Multiple engines on the first stage, good or bad idea?

[JBF]

I thought the Orbcomm boost was scrapped due to safety concerns regarding the ISS, rather than a technical limitation of the SpaceX.

Statistically, the answer is clear: Multiple engines are more reliable as long as the failure of one engine does not cause the failure of others, or the loss of mission.

A 9 engine vehicle is 9 times more likely to have an engine failure than a 1 engine vehicle (see note), so the probability that the failure will effect other engines must be lower than 1 in 9.   

Note: One would also assume that a production run 9 times greater will result in more reliable engines.

Whether it is more cost effective remains to be seen. If the engine is 100% reliable, then a single engine would be best. What is the most reliable engine to date? Would it be the space shuttle SRBs with 1 failure in over 200 launches? However, solids have unpleasant failure modes, so what is the most reliable liquid fuelled engine?

The 2nd burn was automatically canceled due to the fuel check after the release of dragon. There was not enough fuel to get it to the approved orbit.

[peter-b]

The 2nd burn was automatically canceled due to the fuel check after the release of dragon. There was not enough fuel to get it to the approved orbit.

There was not enough fuel to get it to the approved orbit with sufficient margin.

[JohnFornaro]

To the OP:  Good idea.

Agree, on balance. Also the multiple engine set up has turned out fortuitous for the proposed reuseable first stage design. But of course SpaceX had to use multiple engines to get the performance they needed and made a virtue out of a necessity.

I have a bit of a quibble with the grammatical term "fortuitous".  It cannot be inferred that the design was somehow, not an intelligent design, but a "fortuitous" one.  I'm guessing that their design process began with the concept of multiple engines, since they have been talking about the benefits of single or even double engine shut off for quite some time.

In other words, they didn't "have" to use multiple engines, they "chose" to use multiple engines, in order to have a predictable, reliable capability.

There very well may be an unscientific entity out there in the universe, called luck.  I'm not the guy in charge of metaphysics.  Even one of the respected posters around here said something like, "They got lucky".  If luck should be considered the salient factor, does that mean that engineering, a type of intelligent design, can have no predictable, reliable effect in countering the effects of "luck"?  That every successful launch is a "lucky" one?  And the unsuccessful ones "unlucky"?

Great.  Now "they" will call me a Godwinian Grammarian.

Apples-to-oranges, Taurus failed because of a lack of payload fairing separation not engine failure.

Failure is failure.  The cause of failure is certainly in need of discussion, but the F-9 kept on flying.  That is small consolation to Orbcomm, but their insurance has already put a value on that consolation.  From that value standpoint, did Orbcomm suffer a failure?  Did their flight insurance cover loss of future revenue?

Personally, I'm not putting my flyable lunar crater assay probe on one of their rockets without insurance.  I'm in good hands, BTW.

At a 1 in 40 engine failure rate that means a statistical expectation of an engine failure in the Falcon Heavy on more than half the flights.

In other words, they cannot learn from their mistakes and improve the engine to reduce the failure rate?

It was not a design requirement but a fallout

What does this statement mean?

The size of the existing Merlin engine dictated that 9 would be needed to meet mission requirements.

That they designed the rocket with the engines they had?  And this is a bad idea because?

They designed for redundancy. They designed in engine separation barriers. They designed in in-flight recovery. Sure sounds like they designed for loss of an engine.

The distinction between an original specification point and the choice of meeting that specification with redundancy and engine-out ability is precisely what I was referring to as "philosophical wrangling".

Bingo.

Engine out capability is bug and not a designed in feature

Luck has nothing to do with it.  They didn't draw a bullseye around their target and then say that's what they intended to hit.

It would be pointless to test all engines to destruction - you would have no engines left to actually fly.

Which is a twist on the perennial carpenter's dillema:

"I've cut this board three times already, and it's still too short!"

[Proponent]

It really all comes down to whether or not fratricide is common with your typical failure modes, and the jury is out on that one.

You think so?  The Futron study (attached) of 20 years' worth of US space launches cites six failures of liquid-propellant engines, of which I believe only one involved any kind of containment breach.  And SpaceX claims its Kevlar shields would have prevented fratricide in that case.

Or, just to name another statistically-complete sample, consider the set of all US human spaceflight launches, including uncrewed test flights.  I'm aware of about ten engine failures, starting with GT-6 and notably including multiple failures on Apollo 6, but I believe none of those failures involved significant damage to other engines*.

Doesn't that suggest that benign engine failures are roughly an order of magnitude more frequent than catastrophic ones?

*Unless you count Apollo 6, where, as a result of a wiring error, the command to shut-down a malfunctioning engine was sent to the wrong engine, resulting in two engines going out.

EDIT:  Grammar, missing word "launches"

[FuseUpHereAlone]

Well if it was actually designed with engine-out capability as a requirement, then the CRS-1 flight proves that Falcon 9 has not met this requirement. 

Depends on what you mean by "it".  The CRS-1 mission appears OK, and thus F9 met its nominal goals; the rest is up to Dragon and is TBD.  The ORBCOMM mission was not OK and thus F9 did not meet its nominal goals.

Queue interminable argument about "launch" vs. "flight" vs. "mission" vs. etc...

By “it”, I’m mean the launch vehicle (Falcon 9 as it happens to be)… and the launch vehicles “mission” starts when the first stage engines ignite on the launch pad and doesn’t end until the upper stage has been deposited into a disposal orbit.  In the mean time, whether the launch vehicle is depositing one satellite or a dozen, it has only ONE delta-v budget that the entire mission has to fit within.  Because Falcon 9 overran its delta-v budget when it lost a first stage engine, it was not able accomplish the ENTIRE mission.

[FuseUpHereAlone]

Well if it was actually designed with engine-out capability as a requirement, then the CRS-1 flight proves that Falcon 9 has not met this requirement.

I'm choosing to believe that you wrote this as a joke, because the other alternative is too depressing.

Alternatively, could you say that Falcon 9 has engine-out capability for one payload, but not two?

I wish it were as simple as that.  It really depends on the payload size and the amount of delta-v required to get there.  For instance, if you where launching 7500kg from Cape Canaveral to a 2000km circular orbit @28.5 degrees (the max per Falcon 9 user’s guide), then you’re pushing the limits of what a nominally performing Falcon 9 deliver (ie all 9 engines working full duration).  In which case I’d even with one payload you’re probably not going to accomplish the mission with an engine-out.  However, launch the same payload to 500km (which requires a lot less delta-v), then you’ll probably have sufficient performance reserve to lose one engine (or more).

[FuseUpHereAlone]

Well if it was actually designed with engine-out capability as a requirement, then the CRS-1 flight proves that Falcon 9 has not met this requirement.

I'm choosing to believe that you wrote this as a joke, because the other alternative is too depressing.

This isn't a joke, just a simple exercise in determining design requirements, and validating them based on performance.  Otherwise we’re just spinning the truth.

[Lars_J]

Well if it was actually designed with engine-out capability as a requirement, then the CRS-1 flight proves that Falcon 9 has not met this requirement.

I'm choosing to believe that you wrote this as a joke, because the other alternative is too depressing.

This isn't a joke, just a simple exercise in determining design requirements, and validating them based on performance.  Otherwise we’re just spinning the truth.

Hardly - you are just choosing to redefine what "engine-out capability" to fit what you want. (to be "engine out with no performance loss") But this is not how the aerospace industry in general uses the term. Commercial airliners can lose an engine, but are not expected to continue on to reach their destination at the exact time and place they would have otherwise. The designed engine out capability is there to save the payload (passengers) and craft, allowing it to land at the earliest opportunity.

Now let's read your statement again: "if it was actually designed with engine-out capability as a requirement, then the CRS-1 flight proves that Falcon 9 has not met this requirement". Exactly who is "spinning the truth" here?

[Rik ISS-fan]

Well if it was actually designed with engine-out capability as a requirement, then the CRS-1 flight proves that Falcon 9 has not met this requirement. 

Proof: The CRS-1 flight profile called for Falcon 9 to separate from Dragon in one orbit, boost to another orbit, then separate from the Orbcomm satellite.  The first stage suffered an engine-out event.  Falcon 9 successfully releases Dragon, but lacks enough propellant to boost the Orbcomm satellite to its proper orbit (per the flight profile).  If Falcon 9 could not execute the entire mission with an engine-out, then engine-out does not allow Falcon 9 to meet its rated performance.  Therefore, Falcon 9’s rated performance cannot be attained with an engine-out.

The point here is that you can’t just cluster a bunch of engines together and say that it has “engine-out” capability.  If it does, then it better perform as planned when one or more of those engines cuts out.  I suspect that a Falcon 9 with engine-out designed into it would really have performance numbers similar to a Falcon 8 (an imaginary Falcon 9 with one unused engine).  Maybe a Falcon 7 if we wanted 2 engine-out capability.

I totally agree with you FuseUpHereAlone, when the maneuver for the Orbcomm satellite wasn't performed because there wasn't enough propellant (margin) left to execute it. But it they didn't do it for ISS safety reasons, I disagree.
   
I think it is appropriate to call the falcon 9 launch partially failed. Because the Orbcomm satellite didn't reach it's destined orbit. But luckily the most important payload the CRS-1 Dragon reached it's final destination. And I think that no other launcher than the multi-engine falcon 9 could have achieved that while suffering an engine failure. Most rockets wouldn't have reached orbit (a complete failure).

That said, I think using multiple engines on a stage has pros and cones. Others have given good points for this. I think the main benefit are cost savings, and increased reliability over time. And the main disadvantage is that the chance of an engine failure, especially during the first couple of launches, are much higher. SpaceX has designed a good system and process for their falcon 9. This launch proved  one part of that (the desire to have engine out capability).  The other part is increasing the reliability of there merlin engine, that part needs to be seen.

SpaceX knew this disadvantage and that is the reason they emphasize the complexity of launching something. They know a lot of work has to be done to improve the reliability of the merlins. And lets hope they'll learn a lot from this controlled engine failure. 

[cambrianera]

And the main disadvantage is that the chance of an engine failure, especially during the first couple of launches, are much higher.

If you have a fault tolerant system, this isn't entirely a disadvantage.
Let's put in other words what rklahen said in another post:
http://forum.nasaspaceflight.com/index.php?topic=30053.msg963605#msg963605
There is no testbed like the real thing. If you have a system capable of survive a fault you can have the double thing: deliver, and learn from fault.
You put it few sentences after the one I quoted:
"And lets hope they'll learn a lot from this controlled engine failure"
but let me add:
It's sure they'll learn a lot from this controlled engine failure !

[Downix]

People know I am not a SpaceX amazing people by any stretch. I'm rather hard on them in fact. Not because I dislike them, but because launching rockets is hard and a lot of fans fail to take that into account with their praise.

The engine failure was going to happen sooner or later. That is the nature of the business. However, they still managed to achieve a partial mission success with it. That demonstrates something quite positive.

They will learn from this, and will incorporate the lessons from it into their next launch. This is normal in the industry. SpaceX is a new company, these things will happen and it is better to have them happen sooner than later.

That it happened now, and even with the failure the primary mission was a success even if the secondary was not, it overall is a win for SpaceX. Their models and concepts proved themselves with this launch.

Avoiding an issue is good, but addressing what happens when, not if, an issue happens is just as important, and SpaceX clearly demonstrated that with this launch. Well done.

[GalacticIntruder]

I've developed a new pet peeve since the launch. Am I the only that prefers the industry euphemism RUD not be used ever again? Has to be something better.

It's good if it worked. It worked  this time. Is it ideal from a design and mass perspective, probably not. Perhaps we ask the wrong questions.

I would love to see, sometime this week, Musk, Shotwell, Mueller, and others, set up a joint press conference, and answers questions; preferably from knowledgeable journalists. Would it be good PR? Not really since no one outside of the space community cares about this level of detail, specifically engines and Orbcomm.

Agree or disagree?

[Jim]

I've developed a new pet peeve since the launch. Am I the only that prefers the industry euphemism RUD not be used ever again? Has to be something better.

It isn't an industry euphemism, but a Spacex one

[mmeijeri]

It isn't an industry euphemism, but a Spacex one

A NewSpace euphemism I think, but it's intended as humour, just like "engine-rich exhaust".

[MP99]

I totally agree with you FuseUpHereAlone, when the maneuver for the Orbcomm satellite wasn't performed because there wasn't enough propellant (margin) left to execute it. But it they didn't do it for ISS safety reasons, I disagree.

I suspect that if F9 hadn't launched into ISS's orbital plane because of the primary, then Orbcomm could have been lifted into a lower, but still useful orbit. The need to lift above ISS made it all-or-nothing.

That said, I think using multiple engines on a stage has pros and cones. Others have given good points for this. I think the main benefit are cost savings, and increased reliability over time. And the main disadvantage is that the chance of an engine failure, especially during the first couple of launches, are much higher.

Although there's only one US engine (ie no redundancy), I presume that M1vac is under lower stress because it is lower thrust.

It appears that M1Dvac runs at the same chamber pressure as M1Dsl. Wondering whether that makes it more likely to fail, which would increase the chance that a launch will fail because the US fails.

cheers, Martin

[giggleherz]

Awesome link, feels strange going back ten years reading that stuff.

[A_M_Swallow]

{snip}

At a 1 in 40 engine failure rate that means a statistical expectation of an engine failure in the Falcon Heavy on more than half the flights.

In other words, they cannot learn from their mistakes and improve the engine to reduce the failure rate?

That is a different equation.

My commentary.  SpaceX (and FAA) had better learn from their mistakes.

Engine rate	Launch anomaly Rate
1:27	Every launch
1:270	1:10
1:2700	1:100

[giggleherz]

Lets call it "um", for unassuming malfunction.

[beancounter]

{snip}

At a 1 in 40 engine failure rate that means a statistical expectation of an engine failure in the Falcon Heavy on more than half the flights.

In other words, they cannot learn from their mistakes and improve the engine to reduce the failure rate?

That is a different equation.

My commentary.  SpaceX (and FAA) had better learn from their mistakes.

Engine rate	Launch anomaly Rate
1:27	Every launch
1:270	1:10
1:2700	1:100

All the above is simply jumping to conclusions on virtually no data, i.e. speculation.  You all need to wait until SpaceX has examined their data and determined root cause.  That will lead them to whatever fixes they need to make to prevent or reduce the possibility of such a situation in the future.  There's no evidence yet to base any conclusions on whatsoever.

[sdsds]

As I understand the current claim, F9 can meet its performance objectives even if it loses a first stage engine at T+0, i.e. just after first motion. The vehicle is essentially an F8 that by default carries a spare engine.

The exact claim (from the F9 web page) is, "This vehicle will be capable of sustaining an engine failure at any point in flight and still successfully completing its mission."

If spare tires make sense for automobiles, why not spare engines for launch vehicles?