More Constellation Problems

No one who has been following the program will be shocked to learn that the major, fundamental design issues continue, and that they aren’t just “teething pains” of a new program. Despite a lot of happy talk from Griffin and Cook and Hanley over the past few months, thrust oscillation remains a serious problem for the Ares I first stage:

According to a NASA blog, the engineers are still looking at putting a series of passive dampers at the bottom of the rocket and a series of spring-like brackets in the middle to soak up the vibrations like shock absorbers.

Originally the brackets, called a dual plane C-spring isolator system, were too heavy to incorporate into the overall design. An updated version uses titanium, which is as strong as steel but lighter.

However, the fixes are not easy and engineers have yet to settle on a solution. According to NASA officials who attended the meeting, the shaking problem is more difficult to combat than originally thought as each solid rocket burns slightly differently.

You don’t say. That means that a passive solution won’t work, unless they can predict prior to flight exactly what the characteristics will be for each SRB (a longer way of saying…it won’t work). They’ll have to have an active approach that can actually measure the vibrations in real time and try to compensate for them. My solution? Bag the solid first stage. Here’s one that will save even more money. Bag Ares I.

And all is not well at the pointy end of the rocket, either:

An Air Force memo obtained by Todd Halvorson of Florida Today indicates that military safety officials are worried that NASA’s Orion capsule and its crew might not survive an emergency escape off an exploding Ares I rocket.

As I understand it, the concern is that the launch abort system is sized to accelerate away from an exploding upper stage, and to outrun an out-of-control first stage, but not from the flack created by the massive explosion of an SRB. Parenthetically (without the parentheses) it should be noted that one of the ways that NASA put its thumb on the scales when it compared Ares to EELV was to assume that the same LAS would be used in both cases, but the latter has a much more benign failure environment, and could get by with a much lighter LAS, so dinging the EELV for lacking the performance to lift an unnecessary weight was stacking the deck against it.

Anyway, how likely is it that the first stage will explode? Well, I find this sadly amusing:

…the article also has Hanley pooh-poohing the Air Force’s concerns, saying that “supercomputer analyses” will prove that the Ares I rocket is a fine vehicle and Orion’s launch abort system will be able to save the crew in the event disaster strikes.

They have top men looking at it. Who?

Top. Men.

Here’s my question. If they know the results of the “supercomputer analyses” before they have performed them, why are they bothering to perform them? Couldn’t they save some money and just skip them?

Florida Today quotes Hanley saying that the statistical probability of an Ares I first-stage failure is remote. He pinpointed it at 1 in 3,000 to 1 in 3,500.

Gotta love that verb, “pinpointed.”

You know, those were the kinds of numbers that they were claiming for Shuttle, right up until around January 27th, 1986. They got some new data the next day, though, that significantly altered the estimates going forward…

So, once again, show us the numbers, Jeff. Show us your work.

It’s hard to know from this brief news story, so I don’t even know what he means by “failure.” Does he mean spontaneously explode without warning? Well, it’s not unheard of for solid rocket motors to do just that, though it has never occurred in the Shuttle program. But I suspect that what the Air Force is concerned about is a different kind of failure — a guidance failure that requires the Range Safety Officer to destroy the vehicle so that it doesn’t hit any uninvolved areas (e.g., Daytona). And considering that an SRB has never had to do guidance without help from a partner on the other side of the tanks and the SSME gimbals, that’s a non-trivial concern. And when the stage is destroyed (by setting off a linear charge along its length) it could create explosive debris that the LAS may not outrun. I assume that’s the Air Force’s (probably supported by an analysis from Aerospace) concern.

Of course, this all raises the question of whether or not we should even have a launch abort system, as I’ve discussed previously, with further thoughts here. Of course, the whole problem goes back to NASA’s “cargo-cult” engineering approach to Constellation, in which they think that if they just go back and do things the way the Apollo gods did, except “on steroids,” they’ll once again have a successful program.

27 thoughts on “More Constellation Problems”

  1. From the 2nd linked article:

    “We feel we have a very, very, very safe first stage. Very reliable,” the paper quotes Hanley as saying. “We think we have a very robust design for the abort environment.”

    I feel more reassured every time he says “very”.

  2. There is a very interesting technical point that I wonder about.

    The heath monitoring system is supposed to initiate an LAS extraction from the vehicle before it gets blown up. However, in this indefinite time period of x seconds, the LAS is accelerating away, while the SRB is still burning at full power. The SRB, now 100,000 lbs lighter without the Orion on top is surely going to accelerate before it blows. That would be an interesting analysis to see what would happen to the acceleration rate and velocity of the thing before it blows.

  3. I feel more reassured every time he says “very”.

    It reminds me of the line from The English Patient. “‘Yes’ is reassuring. ‘Absolutely’ is not.”

  4. @ Dennis

    I would suspect that simultaneous to the LAS firing, linear charges would rip open the sides of the solid booster causing thrust termination.

    I believe the issue is whether Orion can outrun the shrapnel from that event.

  5. well, the verbs “feel” and “think” are rather useless for establishing factual bases for anything. They are perfect, however, for providing an “out” when facing a board of inquiry.

  6. The guy that worked in the warehouse.

    For all you know, that guy is a top warehouse guy.

  7. If range safety had to blow up Ares I, would it be practical for the safety officer to trigger the abort system shortly before blowing up the launcher? Depending on the speed of the shrapnel from the booster, it might give the capsule enough time to separate a safe distance away. Then again, the amount of time between the trajectory deviation and the need to destroy the booster might be too short to allow a meaningful separation.

    It’s also occurred to me that DIRECT would suffer the same problem, perhaps even moreso, because there’s less vertical separation between the capsule and SRB’s. Maybe we should follow von braun’s original advice on SRB’s and drop them from manned spaceflight entirely.

  8. One would assume any termination command a RSO would give on a manned vehicle with a LAS would automatically initiate the LAS and give a short time for the vehicle to clear the stack before the stack is destroyed.

    One would think the would simply be programmed in by default.

  9. a short time for the vehicle to clear the stack before the stack is destroyed.

    The definition of short time is the crux of the matter. Run the numbers of what the Ares 1 in a full stack accelerates at, and then without the Orion on top. Does the acceleration double? It would be a fair amount for sure and even for a half second that is an appreciable increase in velocity.

  10. Is the basic reason DIRECT’s ‘Jupiter Light’ doesn’t have this exact same vibration problem the spare payload? So they can just say “Yeah, well, we’ll use better springs.”?

  11. No, the reason the Direct and the Shuttle does not have this problem is the SRB’s are coupled thru the support beams that go thru the intertank.

    I gather the vibrations tend to null each other somewaht in this config.

  12. Dennis, what will the removal of the Command Module do for the aerodynamics as soon as the capsule is 50 or so meters away and you aren’t getting an aerospike effect from the capsule plowing the atmo ahead of the booster?(Kinda of like dragging behind a semi I suppose) I suspect the face of the now uncapped Service Module makes a very high drag surface.

  13. In my experiences with solid rockets (admittedly smaller ones than the SRB), the flight termination charge can pretty easily quench the fire (by very quickly reducing the pressure in the rocket) without causing major structural damage. The length of the Ares and the corresponding larger area that has to be breached to cause enough rapid loss of pressure in the rocket may significantly up the likelihood of high speed shrapnel, though.

    The active vibration damping just sounds like a showstopper, though. So they make the thing out of titanium; how’s that going to help them when their actuation system comes in 30% overweight? Or they need to increase the size of the hydraulic fluid reservoir that drives the actuators? Or they get a realistic design from the actuator vendor that includes shipset-to-shipset variations? Just nuts…

  14. What can we learn about SRB shrapnel from the video, telemetry and recovery of the Challenger SRB’s? The Challenger video that we’ve all seen makes it very difficult to see the secondary explosions which occurred when range safety destroyed the SRB’s.

  15. Solid boosters that big don’t make much sense. But if you did insist on running a shuttle derived solid booster in the Ares I configuration, I figure separation at the junctions between segments would be an effective thrust termination.

  16. Does anyone know the magnitude of the thrust oscillations that Space Ship 1 experienced? Is there some feature of a hybrid solid that damps thrust oscillation? Is the thrust to weight ratio of SS1 significantly different from Ares in such a way that oscillations aren’t as significant?


    My solution? Dump Ares 1. 2 launches of Ares 5. Safe, Simple, Soon as hell freezes over.


  17. NASA and its rocket-building programs that need a big red range safety destruct button. As soon as some of the program managers utter the words “supercomputer analysis”, the thrust of the program ought to be terminated.

  18. Some of you missed a key point somewhere along the line. The “thrust oscillation problem” is, oddly enough, not that there is a large oscillating component to the SRB thrust. The point is that the second axial vibration mode of Ares I is very close to the thrust oscillation frequency. The actual oscillation in thrust is relatively small, compared to the average thrust level. But it pumps a lot of energy into the vehicle because the vehicle happens to have a resonance at that frequency. The challenge is that it’s not that easy to change the resonant frequency of the vehicle or of the SRB.


  19. Checkout YouTube and find model rocket launches. One project had a camera mounted on it. Near the middle of one launch, the rocket occillated and vibrated like crazy! I could not believe the oscillations. Solid rockets to have this problem regardless of size. I can’t imagine the problems Ares I will have.

  20. > …And considering that an SRB has never had to do guidance without help from a
    > partner on the other side of the tanks and the SSME gimbals …

    Your forgetting post break up Chalenger, where the two SRBs did stear themself back on course and flew off in parrallel tracks.


    As to
    >…“supercomputer analyses” will prove that the Ares I rocket is a fine vehicle …

    Like then mathmatical analysis that showed Shuttle leading edges won’t break when debre falls on them from the tank?

  21. Your forgetting post break up Chalenger, where the two SRBs did stear themself back on course and flew off in parrallel tracks.

    “Parallel” tracks that diverged and swerved back around independently.

    They were not under guidance after the breakup.

  22. Parallel” tracks that diverged and swerved back around independently.

    If you’l check the Challenger accedent photos they wen’t pretty straight, after first being thrown to the sides. After that as the bad SRBs joint started really leaking adn skiding it around things got worse, but the fact they both excicuted simultanious turns back to parraleel course shows guidance.

  23. but the fact they both excicuted simultanious turns back to parraleel course shows guidance.

    Stopped clocks being right twice a day show they can keep time, too.

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