Some thoughts on the scrub


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Last night Falcon 9’s computers shut the launch down at T minus zero seconds after sensing a high chamber pressure in one first stage engine.

Two thoughts, one good, one not so good.

First the good. This is not an unusual event for SpaceX. They have had a number of previous aborts at launch, with both Falcon 1 and Falcon 9, initiated by computer. What has been very encouraging about all these events is they have always been able to make the rocket safe and recover to launch another time. And that recovery has always been breathtakingly fast, sometimes taking just a few minutes.

This pattern of last second scrubs however suggests something more worrisome. I wonder if we might be seeing a design issue with Falcon 9’s first stage, which uses nine Merlin engines. When they first announced this configuration — chosen to save money as they would not have to design and build a new more powerful engine — my first thought was that it was going to be very complicated pumping fuel and oxidizer to all those engines simultaneously and with precision. One of the reasons the Soviet Union’s N1 rocket failed was it’s first stage had 30 engines, requiring an incredibly complex fuel system. I worry that Falcon 9’s first stage carries with it this same problem. It seems to me, for example, that most of the past launch aborts occurred because of fuel pressure anomalies detected by the control computer.

Having said this, I must add that I am speculating, and really have no idea what the issue is. According to this report, the problem might have less to do with pressure issues and more to do with a fundamental issue with the engine itself. That in itself will be unfortunate, because if SpaceX has to swap out an engine it is going to be very difficult to get this rocket off the ground before May 29. After that date the next launch window will not be until after mid June.

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7 comments

  • Joe

    It is too early for speculating, but since it is fun (and you started it) here is a list of possible causes (according to me based only on publically available press reports).

    The Falcon 9 had a similar problem (again with the number 5 engine) on its first launch. That time they ‘fixed it’ by changing the acceptable pressure levels in the engine. The change caused no catastrophic situation.

    Now (presumably using the more relaxed pressure regime) they have encountered a similar problem.

    The causes I can conjure are as follows:
    – Space X has a problem with faulty pressure sensing (either hardware or software) – reasonable easy to fix.
    – Space X has a problem with quality control (two of the thirty engines used in the three launches attempted have encountered a real pressure problem) – harder to fix.
    – Space X has a basic design problem (either with the Merlin engine itself or with the Falcon 9 design/layout) – the hardest to fix.

    Maybe they will once again pull off a successful flight by changing the acceptable pressure limits, but even if they do, that will not address the core problem and it will be hard to make the vehicle reliable (able to meet the narrow launch windows required for ISS – or other rendezvous – operations).

  • Ron

    “…the narrow launch windows required for ISS – or other rendezvous”

    “The narrow launch windows required for ISS,” on this mission in particular, are dictated by the fact that COTS 2+ is really two missions in one, and in order to complete all of those mission objectives a lot of fuel is required. Hence, as noted by spacenews.com, the fuel reserves are “razor thin.”

    Mission Day Two will burn fuel in rendezvous and station proximity operations.

    Mission Day Three will burn fuel in actual rendezvous and subsequent berthing with the ISS.

    On a typical Dragon resupply mission to the ISS fuel reserves will be more ample and therefore launch windows will no doubt be a lot less “narrow.”

  • Joe

    “On a typical Dragon resupply mission to the ISS fuel reserves will be more ample and therefore launch windows will no doubt be a lot less “narrow.””

    But they would still be far to narrow to allow Space X to trouble shoot and fix this kind of problem. Therefore my original statment stands.

    By the way the combining of two missions into one was Space X idea, they lobbied for it.

  • Ron

    They did lobby for it, and it was a good idea. Obviously under such a scenario you have less margin for error.

    “But they would still be far to narrow to allow Space X to trouble shoot and fix this kind of problem. Therefore my original statment stands.”

    That depends. Would they have to wait 3 days for the next launch window? Or after trouble shooting would the additional fuel reserves allow them to launch the next day and still leave them with a good margin for rendezvous? Off-hand I don’t have the answer to that one. Do you?

    BTW SpaceX has already identified the problem:

    http://www.parabolicarc.com/2012/05/19/spacex-identifies-falcon-9-problem-could-launch-on-tuesday/?utm_source=twitterfeed&utm_medium=twitter

    “Today’s launch was aborted when the flight computer detected slightly high pressure in the engine 5 combustion chamber. We have discovered root cause and repairs are underway.

    “During rigorous inspections of the engine, SpaceX engineers discovered a faulty check valve on the Merlin engine. We are now in the process of replacing the failed valve. Those repairs should be complete tonight. We will continue to review data on Sunday. If things look good, we will be ready to attempt to launch on Tuesday, May 22nd at 3:44 AM Eastern.”

    Their intent is to complete ALL repairs by tonight. That means with adequate fuel margins on a typical ISS mission profile they could launch as early as tomorrow.

    The original spacenews.com article:

    http://www.spacenews.com/launch/120518-propellant-requirements-force-razor-thin-window-for-spacex-launch.html

    Please note that in addition to the next launch window on 22 May there is also one on 23 May, the very next day.

  • Joe

    “Or after trouble shooting would the additional fuel reserves allow them to launch the next day and still leave them with a good margin for rendezvous? Off-hand I don’t have the answer to that one. Do you?”

    I do not have the exact launch window parameters in front of me, but I know (from Shuttle days) that the windows are too narrow to allow getting people on to the pad to examine and change out hardware. The Space X statement you quote says: “Those repairs should be complete tonight.” That time frame is far too long to stay within even a more standard launch window.

    “Their intent is to complete ALL repairs by tonight. That means with adequate fuel margins on a typical ISS mission profile they could launch as early as tomorrow.”

    No, they would have to wait for the next launch window, which is determined by orbital mechanics; not just hardware availability.

    “Please note that in addition to the next launch window on 22 May there is also one on 23 May, the very next day”

    Launch windows (like Buses) come along at regular intervals, the question is how many will you miss (on each attempt) before you catch one.

  • wade

    it will most likely Go on the 22nd. they are reporting a faulty check valve. plus this little nail-biter draws attention to the actual launch for those not so understanding of importance of this launch. it will be a strong statement to the entire world as to a proven idea beyond theory and truly steps up the “game”, so to speak. and advances Space X from successful Test Flight to a real workhorse.

  • Ron

    “Their intent is to complete ALL repairs by tonight. That means with adequate fuel margins on a typical ISS mission profile they could launch as early as tomorrow.”

    “No, they would have to wait for the next launch window, which is determined by orbital mechanics; not just hardware availability.”

    Granted. Launch windows don’t always come back to back, on consecutive days. (Even allowing for generous fuel reserves.)

    “Please note that in addition to the next launch window on 22 May there is also one on 23 May, the very next day”

    “Launch windows (like Buses) come along at regular intervals, the question is how many will you miss (on each attempt) before you catch one.”

    Looks like they just caught one! Have a nice day!

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