Why is adding a second stage not increasing the velocity?

[Nv7]

The rocket hits mach 1.2 with 2 stages:

And it hits Mach 1.01 with just the top stage:

I'm trying to maximize the max velocity, how would you recommend I do that? I made it such that it hot stages with the second stage firing right when the first stage hits its peak velocity, and I'm using G80T-13s which I believe is the largest non-level 1 cert motor I can get.
Velocity graph of the top stage:

 

[neil_w]

Hard to tell unless you provide the file.

Are the stages separating?

 

[dhbarr]

You might also try a better fin shape, and make sure they're set to airfoil.

 

[neil_w]

Also: if you stage higher, drag will be lower. Also: don’t two G80s add up to high power? Also: stability is very marginal for a supersonic rocket.

 

[Art Upton]

If I am not mistaken, I see the velocity increase .2 Mach. That would be close to normal, but what are you using to light the upper stage with electronics? Getting Mach 1,01 out of a single G80 can be done but it is hard.

 

[Art Upton]

Also: if you stage higher, drag will be lower. Also: don’t two G80s add up to high power? Also: stability is very marginal for a supersonic rocket.

LPR is limited to 2 G motors in use with a total of under 320ns. A single motor is limited to 160ns.

 

[neil_w]

LPR is limited to 2 G motors in use with a total of under 320ns. A single motor is limited to 160ns.

I stand corrected.

 

[Art Upton]

I stand corrected.

No worries, it was simply back in my mid from the mid-late 1990s. There was a max for a single motor and a max for multiple motors like clusters or staged where single motor limit was 160ns, and multi motor was 320ns.

Why that was put in back then I have no Idea.

 

[cerving]

Look how fast the velocity drops as soon as you burn out... the drag is slowing it down like crazy. Reducing drag is about your only choice, I'm assuming that you have already reduced the weight as much as possible. Note that this strategy is to maximize velocity, not altitude... a light rocket will slow down faster than a heavier one from the same burnout velocity.

 

[Art Upton]

So like you could take the LOC Heavy Duty Beauty which was/is a LOC IV with 4 24mm mounts around the 29mm center and still a L0 rocket cluster using a G40/80 in the center and D12s on the sides.

 

[cerving]

So like you could take the LOC Heavy Duty Beauty which was/is a LOC IV with 4 24mm mounts around the 29mm center and still a L0 rocket cluster using a G40/80 in the center and D12s on the sides.

Nope, you'd be exceeding the amount of propellant allowed for a L0 cluster. 4 x 21 g for the D's = 84g, 63g for the G80, total is 147g which is over the 125 g limit.

Things are so much easier once you get certified...

 

[Art Upton]

Look how fast the velocity drops as soon as you burn out... the drag is slowing it down like crazy. Reducing drag is about your only choice, I'm assuming that you have already reduced the weight as much as possible. Note that this strategy is to maximize velocity, not altitude... a light rocket will slow down faster than a heavier one from the same burnout velocity.

I think he should get the L1 first and then use the G125T for the booster.

 

[Art Upton]

Nope, you'd be exceeding the amount of propellant allowed for a L0 cluster. 4 x 21 g for the D's = 84g, 63g for the G80, total is 147g which is over the 125 g limit.

Things are so much easier once you get certified...

Thanks for that, I forgot about the propellent weight issue it's been so long, anyway I flew mine after I got "Confirmed" back in those old days.

 

[Nv7]

Hard to tell unless you provide the file.

Are the stages separating?

I was thinking the coupler would actually go around the outside of the top stage but I don't think that can be modelled in OpenRocket, that way I could keep the tube diameter as low as possible to reduce drag.

I'm thinking the booster could just fall down without a parachute since its pretty light when burnt out and not stable so will probably just tumble

Not sure whether to use flight computer to trigger parachute deployment or just ejection charge will have to see how the design works out, right now just seeing how fast I can get it

 

[bjphoenix]

Well the first stage is pushing more weight than the sustainer by itself. The resulting velocity accounts for 44% more kinetic energy in the sustainer so it's more than what you think.
Is this just an exercise or is there a reason for more velocity? Even mach 1 in a model rocket is darned fast. Consider that a lot of bullets from firearms are below mach 1 and we don't consider them slow. Launch a small rocket anywhere close to mach and it just disappears, there is not enough tracking smoke to see where it went and the rocket itself will be out of sight. You would need a tracker or a lot of luck to find it.

 

[Nv7]

You might also try a better fin shape, and make sure they're set to airfoil.

I thought elliptical was the most drag-efficient? I can round them but not sure how airfoil works with elliptical fins

 

[Nv7]

Look how fast the velocity drops as soon as you burn out... the drag is slowing it down like crazy. Reducing drag is about your only choice, I'm assuming that you have already reduced the weight as much as possible. Note that this strategy is to maximize velocity, not altitude... a light rocket will slow down faster than a heavier one from the same burnout velocity.

How can I reduce drag anymore? Already using a long, pointy nose cone, elliptical fins that are pretty short

 

[Art Upton]

Super Sonic Fins are sorta swept back, not Elliptical like sub-sonic propeller planes; or look like Nike Smoke Fins with their special taper.

Once you get into transonic range those fins are giving you drag you can model in Ras-Aero. OR will not do that, but they will export a file to Ras-Aero for you.

 

[Nv7]

Well the first stage is pushing more weight than the sustainer by itself. The resulting velocity accounts for 44% more kinetic energy in the sustainer so it's more than what you think.
Is this just an exercise or is there a reason for more velocity? Even mach 1 in a model rocket is darned fast. Consider that a lot of bullets from firearms are below mach 1 and we don't consider them slow. Launch a small rocket anywhere close to mach and it just disappears, there is not enough tracking smoke to see where it went and the rocket itself will be out of sight. You would need a tracker or a lot of luck to find it.

Flight time is 150s for first booster so I think that can be recovered pretty easily, might try some dual deployment stuff with the electronics to reduce that flight time as much as possible

 

[Nv7]

Super Sonic Fins are sorta swept back, not Elliptical like sub-sonic propeller planes; or look like Nike Smoke Fins with their special taper.

Oh I didn't realize fins have to be different after supersonic, will fix that and make sure it works. Also what would you recommend the stability that I should target?

 

[bjphoenix]

Flight time is 150s for first booster so I think that can be recovered pretty easily, might try some dual deployment stuff with the electronics to reduce that flight time as much as possible

Yes but after motor ignition you will not see it again. You won't know if it went straight up or deviated a little bit. That size rocket will go out of sight before 1000' and I'm guessing your simmed altitude is maybe 4000', it will be out of sight in less than one second. It could come down many hundreds of feet away in any direction so spotting it once it drops below 1000 or 2000' will be pure luck.

 

[Art Upton]

Oh I didn't realize fins have to be different after supersonic, will fix that and build your make sure it works. Also what would you recommend the stability that I should target?

I'm can't answer your second question, too long ago for me. I do know that Ras-Aero will give you better numbers for trans-sonic and super-sonic then Open Rocket and why the developers of Open Rocket said during vNarcon this weekend that you should use their Export to Ras-Aero to check those numbers after you built your rocket in Open Rocket.

 

[Nv7]

Would these fins be ok? I was thinking booster fins/stability don't need to be as high since it only reaches Mach 0.69, and top stage has swept back fins with stability of 2.18.

Also, where could I find a site to launch this? Doubt it would land in the constraints of 60 Acres Field which is near me.

 

[Nv7]

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.

 

[dhbarr]

Try a vonn karman profile nose, and shorten your body tube. But also, fly some D-E-F motors minimum diameter and recover them successfully before trying something that requires electronic staging.

 

[Voyager1]

View attachment 626819
Would these fins be ok? I was thinking booster fins/stability don't need to be as high since it only reaches Mach 0.69, and top stage has swept back fins with stability of 2.18.

Also, where could I find a site to launch this? Doubt it would land in the constraints of 60 Acres Field which is near me.

I would increase the fin size for the booster stage. Your combined stability is marginal. I would aim for 1.5 - 2.0.
Also, you haven’t included a coupler section for the nosecone. Including that will shift the CG forward, which is desirable. I can’t check your sim file at present, but have you set the nosecone, airframe and fin finish. Are the fin leading edges flat, rounded or aerofoiled? That will affect drag and velocity.

 

[Nv7]

I would increase the fin size for the booster stage. Your combined stability is marginal. I would aim for 1.5 - 2.0.
Also, you haven’t included a coupler section for the nosecone. Including that will shift the CG forward, which is desirable. I can’t check your sim file at present, but have you set the nosecone, airframe and fin finish. Are the fin leading edges flat, rounded or aerofoiled? That will affect drag and velocity.

Using rounded fins, was thinking stability margin of 1 would be ok for the sub-supersonic flight of the booster but I can increase it

 

[cls]

The rocket hits mach 1.2 with 2 stages:
View attachment 626778
And it hits Mach 1.01 with just the top stage:
View attachment 626779
I'm trying to maximize the max velocity, how would you recommend I do that? I made it such that it hot stages with the second stage firing right when the first stage hits its peak velocity, and I'm using G80T-13s which I believe is the largest non-level 1 cert motor I can get.
Velocity graph of the top stage:
View attachment 626780

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.

 

[NTP2]

I’m going to tell the story of a me and a small rocket, I had a rocket it was quite small I wanted to go fast,and I did I was so fast my fins came off then being fast was no fun anymore.
That is a pretty accurate assessment of what will happen.

Ps but feel free to try it.

 

[Nv7]

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.

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?