Why is adding a second stage not increasing the velocity?

[Alan15578]

Drag increases hugely with velocity and is maximum at transonic speeds. You (the G80) don't have enough thrust to punch through to supersonic and keep it there. So yeah, the second motor also does nothing. Most likely, with those ridiculous tiny ineffective fins, the vehicle will suffer transonic instability and fall apart. Look really carefully at those graphs from the simulation, they're talking to you.

It is Cd that peaks at transonic speed, not drag. The the measured and documented 80 NS record is Mach 1.42 with an AT F80. Even the old AT G60 could go supersonic, not faster than the G80, but carrying instrumentation. The AT G80 is actually more like a G100 with a thrust tail that lowers the average thrust to a modern legal MR motor. The AT G80 is the way to go for max speed or Mach with a model rocket motor.

The Nv7 design is another matter. It is rather heavy. I would suggest starting with the Apogee Aspire and reading all the anecdotal reports of other rocketeers. Two stage might be fun to sim, but is absolutely nuts if you want observe or measure supersonic MR performance.

 

[jderimig]

It certainly increases the velocity compared to what the velocity would be after 1st stage burnout without a second stage motor.

 

[dhbarr]

g74w is an interesting one

• peak thrust is 95N
• burnout weight is only 48g
• overall length is only 93mm

I think it can just barely almost get over M1 ( but not fully supersponic ) by itself, maybe.

 

[bjphoenix]

If you can find someone that has a 24mm F72...

 

[Art Upton]

Actually I realized the OpenRocket staging was setup wrong (stage separation was set to ejection charge instead of when top stage motor ignited) so it was carrying the booster up, I fixed that and now it hits Mach 1.3. Do you think the booster stage needs larger fins or the top stage?

I think you need to test this on 24mm motors first and then on how to do staging with AP motors

 

[Dave P]

The Handbook of Model Rocketry by G Harry Stine has a whole chapter on this.

It boils down to staging velocity.

He explains why staging should always be at booster burnout with no delay for max altitude.

 

[dhbarr]

The Handbook of Model Rocketry by G Harry Stine has a whole chapter on this.

It boils down to staging velocity.

He explains why staging should always be at booster burnout with no delay for max altitude.

Max speed could actually improve a bit if you use a timer to stage before there's a detectable dropoff in thrust.

Max altitude is definitely not zero-delay staged.

 

[Dave P]

So you read the chapter?

 

[cerving]

BP motors HAVE to stage at burnout, no choice. If you stage electronically you have choices... and if you do some sims you'll find that you get more altitude if you delay staging. You have to keep pointing "up" however... that's the trick.

 

[Dave P]

Model rocketry development is empirical. You have to do the work. Sims are great but the mesh is not 100% accurate yet.

The ground truth of build and material quality, environment, and unexpected variables effect the results.

I am not talking about space launch vehicles, even though the Starship is moving from a delayed ignition to zero delay ignition of second stage. The explanations why are out there.

The fundamentals in Handbook are largely overlooked today in the rush to certify. They still apply; while a revision of physics is needed, those rules work well in most hobby rockets.

Model rocketry is empirical. You can do everything right and still get a failure. Motors don't burn symmetrically, and and BP explosions are asymmetrical.

 

[Lotronex]

BP motors HAVE to stage at burnout, no choice. If you stage electronically you have choices... and if you do some sims you'll find that you get more altitude if you delay staging. You have to keep pointing "up" however... that's the trick.

If only someone could make a replacement for the RTOM3. I'd love to do airstarts.

 

[cls]

The fundamentals in Handbook are largely overlooked today in the rush to certify. They still apply; while a revision of physics is needed, those rules work well in most hobby rockets.

this can't be overstated!!! so many times as mentor I have insisted the groups read the Handbook and understand the relevant sections, and gotten nowhere... blank looks all around. what, read an old book? but I just want the answer right now on my cell phone!?!? witness also the recent discussions about elliptical fins, that's thoroughly discussed in the Handbook.

TVM's Model Rocket Design and Construction has a lot of the same good info, and more. There's plenty of good information available.

 

[Dave P]

It's far faster, cheaper, and comprehensive to buy the book and read it to learn all those lessons than do it yourself. After that Sutton.

My first composite 2 stage was a US Rockets Dream Machine on two J's. Used all methods to stage it.

It was called "Whatever Happened to Baby Js."

 

[Spacedog49Krell]

Could I use rail guides on the booster or would a fly-away rail guide be necessary? It only hits mach 0.69, wondering how much drag the rail guides create at that velocity.

Paper tubes and balsawood fins will not withstand supersonic flight dynamics. I took your OR file and made the basic changes for Mach 1.5 flight. There are fine details that are still required for an actual flight. I changed your body tube to 0.5mm wall carbon fiber. A single layer of polyester glass over a BT-55 tube could also be used. Fins made from 1.5mm thick G-10. I replaced your external tube coupler with four fins that extend over the booster stage to form a slip fit stage coupler. This also improves the second stage stability. The second stage motor should be ignited 1.3 seconds after the booster. This is where the booster acceleration starts to deteriorate. Ignition of the second stage at this time will propel it to ~1.5 Mach with an altitude of 9500-10100 feet. A reduction of the flight computer mass will result in a higher Mach velocity. An increase in overall mass will increase the peak altitude.

This flight will require a very large field, 12-16 sq miles, and a tracking locator. It will not be visible after the second stage burnout. You may need to lengthen the booster to include a streamer for its recovery.

As for fly-away rail guides, the high launch velocity will result in all three or four fins striking the rail guide before it clears the fins. This would not be good for velocity or flight direction.
Edit: I recommend only flying this rocket from a tower.

Attaining supersonic velocity was easier in the early years of APCP motors. Today's, motors are engineered for reliability and safety. They are regressive thrust profile motors designed to lift heavy model rockets to a stable launch velocity. The early EnerJet and PlasmaJet motors had progressive thrust profiles delivering maximum thrust just before burnout. Even F motors, in light weight vehicles, could reach Mach 1 for a few milliseconds.

 

[Nv7]

Paper tubes and balsawood fins will not withstand supersonic flight dynamics. I took your OR file and made the basic changes for Mach 1.5 flight. There are fine details that are still required for an actual flight. I changed your body tube to 0.5mm wall carbon fiber. A single layer of polyester glass over a BT-55 tube could also be used. Fins made from 1.5mm thick G-10. I replaced your external tube coupler with four fins that extend over the booster stage to form a slip fit stage coupler. This also improves the second stage stability. The second stage motor should be ignited 1.3 seconds after the booster. This is where the booster acceleration starts to deteriorate. Ignition of the second stage at this time will propel it to ~1.5 Mach with an altitude of 9500-10100 feet. A reduction of the flight computer mass will result in a higher Mach velocity. An increase in overall mass will increase the peak altitude.

This flight will require a very large field, 12-16 sq miles, and a tracking locator. It will not be visible after the second stage burnout. You may need to lengthen the booster to include a streamer for its recovery.

As for fly-away rail guides, the high launch velocity will result in all three or four fins striking the rail guide before it clears the fins. This would not be good for velocity or flight direction.
Edit: I recommend only flying this rocket from a tower.

Attaining supersonic velocity was easier in the early years of APCP motors. Today's, motors are engineered for reliability and safety. They are regressive thrust profile motors designed to lift heavy model rockets to a stable launch velocity. The early EnerJet and PlasmaJet motors had progressive thrust profiles delivering maximum thrust just before burnout. Even F motors, in light weight vehicles, could reach Mach 1 for a few milliseconds.

Thank you! I was thinking of 3d-printing it out of something like ABS but perhaps I should attempt to make it out of the materials suggested instead.

I will definitely wait to launch this and develop better electronic systems that could handle the difficult requirements of this flight. (need to add better pyro capabilities, GPS, radio or some sort of ground signaling system)

 

[dhbarr]

So you read the chapter?

Yes, I have. If the entire world of model rocketry were direct staged pintle burning black powder, my points would not stand.