Two-Stage Rocket Stability and Stage Separation

[Marvin562]

Hey guys I'm a college student part of my school's rocket team. This year we are building a two-stage rocket and expected to reach 10,000ft apogee. One of my concerns is the rocket's stability for a two-stage. I was told a good stable rocket can range from 1.5-3.5 cals. Is the stability the same for two-stage rockets? Reading up on multiple two-stage rocket reports most people start off with 1.8 cals and above, why is that? My team is also having trouble with the stage separation. We are thinking of just doing drag separation but don't know how precise that'll be, so another option is just doing a small black powder charge. What do you guys think, would appreciate your input.

 

[lakeroadster]

Reading up on multiple two-stage rocket reports most people start off with 1.8 cals and above, why is that?

The short answer is Fear of Failure.​

I build a lot of scratch build / scratch design odd-rocs with stability around 0.7 calibers.​

Launch Rail Length and Rocket Exit Speed are keys to successful launches.​

 

[Titan II]

https://www.rocketryforum.com/threads/separation-charge.184185/

 

[Rschub]

The short answer is Fear of Failure.​

I build a lot of scratch build / scratch design odd-rocs with stability around 0.7 calibers.​

Launch Rail Length and Rocket Exit Speed are keys to successful launches.​

I would add that the upper stage can go supersonic on many 2-stage projects and stability is reduced at those speeds.

 

[waltr]

Stability must be analyzed on the full stack (booster + sustainer) and then for the Sustainer.
Less than 1 cal is OK if not approaching Mach1. Else an additional margin is need since CP tends to move back around Mach1.

Many have tried stage separation by drag but typically this does not happen. Therefore a small separation charge is desired. Separation can be obtained from the sustainer motor firing but may fry the ISC.

Keep going through and reading the various threads on 2-stage rockets. Lots of good info in these threads.

Keep this thread going please by adding details of your design and build.

 

[MClark]

Calibers is ok measurement for a typical length rocket, not long or short.
For long rockets 10% of total length or greater is better.
Heavy rockets the minimum stability sometimes doesn’t work. It may be stable but does not have enough righting force and can tumble out of the tower. Fins a bit large may have more drag but only attached for a few seconds.

Nothing like watching a multi stage O motors going end over end and hoping the sustainer doesn’t light.

 

[JimJarvis50]

Calibers is ok measurement for a typical length rocket, not long or short.
For long rockets 10% of total length or greater is better.
Heavy rockets the minimum stability sometimes doesn’t work. It may be stable but does not have enough righting force and can tumble out of the tower. Fins a bit large may have more drag but only attached for a few seconds.

Nothing like watching a multi stage O motors going end over end and hoping the sustainer doesn’t light.

Yeah, what he said. If you think about stability as a percentage of the length, then you might conclude that you want 2-3 calibers on the stack.

Regarding a separation charge, I typically use them just in case drag separation doesn't occur. One thing you can do to investigate drag separation is to perform your simulation where you delay stage separation for a second or so after motor burnout, and then look at the acceleration profile from the simulation. It is not an exact way to perform the calculation, but if you see less negative acceleration after stage separation, such as in the attached picture, then the pieces will want to separate, and vice versa. Even if that is the case, though, remember that there are forces on the joint during an actual flight that may prevent drag separation from occuring (e.g., an angle of attack, fins that aren't straight, rotation, etc). A separation charge is cheap insurance.

Jim

 

[Jmhepworth]

I use about a half gram of black powder in a glove tip for a separation charge, setting it off from the booster. You want to lose the booster as soon as possible. It’s just a drag on altitude otherwise.

 

[SolarYellow]

There's an older rule of thumb that stability factor of 8-15% of the rocket's overall length is desirable. That's a lot more useful in adapting from short/fat to long/skinny rockets than the "caliber" rule. It's one example of error in denominator selection getting embedded in this hobby. I usually shoot for 10-12%. Discussing this with @neil_w is one of the things that led to the current release of Open Rocket reporting both bases at the same time.

Also, this (Please click through to the thread to read the rest of the discussion, but this is the meat relevant to this one):

For those on this thread who have actually flown supersonic aircraft, remember that for most of our rockets they get more stable from Mach 0.90 to Mach 1.05, then the rocket CP moves forward, but it usually only gets back to its subsonic value by around approximately Mach 2. As the Mach number increases above approximately Mach 2, that is when the CP moves forward of the subsonic value. If you were only flying at Mach 1.5 to Mach 1.8, you'd still be in the "more stable than subsonic" range. (With a whole bunch of details for an aircraft versus a rocket.)


Charles E. (Chuck) Rogers
Rogers Aeroscience

Also, read up on wind-caused instability. Especially with a long, multistage rocket, the forward CP shift at rail clearance due to any cross wind can be significant, which is another reason leading to bigger stability factors in carefully designed rockets.

 

[JimJarvis50]

I use about a half gram of black powder in a glove tip for a separation charge, setting it off from the booster. You want to lose the booster as soon as possible. It’s just a drag on altitude otherwise.

There are several reasons to separate quickly, but higher altitude isn't necessarily one of them. A good example might be the rocket of the OP. A 10K two-stage flight will often have a larger booster motor to get the rocket moving and then a smaller sustainer motor to limit altitude to 10K (10K two-stagers are difficult to design in that respect). Such a rocket would go higher with a delayed separation. The main reason to separate quickly is because the stack can get very overstable after the booster motor burnout, so it's a good idea to separate the stages. However, be careful not to accidently separte before burnout actually occurs. Burnout detection typically occurs when no acceleration is measured. However, the booster motor may still be burning and producing thrust. For some motors, this can go on for a second or so, or even longer. You can examine the thrust curve of commercial motors, but research motors may be less certain. It's a good idea to delay separation for just a bit to avoid separation while there is still thrust. That never ends well.

Jim