Mach Pressure Change

Does anyone know how big the pressure spike is when you approach Mach 1, and what the perceived drop in altitude is due to that spike? I've been looking all over for something to quantify this, and haven't had any luck...

Check out https://www.rocketryforum.com/showthread.php?16044-MAWD-crash-with-strange-data&p=140733#post140733 from https://www.rocketryforum.com/showthread.php?16044-MAWD-crash-with-strange-data

In the case I plotted the subsonic to supersonic Mach transition wave looked like a 500' drop in altitude. The supersonic to subsonic Mach transition wave looks like a 300' drop in altitude.

The magnitude of the Mach transition wave depends on the rocket shape and acceleration at the time of the Mach transition.

Bob

Thanks Bob. The interesting thing about that graph is that the subsonic-supersonic transition happens at a low altitude, a few hundred feet, so the pressure altitude drops way below AGL zero. It also occurs very early, about T+1, which makes sense since this transition is most likely to occur during motor burn, and that's a pretty high thrust motor. I can see how this would have screwed up the deployment on baro-only altimeters without any kind of adjustment for this behavior. They're thinking, whoa, we're way past apogee, pop the chutes.

Is it possible for the shock wave to occur AFTER burnout? I'm thinking not, because at that point the rocket is going to start decelerating, so if it hasn't built up at that point it probably won't...

bobkrech said:

Check out https://www.rocketryforum.com/showthread.php?16044-MAWD-crash-with-strange-data&p=140733#post140733 from https://www.rocketryforum.com/showthread.php?16044-MAWD-crash-with-strange-data

In the case I plotted the subsonic to supersonic Mach transition wave looked like a 500' drop in altitude. The supersonic to subsonic Mach transition wave looks like a 300' drop in altitude.

The magnitude of the Mach transition wave depends on the rocket shape and acceleration at the time of the Mach transition.

Bob

Click to expand...

bobkrech said:

Check out https://www.rocketryforum.com/showthread.php?16044-MAWD-crash-with-strange-data&p=140733#post140733 from https://www.rocketryforum.com/showthread.php?16044-MAWD-crash-with-strange-data

In the case I plotted the subsonic to supersonic Mach transition wave looked like a 500' drop in altitude. The supersonic to subsonic Mach transition wave looks like a 300' drop in altitude.

The magnitude of the Mach transition wave depends on the rocket shape and acceleration at the time of the Mach transition.

Bob

Click to expand...

I concur it varies a lot depending on the rocket. The effect is sometimes filtered out entirely by having a relatively small pressure port and a large-volume av-bay.

Here are a few more sets of data:

https://www.rocketryforum.com/showthread.php?3499-I600-flight-to-15965-feet
https://www.tripoli.org/Membership/CompetitionsandRecords/LSingleMotorRecord/tabid/312/Default.aspx
https://www.tripoli.org/Membership/CompetitionsandRecords/HSingleMotorRecord/tabid/302/Default.aspx
https://www.tripoli.org/Membership/CompetitionsandRecords/JSingleMotorRecord/tabid/328/Default.aspx

It's also pretty common for acceleration-related effects to mess up the pressure readings during the burn, too. In particular, just about everyone's chute shifts backwards under thrust, and if there is any leak from the forward compartment into the av-bay, the chute acting like a piston can change the pressure in the av-bay noticeably. If there is a long av-bay, just the weight of the accelerating air can add apparent pressure. (an altimeter sitting at the bottom of a 1 foot column of air under 50 Gs sees a pressure change of 50' at 1 G)

Adrian A said:

I concur it varies a lot depending on the rocket. The effect is sometimes filtered out entirely by having a relatively small pressure port and a large-volume av-bay.

Here are a few more sets of data:

https://www.rocketryforum.com/showthread.php?3499-I600-flight-to-15965-feet
https://www.tripoli.org/Membership/CompetitionsandRecords/LSingleMotorRecord/tabid/312/Default.aspx
https://www.tripoli.org/Membership/CompetitionsandRecords/HSingleMotorRecord/tabid/302/Default.aspx
https://www.tripoli.org/Membership/CompetitionsandRecords/JSingleMotorRecord/tabid/328/Default.aspx

It's also pretty common for acceleration-related effects to mess up the pressure readings during the burn, too. In particular, just about everyone's chute shifts backwards under thrust, and if there is any leak from the forward compartment into the av-bay, the chute acting like a piston can change the pressure in the av-bay noticeably. If there is a long av-bay, just the weight of the accelerating air can add apparent pressure. (an altimeter sitting at the bottom of a 1 foot column of air under 50 Gs sees a pressure change of 50' at 1 G)

Click to expand...

Yes! Two excellent and little known points.

Wouldn't the small port/large bay cause a lag in the pressure readings? I'm guessing that it's actually this lag that prevents the perceived pressure spike, since it's a transient event and might pass before the pressure in the bay has a chance to catch up with ambient pressure. Kind of a mechanical low-pass filter...

Adrian A said:

I concur it varies a lot depending on the rocket. The effect is sometimes filtered out entirely by having a relatively small pressure port and a large-volume av-bay.

Here are a few more sets of data:

https://www.rocketryforum.com/showthread.php?3499-I600-flight-to-15965-feet
https://www.tripoli.org/Membership/CompetitionsandRecords/LSingleMotorRecord/tabid/312/Default.aspx
https://www.tripoli.org/Membership/CompetitionsandRecords/HSingleMotorRecord/tabid/302/Default.aspx
https://www.tripoli.org/Membership/CompetitionsandRecords/JSingleMotorRecord/tabid/328/Default.aspx

It's also pretty common for acceleration-related effects to mess up the pressure readings during the burn, too. In particular, just about everyone's chute shifts backwards under thrust, and if there is any leak from the forward compartment into the av-bay, the chute acting like a piston can change the pressure in the av-bay noticeably. If there is a long av-bay, just the weight of the accelerating air can add apparent pressure. (an altimeter sitting at the bottom of a 1 foot column of air under 50 Gs sees a pressure change of 50' at 1 G)

Click to expand...

cerving said:

Is it possible for the shock wave to occur AFTER burnout? I'm thinking not, because at that point the rocket is going to start decelerating, so if it hasn't built up at that point it probably won't...

Click to expand...

You would be thinking wrong.

An object does not have to be traveling at Mach 1 or greater to generate a shock wave. Shock waves can be generated in the transonic region which is normally considered to begin around Mach 0.8. Air flowing over curved surface such a a nose cone, wing or fin on an air vehicle will accelerate and speed up. For air vehicles traveling in the transonic zone, at some velocity below Mach the air passing over the curved nose cone or wing will reach Mach 1 and form a shock wave. As the vehicle increase speed to Mach 1 this shock wave moves forward toward the leading edge where it attachs at velocities of Mach 1 or greater. Another shock wave moves aft and attachs to the trailing edge and base of the rocket.

Similarly as a rocket or aircraft decelerate from supersonic to subsonic speed, the shock wave moves in the opposite direction over the airframe.

The shock waves don't care if the motor is burring or not. They only depend on vehicle speed, and they only move up and down the airframe it the transonic zone. Once the vehicle exceeds Mach 1 they attach to any discontinuities on the airframe and stay there. Check out https://en.wikipedia.org/wiki/Schlieren_photography for shock wave visualization.


[YOUTUBE]atItRcfFwgw[/YOUTUBE]

Bob

But the rocket should be slowing after burnout... so shouldn't the shock wave begin subsiding assuming that it never makes it to Mach 1 during the motor burn?

Yes, I know that the shock wave does build up way before Mach 1... the question is, how much before, and what is the pressure altitude differential caused by it?

cerving said:

But the rocket should be slowing after burnout... so shouldn't the shock wave begin subsiding assuming that it never makes it to Mach 1 during the motor burn?

Yes, I know that the shock wave does build up way before Mach 1... the question is, how much before, and what is the pressure altitude differential caused by it?

Click to expand...

If your rocket does not exceed Mach 1, you should be able to find a location where there isn't a mach wave moving across the sampling port. The location and diameter of the altimeter sampling port(s) greatly effect the observed pressure differential as does the physical size of the rocket. There is not simple way to predict what pressure change will be observe on the passage of a mach wave.

If you do observe a transonic pressure disturbance as the velocity increases, you should see a corresponding disturbance when the velocity increases. You are not observing the the shock wave directly, but rather the pressure disturbance produced at the shock wave attachment point as it moves along the airframe. This only occurs in the transonic region at or near Mach 1 and they are transient. Once you are above Mach 1, the shocks attach to discontinuities on the rocket: The nose cone tip, the transition between the base of the nose cone and the top of the airframe, the top and bottom of launch lugs or rail buttons, the leading and trailing edges of fins, and the base of the rocket are all locations where shocks attach. At Mach 1, the shock angle is perpendicular (90 degree) to the centerline axis of the rocket. As the Mach number increases, the attachment points don't change, what changes is the Mach angle (shock angle).

NASA has several tutorials that explain sound speed and shockwaves. https://www.grc.nasa.gov/WWW/k-12/airplane/guided.htm The details of the velocities and locations where shock form below Mach 1 are too complex to go into here. If you are interested, google is your friend.

Bob