, will keep the streamer from fracturing and breaking. Had that happen a few times. The fanfolding gets the streamer to rattling and out in the country, unless you have a hearing impediment, I've had my "ears" direct my "eyes" to get that all-important visual fix.

I remember one flight with a two stage where the rattle caused me to turn my head 120 degrees to see the rocket coming in. Landed in standing corn and I had the foresight to have a beeper on the thing too. Walked in 8 rows towards the sound and picked it right up. Remember, one's ears can be an important recovery tool.

The only other advice is you could get lucky by flying at a large launch with many folks running around recovering rockets. You lose yours and someone finds it, most fliers are honest and will return them to the RSO/LSO.
If I stumble on a large rocket, I mark it with a waypoint on my handheld GPS. If it's small, I say screw it and take it back to the table. I returned a rocket with a tracker one time and the flier was flabbergasted the RSO paged
him to get his rocket. He flew it and was going to hunt for it later. Kurt

 

[Bat-mite]

I don't know squat about Mach speeds, so if I may ask, once you have determined it to be oblique or bow shock, how will you use that information?

 

[soopirV]

I don't know squat about Mach speeds, so if I may ask, once you have determined it to be oblique or bow shock, how will you use that information?

Subscribed! I'm curious about this too...

 

[aerostadt]

I don't know squat about Mach speeds, so if I may ask, once you have determined it to be oblique or bow shock, how will you use that information?

Good question. I don't think it will provide a model rocketeer with very much practical information. There are no analytical solutions for finding the location of a detached bow shock, but, of course, a computational fluid dynamics (cfd) computer code can tell you. If you nose cone has a point and is gently curved, there might still be some trouble in determining the shock shape. There are analytical methods readily available in compressible flow text books for finding the oblique shock angle for a 2-D wedge. These are inviscid solutions and do not include a boundary layer. There is an axi-symmetric conical analytical shock solution, but it is hard to find. I think I got one from a government source years ago. Needless to say, the conical shock has more relevance for modelers than a wedge solution. In either case, though, I don't see much practical information here, except one can say that the more oblique the nose cone, the more oblique the shock will be and the strength of the shock will be lower. The Nike Smoke has a very oblique nose cone, which I assume is to take advantage of a oblique shock and lower pressure on the nose. Canted intakes on supersonic jet fighters is quite common. This is to initiate a oblique shock at the inlet, which slows the air flow going into the engine without a large drop in total (or stagnation) pressure.