Ideal Two Stage Burn Time

[Sparkyflyer14]

Quick, quick question: Which motor in a two stager should have longer burn-time, impulse, ect? Is there a general rule of thumb? Like high impulse, short burn first stage with long weak upper stage or something?
Thanks!

 

[Charles_McG]

The first stage is usually high thrust.

Upper stages - it depends? How big? How high? How fast?

 

[smstachwick]

Quick, quick question: Which motor in a two stager should have longer burn-time, impulse, ect? Is there a general rule of thumb? Like high impulse, short burn first stage with long weak upper stage or something?
Thanks!

It really depends on what your desired flight characteristics are. Obviously you need a sufficient thrust-to-weight ratio at liftoff, reliable sustainer ignition, and workable recovery. Beyond that, anything goes.

For maximum altitude though, it’s generally accepted that long burns are best to reduce maximum speed and thus reduce aerodynamic drag forces. But you also have to think about how winds will make your rocket weathercock and rob it of vertically-directed thrust during the burn, so it can be a tradeoff depending on conditions.

Some high-power supersonic multistagers will delay upper stage ignition to coast for a bit and bring down the maximum speed, though they’re generally designed so that the rocket won’t dip back into the transonic speed range where drag is highest.

This is a far cry from LPR, where a lot of times you’ll see multistagers deliberately kept low with small motors for simple tracker-free recovery.

 

[waltr]

Yes, it depends.
To ensure stability off the rail at least a 10:1 WTT is recommended. Also to ensure enough velocity at burn-out to stay vertical until 2-nd stage lights.

If you are just sport flying then high thrust 10-20:1 in booster so staging is low enough to see, then lower thrust 2nd stage of either short or long burn depending on Wavier or how high you want to go.

Sims are your friend.

 

[Sparkyflyer14]

Sorry, should have mentioned this scenario is for maximum altitude.
Thanks for the responses!

 

[Steve Shannon]

Fred Taverni has written some absolutely excellent articles on designing multiple stage flights (the design being the things you’re asking.) They’re in the last and next HPR Magazine, which are online magazines available to all Tripoli members for free.

 

[Sparkyflyer14]

Fred Taverni has written some absolutely excellent articles on designing multiple stage flights (the design being the things you’re asking.) They’re in the last and next HPR Magazine, which are online magazines available to all Tripoli members for free.

Thanks! Good thing I recently became a member.

 

[Jmhepworth]

My simulations nearly always tell me that a long burn in the sustainer gives me the most altitude. Another interesting question is how long should I make the delay between 1st stage cutoff and sustainer ignition? On his 293,000 foot flight, Kip had 17 seconds between the two. I’ve never gone more than a couple of seconds.

 

[Sparkyflyer14]

My simulations nearly always tell me that a long burn in the sustainer gives me the most altitude. Another interesting question is how long should I make the delay between 1st stage cutoff and sustainer ignition? On his 293,000 foot flight, Kip had 17 seconds between the two. I’ve never gone more than a couple of seconds.

I agree, it is interesting that so much more altitude can be achieved through longer delays. Is this just because the decrease in speed causes less drag? It doesn't seem like a worthwhile tradeoff.

 

[smstachwick]

I agree, it is interesting that so much more altitude can be achieved through longer delays. Is this just because the decrease in speed causes less drag? It doesn't seem like a worthwhile tradeoff.

That’s precisely why. Drag increases with the square of airspeed, although there are some unusual phenomena once you get into the transonic range.

 

[jderimig]

I agree, it is interesting that so much more altitude can be achieved through longer delays. Is this just because the decrease in speed causes less drag? It doesn't seem like a worthwhile tradeoff.

That and air density decreases at higher altitudes. If you can delay the ignition of the sustainer as long as possible you get into thinner air and get a futher drag reduction.

 

[OverTheTop]

From a gravity loss point of view you want to burn the motor in as short a time as possible. That saves the energy required lifting the fuel to higher altitude to burn it later. As others have pointed out drag increases (square law) with the higher velocities, so you are trying to strike a balance between the two.

I did an O3400 to M2020 with a 15.5 second delay. That kept the sustainer to maximum of M2.14. Things get really serious really quickly, typically after M2, IMHO.