Air-starts

[MetricRocketeer]

Hi TRF colleagues,

I have begun experimenting with air-starts. The example in OpenRocket opened my eyes to this technique in rocketry.

I copied the Patriot Missile in the OR example to RockSim because I wanted to see an animation of an air-start using RS’s flight profile. When I copied the Patriot Missile file to RS, some items changed in the rocket design (although the design remained more of less the same, for the most part). I am still tweaking a few things.

I have uploaded my RockSim file for anyone to please look at and run. I always appreciate your comments. I have used the same motors in RockSim as in OpenRocket. Namely, the central motor is a plugged AeroTech K550W, and the cluster of three motors uses an AeroTech I211W each having a six-second ejection delay and a four-second ignition delay.

I have this general question about air-starts. When I have looked at news stories about an ICBM launch, I see the rocket being thrust out of a missile silo (I guess that’s what’s happening). Then that first motor gets close to burning out, and next — while the rocket is more or less suspended in midair or perhaps still ascending slowly — another motor or motors ignites and sends the ICBM to great heights and distances. This is an example of an air-start, right?

And what are other examples of air-starts?

Thank you.

Stanley

 

[Antares JS]

I have this general question about air-starts. When I have looked at news stories about an ICBM launch, I see the rocket being thrust out of a missile silo (I guess that’s what’s happening). Then that first motor gets close to burning out, and next — while the rocket is more or less suspended in midair or perhaps still ascending slowly — another motor or motors ignites and sends the ICBM to great heights and distances. This is an example of an air-start, right?

ICBM's are multistage vehicles. What you are talking about is simply them staging. See below.



No non-hobby rocket that I know of ignites additional motors without dropping the burned out ones first. Such an arrangement would waste volume and mass, as well as drag additional unwanted inert mass with it as the airstarts burn.

I have uploaded my RockSim file for anyone to please look at and run. I always appreciate your comments. I have used the same motors in RockSim as in OpenRocket. Namely, the central motor is a plugged AeroTech K550W, and the cluster of three motors uses an AeroTech I211W each having a six-second ejection delay and a four-second ignition delay.

Strongly, STRONGLY advise putting motor ejection completely out of your mind when contemplating a flight like this. What happens if your airstarts fail to light?

 

[Steve Shannon]

Yes, amateur rockets have been built that do air starts, also called parallel staging. But theyAs @Antares JS stated It’s not as efficient as dropping spent stages, but it is an interesting effect. In order to do it safely the staging must happen while the airspeed is still high enough for stable flight and the rocket must be reasonably vertical.
One way I t’s done by incorporating a staging timer which is wired up to igniters appropriate for the staging motors. It’s armed by acceleration or a change in altitude over some time usually.
Ken Good has done some interesting work with “rack staging” which has motors that are fed to replace the spent ones.

 

[MetricRocketeer]

Hi @Antares JS and @Steve Shannon,

Thank you for your informative responses.

Given what I am trying to do — which I thought was an air-start — then how should I modify my RockSim settings?

Here is what I have in RockSim:

> 54 mm center motor: AeroTech K550W; plugged ejection delay; no ignition delay.

> 3 X 38 mm motors in a triangular cluster: AeroTech I211W; 6-second ejection delay; 4-second ignition.

Could you please tell me how I should change my settings to do a successful air-start?

Thank you.

Stanley

 

[Steve Shannon]

Hi @Antares JS and @Steve Shannon,

Thank you for your informative responses.

Given what I am trying to do — which I thought was an air-start — then how should I modify my RockSim settings?

Here is what I have in RockSim:

> 54 mm center motor: AeroTech K550W; plugged ejection delay; no ignition delay.

> 3 X 38 mm motors in a triangular cluster: AeroTech I211W; 6-second ejection delay; 4-second ignition.

Could you please tell me how I should change my settings to do a successful air-start?

Thank you.

Stanley

I don’t know.

 

[Titan II]

Google rocksim air start. The info you need is readily available.

 

[mikec]

No non-hobby rocket that I know of ignites additional motors without dropping the burned out ones first.

I believe that some variants of the Titan III ignited their solid rocket boosters on the ground and started the center liquid core up before the solids burned out. The nine-solid variant of the Delta II ignited six of the solids on the ground and started the other three after the first six were dropped (as you say), with the core running the whole time.

Many ICBMs, the sub-launched ones for sure and perhaps some of the silo-based ones, are ejected out of a canister/silo by an external gas generator and then the first stage of the rocket is ignited in the air, which is kind of an air start I suppose.

 

[mikec]

Could you please tell me how I should change my settings to do a successful air-start?

In OR, you do an airstart by setting motor(s) to ignite at launch plus some fixed delay in the "select ignition" part of "motors and configuration".

 

[MetricRocketeer]

No non-hobby rocket that I know of ignites additional motors without dropping the burned out ones first. Such an arrangement would waste volume and mass, as well as drag additional unwanted inert mass with it as the airstarts burn.

Strongly, STRONGLY advise putting motor ejection completely out of your mind when contemplating a flight like this. What happens if your airstarts fail to light?

Hi @Antares JS and everyone else,

Thank you for your reply, but then I am confused.

I took this design of a half-scale Patriot Missile from OpenRocket's example section. I then proceeded to copy the OR rocket into RockSim, and I also did a few tweaks. So this is for a sport rocket, right?

Stanley

 

[Antares JS]

Hi @Antares JS and everyone else,

Thank you for your reply, but then I am confused.

I took this design of a half-scale Patriot Missile from OpenRocket's example section. I then proceeded to copy the OR rocket into RockSim, and I also did a few tweaks. So this is for a sport rocket, right?

Stanley

Your OP implied to me that you thought ICBM's did airstarts in this hobby rocket style, and that's what I was replying to.

 

[MetricRocketeer]

Your OP implied to me that you thought ICBM's did airstarts in this hobby rocket style, and that's what I was replying to.

Hi @Antares JS,

Right. I did think that. So, now you have explained to me that the ICBM launch is not an air-start but rather the launching of the second stage of a multistage rocket. I think that I am stating that correctly. And thank you for setting me straight on that.

However, I would like to be clear on this. I don't know if you looked at the RockSim file that I uploaded. In that file, I am simulating a plugged AeroTech K550W as the central motor and a three-motor cluster of AeroTech I211Ws each having a six-second ejection delay and a four-second ignition delay. The central motor ignites first, and after that motor burns out the three clustered motors ignite.

That is an air-start. Is that correct?

Stanley

 

[mikec]

I don't know if you looked at the RockSim file that I uploaded.

When I load that file into OR (I don't have Rocksim) it doesn't have any motors defined or any ignition conditions, but it sounds like what an airstart would look like. But you probably don't want to have ejection charges in airstarted motors at all, and you certainly don't want to rely on them for safe recovery in case the airstarts fail.

And I think you're still confused about how ICBMs work, try reading my posts upthread, but that's perhaps not important. For example, see below which shows an MX missile being ejected from a silo by a gas generator and then the first stage igniting in the air. This is not the same thing as a hobby rocket air-start.

 

[waltr]

If you are igniting a motor(s) after rocket leaves the pad then this is an Airstart....

The modern method is with a flight computer (typically called DD altimeters) that have Airstart feature.
I have seen several Rockets launched at MDRA that airstart additional motors. Very neat to watch.

Another comment above involved motor ejection charges not recommended. Reason is if those motor do not ignite then no recovery system deployed. This is overcome by using a flight computer to depoly reccovery.

 

[MetricRocketeer]

For example, see below which shows an MX missile being ejected from a silo by a gas generator and then the first stage igniting in the air.

And when that first stage ignites, is that an air-start?

 

[mikec]

Depends on how you define "air-start". https://www.rocketreviews.com/airstart-180703140819.html just says "Airstart is the practice of starting one or more rocket motors after the rocket is already in the air." By that definition, yes. I would define airstart as used in hobby rocketry as starting a motor without dropping a part of the rocket, so that the ignition of the second stage in a two-stage rocket would not be an airstart, and I've never seen a hobby rocket use a gas-generator silo launch like that MX. But that's just me, and this discussion is veering into uninteresting semantics.

 

[MetricRocketeer]

Depends on how you define "air-start". https://www.rocketreviews.com/airstart-180703140819.html just says "Airstart is the practice of starting one or more rocket motors after the rocket is already in the air." By that definition, yes. I would define airstart as used in hobby rocketry as starting a motor without dropping a part of the rocket, so that the ignition of the second stage in a two-stage rocket would not be an airstart

So far, so good. I like that answer.

 

[MetricRocketeer]

Strongly, STRONGLY advise putting motor ejection completely out of your mind when contemplating a flight like this. What happens if your airstarts fail to light?

But you probably don't want to have ejection charges in airstarted motors at all, and you certainly don't want to rely on them for safe recovery in case the airstarts fail.

Hi @Antares JS and @mikec,

Now I understand what you both are saying about ejection charges in the air-started motors. Those motors (the clustered I211Ws) should be plugged, just as the central K550W is.

That's what you are saying, right?

Stanley

 

[Antares JS]

Hi @Antares JS and @mikec,

Now I understand what you both are saying about ejection charges in the air-started motors. Those motors (the clustered I211Ws) should be plugged, just as the central K550W is.

That's what you are saying, right?

Stanley

Yes, and use electronic deployment.

 

[Reinhard]

And when that first stage ignites, is that an air-start?

The procedure is called cold launch. The most prominent operational example in the US is probably the Trident D5 (the Minuteman is launched hot).

The Russians seem to be quite fond of it. They use it also on tactical systems like the S-300 family and K-300P besides the usual strategic suspects (R36, RS12 Topol, RS28 Sarmat,...).

Although a cold launch could be described as a type of air-start, I've never seen it being referred as such. In my experience, the term air-start implies some clustered arrangement where additional motors are ignited well into the flight. It also appears to be more of a hobby thing, with the Delta III being a notable exception.

Reinhard

 

[MetricRocketeer]

The Russians seem to be quite fond of it. They use it also used on tactical systems like the S-300 family and K-300P besides the usual strategic suspects (R36, RS12 Topol, RS28 Sarmat,...).

Thank you, @Reinhard. Your post was very instructive, and it has led me to interesting YouTube watching.

 

[Donnager]

NM..

 

[BABAR]

For all practical purposes, in model rocketry we never “cold launch”, so “air starts” are always secondary motors igniting AFTER the rocket has left the pad launched by ignition of a primary motor (or in the case of a cluster a set of primary motors.). Probably safe to say air started motors should be considered auxiliary motors which provide a spectacular show when they work, but the rocket should be able to successfully fly to an acceptable height AND deploy a recovery device completely independent of whether the air started motors light or not.

so while they certainly can increase altitude, they are primarily for show and are never “critical systems” that, if they fail, will cause a dangerous situation such as a ballistic recovery. Ideally there should also be a lockout mechanism that actively PREVENTS air start ignition if the rocket lacks appropriate trajectory and velocity at the planned air start time. Air started motors that ignite in a model rocket with a significantly non-vertical trajectory are at best “poor form.”