Should I use sheer pins on Hot Stage Separation

[Alan Offer]

Good morning, my name is Alan Offer and I want to build a two stage rocket using Hot stage Separation to deploy. My design criteria is that I need to build a separation mechanism for two stage having a minimum diameter and a 98mm motor. I made a CAD (see the figure) of how the separation mechanism will work. First the sustainer stage will be ignited using the booster flight computer that you see on the bottom of the bulkhead ( two ignition wires will go up the upper stage, one from a flight computer made by me and the other is a Blue Raven from Featherweight altimeters). Then the nozzle of the Pro 98 N1100 will be insert into the conical support made in aluminium. Finally I will drill 6 vent holes so that the pressure will be evacuated. My question is, in order to ignite the second stage I need the first stage avionic, should I put sheer pins to prevent it to stage prematurely? Or do you think is ok ? If I have to put sheer pins, where should I place them.

Thank you and have a great day,
Alan from IMF.Rockets

 

[cbrarick]

i guess the first question is "why would you do that?"

Hot staging a 2 stage MD 98 mm rocket seems to defeat the purpose of the second stage. look at sims using different coasting times before lighting the sustainer...........

 

[Alan Offer]

i guess the first question is "why would you do that?"

Hot staging a 2 stage MD 98 mm rocket seems to defeat the purpose of the second stage. look at sims using different coasting times before lighting the sustainer...........

Thanks for the response. You think is better to have either two diameters or two diameters greater than the motor diameter ? Because either case it's going to decrease the apogee of the rocket

For the sims, what should I look when changing the coasting time before lighting the sustainer ? Thanks

 

[WizardOfBoz]

Simulation engineer here. That’s the beauty of simulation. You can run several scenarios and choose the best compromise between maintaining enough velocity to retain stability, and maximum apogee. And other stuff that others with more experience identify here. Given that the math models aren’t calibrated for your rocket, leave a little breathing room to ensure that the rocket is still pointing up when the sustainer is ignited.

 

[OverTheTop]

Hot staging is great if you want to carry the velocity to the second stage. It is not usually done in HPR.

The delay between staging allows the velocity to come down, reducing peak velocities and drag on the sustainer. I did an O3400-M2020 and it had a 15.5 second staging delay.

It just depends on what your goals are.

 

[DaHabes]

Hot staging is great if you want to carry the velocity to the second stage. It is not usually done in HPR.

The delay between staging allows the velocity to come down, reducing peak velocities and drag on the sustainer. I did an O3400-M2020 and it had a 15.5 second staging delay.

It just depends on what your goals are.

If you hot stage the sustainer, then you better do a finsim to make sure you don't shred the fins on the sustainer, especially if this is a MD design. What is the max velocity of this design?

 

[Alan Offer]

If you hot stage the sustainer, then you better do a finsim to make sure you don't shred the fins on the sustainer, especially if this is a MD design. What is the max velocity of this design?

The maximum velocity of the rocket is 902m/s and the velocity at stage separation if we have a delay of 1s is 470m/s. Is there a way to know at what speed should the separation occurs. People always told me that the more the better because of the stability. But I guess that too much is not good either right ?

 

[Alan Offer]

Hot staging is great if you want to carry the velocity to the second stage. It is not usually done in HPR.

The delay between staging allows the velocity to come down, reducing peak velocities and drag on the sustainer. I did an O3400-M2020 and it had a 15.5 second staging delay.

It just depends on what your goals are.

Should I wait for the rocket to become subsonic before igniting ? If I wait 15s the rocket will ignite at a speed of 200m/s . I'm essentially going for altitude ( I want it to leave the troposphere and a little more) but safety is the most important factor.

 

[OverTheTop]

I think the general advice would be to run a shorter delay and keep it supersonic. Transonic region consumes more energy due to turbulence, so keep above that.

The 15+ seconds on mine is a rare stretch.

 

[Alan Offer]

I think the general advice would be to run a shorter delay and keep it supersonic. Transonic region consumes more energy due to turbulence, so keep above that.

The 15+ seconds on mine is a rare stretch.

Thanks for the advice, 1.2 Mach is about 396m/s. So if the stage delay is 3s, the simulations tell me that the separation speed is 410m/s, I think I will try to aim that speed then