Ares-1 stability AFTER staging?

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Well-Known Member
Jan 18, 2009
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The earlier discussion about why the Ares-1 (or more specifically, the Ares-1X test vehicle) has no fins made me wonder about a related question:

On the full-scale rockets, what keeps the rocket stable during the few seconds between when the first stage engine(s) cut off and when the second stage ignites?

During that time, there's no thrust being produced with which to control the vehicle (with gimballed motors or vectored thrust), before or after separation of the first stage, until the second stage ignites.

What made me wonder was what happened in the first few seconds after separation of the Ares-1X booster from the upper-stage simulator. The dummy upper stage almost immediately began tumbling, and although no recontact between it and the booster occurred, it sure looked like they (nearly) hit from the ground camera's point of view.

Now I understand the builders expected the upper stage to tumble upon separation, although none of the animations they kept showing us showed any tumbling. So maybe they expected it, maybe they didn't. That's not the point though.

So possible answers to my original question include:
  1. With the upper stage full and the booster empty, it's just plain statically stable enough to coast without tumbling for the short time spanning the staging process.
  2. Separation happens high enough above the atmosphere that static stability is not an issue, it's purely momentum keeping it going straight. (Which would still make me wonder why Ares-1X behaved so differently. Was it much lower in the atmosphere at staging?)
  3. Reaction control systems in the upper stages come in play after staging and before second-stage ignition.
I know there are some rockets (liquid-fueled ones primarily) that use small ullage motors to accelerate the upper stage away a little from the booster, and to force the propellants to settle to the bottoms of the tanks before ignition of the upper stage motor(s). The best example I can think of is the Saturn V, which if I remember correctly had a ring of 8 such motors on the interstage between the first and second stages. I seem to remember the third stage may have had some too. But I believe these are very simple motors, probably solids, so I don't think they enter into the control of the vehicles.

So does anyone have any real knowledge here as to how the big rockets handle this?
I can't speak for all rockets, but the ones I have worked on either do a hot sep where the separation charge and motor ignition are fired at the same time, or we use cold gas thrusters to maintain the attitude of the vehicle until ignition.
Generally speaking, there are two types of staging--
1) fire in the hole
2) delayed or sequential staging.

Fire in the hole is the system used on the Titan II's-- the upperstage ignites while still coupled to the lowerstage. The lower stage usually has vents to allow the rocket exhaust to escape until the stages clear. The upperstage then of course has engine thrust to direct through gimballing for stability.

Sequential staging uses a sequence of events like the Saturn V's-- first stage shutdown, followed by retrofire of the first stage retros, first stage to interstage explosive bolts fired, allowing the stages to seperate, interstage ullage rockets fire, pushing the upperstage away, second stage engine startup, and finally interstage to second stage explosive bolts fire to drop the interstage. Ullage rockets are, AFAIK, all pretty much small solid rockets strictly used to impart a little velocity change (delta V impulse, as either prograde (pushing the rocket forward) or retrograde (slowing the lower stage down) for a clean seperation.) The stack's stability is handled by another system, probably some sort of RCS system.

Then you've got the Atlas, which when it dropped it's skirt and two outboard booster engines, they slid down guide rails past the center sustainer engine bell before they were 'released' to prevent any possible contact with the center engine bell.

If the rocket is flying VERY stable, and at null rates at seperation, it will continue with that for a little while-- remember Newton's first law-- a body at rest will remain at rest unless acted upon by an outside force. When you consider the mass of some of these rockets, if the rates are zero at shutdown, it would take some substantial energy to get them moving off course. If, however there are ANY rates at all at shutdown or seperation, those rates will be magnified at seperation due to the single rocket becoming two seperate bodies in close proximity, each with their own CG and moments of inertia. Case in point-- look up the video of the first Spacex Falcon 1 orbital attempt-- the one that failed due to contact between the first stage and the upperstage engine nozzle extension. If you sight along the rocket and look at the Earth below, you will notice an ever-so-slight motion (rotational rate) at first stage shutdown and upperstage seperation. This slight motion becomes GREATLY magnified when the two stages seperate, because each becomes a much shorter unconnected body (remember Stine's example in the Handbook, about a long stick thrown end over end rotating more slowly than a shorter stick tossed end over end-- when the explosive bolts fire seperating the stages, the "long stick" of the rocket stack becomes two seperate "short sticks" (with much shorter moments of inertia) rotating significantly faster than they did when combined. Additionally, when combined, they rotated around the COMMON center of gravity-- when the bolts fire, the TWO stages keep rotating, but EACH AROUND IT'S OWN CENTER OF GRAVITY. Combine that with the fact that the two stages now rotate IN THE SAME DIRECTION as the original rotation, and since the top of one is next to the bottom of the other, with both turning in the same direction, the stage ends closest together are actually moving in opposite directions, which puts them at much high risk of collision. That's where guide rails or something would be very worthwhile, to prevent those independent motions until the critical parts of the stages (like nozzle extensions) are clear of each other.

I'm sure there's a LOT more to it than that, but it's something to think about. I'm sure it played into the Ares I-X "jackknife" at seperation although let's remember that the Ares I-X had virtually NOTHING in common with the eventual flight hardware for the proposed crew rocket. The upperstage, Orion, and LES tower were all boilerplate simulators; even the first stage was a four-segment SRB with a fifth dummy segment simulator. So the stability of the upperstage was always in question anyway, since NONE of the upperstage systems were there to provide stability for it at or after seperation. Only the SRB "sep motors" and "tumble motors" (basically, small solid retrorockets) were there. Additionally I think I read somewhere that an instrument wiring harness between the upper and lower stages that was to be cut by a guillotine at seperation did not cut completely through, and the wires "dragged" the upperstage around before they ripped free of the upperstage simulator by the forces involved.

Interesting stuff.... OL JR :)
I don't know anything about actual rockets, but wouldn't it be possible to simply have the CG in front of the CP without fins, rendering the rocket stable even when it's not under thrust? Plus if they happen to be tapered, that'd add some amount of stability too...
I don't know anything about actual rockets, but wouldn't it be possible to simply have the CG in front of the CP without fins, rendering the rocket stable even when it's not under thrust? Plus if they happen to be tapered, that'd add some amount of stability too...

It is possible, but it is difficult to obtain any significant restorative moment without fins or some sort of conical taper.

(As far as full size rockets are concerned, there's also the issue that staging typically occurs above almost 100% of the atmosphere)