Ideal Airstart Delay Time?

[Sparkyflyer14]

I simulated a flight with a upper stage ignition time of 3 seconds after booster ignition. To my suprise, when I did the same flight with 4 or 5 seconds, the projected hight was higher then the 3 seconds. I was under the impression that you wanted as quick of an airstart as possible so you utilized all possible speed. Why then are the flights with longer delays going higher? In addition to this, the simulated flights that have longer delays end up starting to tilt dramatically due to the wait for sustainer ignition. Yet despite this, it still goes higher? Please, someone explain the science of this to me so that I can get full performance from my rockets. Thanks!

 

[Charles_McG]

I airstart fast to limit the increasing tilt due to simple gravity turns. I’ve been running 0.7sec recently.

Increasing speed increases drag. For the same impulse, a slower burning motor goes higher than a faster burning motor.

If you can stay vertical during the coast, you get all that altitude and bleed off speed so the next motor is fighting less drag.

 

[OverTheTop]

If you can stay vertical then you can use long delays. Any gravity turn can put you off vertical and easily bust the waiver cylinder.

FYI I staged an O3400 to a M2020 with a delay of over 15 seconds.

 

[heada]

Drop the previous stage ASAP to reduce drag but next stage ignition should be delayed as long as possible with the assumption that you remain vertical and you don't drop into a high drag speed like transonic.

 

[Charles_McG]

Drop the previous stage ASAP to reduce drag but next stage ignition should be delayed as long as possible with the assumption that you remain vertical and you don't drop into a high drag speed like transonic.

Separation timing is really design sensitive. If the rocket is all the same OD, the spent stage may contribute more momentum than drag - and lead you to separation just before ignition. Your sim will inform that.

This is all assuming you’re staging for altitude. I fly staged rockets for the effect, and the challenge. I have slightly different hurdles- keep the flight as low as I can safely. That really limits motor selection and crunches the timing sequence.

 

[waltr]

Charles explained it well.

I have also been going as low as I can on air start 2-stage for effect.
Been through all the issues of finding just enough motor in booster to have enough speed to stay vertical. But with finial apogee within our field limits.

 

[Johnly]

I fly a 3" minimum diameter 2 stage rocket with a L1520 or M1780 in the booster and a M685 sustainer. The booster separation charge is 2 seconds shorter than the sustainer ignition, but that is a backup to drag separation of the two sections. The crux of HPR multistage flights is obtaining the shortest and repeatable delay between the firing of the sustainer igniter and the motor coming up to nominal operating pressure.

 

[waltr]

The crux of HPR multistage flights is obtaining the shortest and repeatable delay between the firing of the sustainer igniter and the motor coming up to nominal operating pressure.

Yep, that can be cause all sorts of problems.
I did have one flight where the sustainer motor took almost 4 seconds to come up to pressure. By then the rocket arced over and was pointed in a downward direction. It then powered through a tree and ejected chute within the tree. Lots of damage but all was repairable. The Quantum did fire the ignited at proper time. I run a 10DOF sensor package so could measure time from accelerometer data.
I have been tilting the launch rail a few degrees away from the flight line just in case something like that happens. Trajectory was away from flight line so all ok.
My 2-stage is 3" diameter kit bashed from the BMS 3" school rocket. Weight under 1300gram but ran a F67 in booster on the bad flight. Now using an F74 which gives it a bit more velocity at burnout. Sustainer is AT D15T (Blue thunder which is supposed to be easy to light). This combination has apogee in the 600 foot range.

 

[jderimig]

Yet despite this, it still goes higher? Please, someone explain the science of this to me so that I can get full performance from my rockets. Thanks!

Lower average speed = lower average drag for the same impulse input = higher altitude. Coasting as long as possible (while still remaining close to vertical) lowers the average speed/drag.

If your rocket wants to drag separate (from a physics analysis) then separate your booster right at booster burnout.

If your rocket doesn't want to drag separate (booster still pushing on the sustainer) you want to separate the stages as late as possible.

 

[Adrian A]

I simulated a flight with a upper stage ignition time of 3 seconds after booster ignition. To my suprise, when I did the same flight with 4 or 5 seconds, the projected hight was higher then the 3 seconds. I was under the impression that you wanted as quick of an airstart as possible so you utilized all possible speed. Why then are the flights with longer delays going higher? In addition to this, the simulated flights that have longer delays end up starting to tilt dramatically due to the wait for sustainer ignition. Yet despite this, it still goes higher? Please, someone explain the science of this to me so that I can get full performance from my rockets. Thanks!

There are two advantages to longer delay times: 1. The average airspeed is lower, reducing the drag losses. This is like using a long burn motor. 2. The high-speed part of the sustainer flight happens at higher altitudes where the air is thinner, again reducing the drag. Of course, if you wait too long, the angle will get to be too large and your rocket will flight out of your recovery zone and/or you'll get more pointing loss (cosine loss) to be worth it.

 

[waltr]

I ran a bunch of sims on my 2-stage. Sustainer motor a D15T but changed the booster motor trying F67, G74, H165, H210, each with higher total impulse. The increment of booster motors gave higher velocity at booster burn-out.
With the F and two G's highest apogee was with no delay for sustainer ignition. However, on the H's a delay added altitude.
It was a burn-out velocity of around 200f/s and higher that delayed sustainer ignition added altitude with higher speeds wanting longer delay.
Since the drag force increases as velocity increases this make sense. Keeping the average flight velocity lower is less drag force thus higher altitude.

This delayed ignition is very much like the concept of a boosted dart. The dart has no motor and simply coasts to apogee. But it drops the high drag booster. It is also heavy carrying lots of inertia.

At what the velocity is better to delay sustainer ignition is dependent of the rocket design, Drag and mass being to two characteristics of importance. If you want highest apogee then doing multiple sims is helpful to determine best ignition delay. Also try varied mass (weight) of sustainer. There is an optimal mass for each rocket to maximize the coast time and distance.
Open Rocket can plot the Drag force, Acceleration,. Velocity, etc to help understand
the dynamics.

 

[G_T]

Just for reference, very very roughly speaking:

Aerodynamic forces are proportional to the square of the velocity. This includes drag.

Aerodynamic heating is proportional to the cube of the velocity, at least until the onset of hypersonic speeds.

Both are proportional to atmospheric pressure.

Atmostpheric pressure decreases rather rapidly with altitude. Beware supersonic shockwaves though... they are not at ambient pressure for that altitude. So some regions of a rocket may see greater than ambient atmospheric pressure. And aero effects will certainly cause a pressure distribution around the rocket which is not equal to ambient atmospheric pressure, at least while the rocket is moving. But you can sort of pretend as a high level concept that atmospheric pressure as experienced by the rocket decreases rather rapidly with altitude.

Atmospheric temperature decreases with altitude, at least within the range of atmosphere we are generally flying in!

Drag is higher after the motor cuts off.

Rocket total energy lost due to gravity is the integral of gravity_at_that_altitude_and_location over time. Or roughly gravity times time, for our purposes.

Rocket total energy lost due to drag is the integral of drag over time. Sorry; no shortcut on this one.

In our small size rockets, drag losses dominate, limiting altitude. For extremely large rockets, gravity dominates, limiting altitude. So for super large rockets for high altitude, they just have to be slow enough to survive Max Q. Otherwise they are better off getting up there as quickly as possible. Little rockets like ours though are better climbing more slowly so more of the available propulsive energy is converted to altitude rather than heat and drag. It's actually a minimization function but hopefully you get the idea.

And the slower the rocket has to go, the lighter it can be built, which improves the propellant mass fraction and/or improves the odds of survival.

Climbing slowly generally requires multiple stages and/or rather long burn motors. So I'm personally looking at developing a sustainer motor that will burn 13s or longer, possibly somewhere around 18s, at 88mm diameter. It looks quite feasible from the work I've already done. I still need to do the sims to see how long I really want it to burn. I expect 13s would be the low end of the range.

Note, climbing slowly at very high altitude though costs too much stability if a motor is going to be burning. Lighting a third stage at 100Kft without active stabilization or plenty of spin would not be smart.

Gerald

 

[rocket_troy]

caveat: below is only applicable for very high altitude, high performance staging requirements.

While we're delving deep into this, I'll also add that igniting solid propellant in very low pressure conditions is more challenging and not only is the atmosphere thinner the higher you go, the dynamic chamber pressure (before ignition) will also reduce with velocity (up to a point).
You can avoid this by blocking the nozzle, however, that can also present ignition challenges as there could be an initial pressure spike followed by a dip as the throat is opened up thereby potentially extinguishing combustion from the propellant's diffusion layer expanding and pushing the thermal transfer back from the propellant surface. This can also be an issue with igniters even.
There are ways to overcome these challenges like igniting before stage separation (sub optimal aerodynamically) or employing staged (choked) motor ignition or being quite diligent to igniter/ignition properties at the expected conditions.

TP

 

[G_T]

I'm figuring on something like a 3d printed nozzle throat insert to boost the initial Kn, probaly with a thin foam ring in that (the staged choked approach). I don't want any sudden pressure drops. I road-flared a dual-thrust test motor that way. It almost went out when the thrust dropped. It EVENTUALLY came back up to pressure... Now I design with a ramp-down rather than a drop.

But if I have the sustainer burn duration long enough then I don't have to light it quite as high. Bonus points. We'll see. Lots of simming to be done! I will be giving up some Density ISP in exchange for much slower burn rate than normal APCP. Altenative is wired end burner and I haven't completely ruled that out yet. I've just been working on the other approach. Very slow burn propellant + asymmetric core, or very slow burn propellant + go up to 114mm for the sustainer.

Gerald

 

[rocket_troy]

I'm figuring on something like a 3d printed nozzle throat insert to boost the initial Kn, probaly with a thin foam ring in that (the staged choked approach).

Interesting concept. It's not what I meant (apologies for the ambiguity): I was simply referring to using a smaller motor to ignite the larger one eg. utilising something like a small Estes BP motor or something with a choked throat to provide the ignition energy. In that case, there's more tolerance and margin for pressure fluctuations within the chamber as your ignition energy source is itself choked.
But as you've highlighted, there's plenty of room for different solutions and concepts that could very well be superior.

TP