Author Topic: Multiple engines on the first stage, good or bad idea?  (Read 52275 times)

Offline Star One

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After today's events can we form any kind of view whether it is better to have multiple engines on the first stage of a modern rocket or if it is best to have just one or two?

To me having multiple engines equals more things that could go wrong but also means on the other hand more redundancy if something does go wrong.

Offline peter-b

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #1 on: 10/08/2012 07:03 pm »
After today's events can we form any kind of view whether it is better to have multiple engines on the first stage of a modern rocket or if it is best to have just one or two?

To me having multiple engines equals more things that could go wrong but also means on the other hand more redundancy if something does go wrong.

It seems to me that there are benefits and drawbacks to both approaches, from the point of view of performance, reliability, and cost. And the jury's still out.
Research Scientist (Sensors), Sharp Laboratories of Europe, UK

Offline mduncan36

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #2 on: 10/08/2012 07:05 pm »
I don't think it really matters. You have the number of engines you design for based on what you have to work with. Yes, having nine engines means you have nine times the chance for failure but it also means you only lose one ninth of your thrust if you lose one. Having a single big engine means that you have only one ninth the risk of failure but one hundred percent of thrust is gone if you lose it.

Build them all the best you can and fly.

Offline Star One

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #3 on: 10/08/2012 07:07 pm »
After today's events can we form any kind of view whether it is better to have multiple engines on the first stage of a modern rocket or if it is best to have just one or two?

To me having multiple engines equals more things that could go wrong but also means on the other hand more redundancy if something does go wrong.

It seems to me that there are benefits and drawbacks to both approaches, from the point of view of performance, reliability, and cost. And the jury's still out.

To me it seems as if the long shadow that the N1 and its multiple failures casts is one that has maybe set some against the idea of rockets with multiple engines on the first stage. Though to me this is somewhat unfair considering the time and circumstances that contributed to the issues of the N1.

Offline strangequark

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #4 on: 10/08/2012 07:17 pm »
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. Engine redundancy works well for aircraft because uncontained failures are very rare. Rockets are higher pressure systems with lower margins.
« Last Edit: 10/08/2012 07:19 pm by strangequark »

Offline savuporo

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #5 on: 10/08/2012 07:38 pm »
Soyuz-U, the worlds most reliable booster, has multiple engines.
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Offline JohnFornaro

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #6 on: 10/08/2012 08:05 pm »
To the OP:  Good idea.
Sometimes I just flat out don't get it.

Offline IRobot

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #7 on: 10/08/2012 08:07 pm »
Aircrafts have multiple engines. Elon loves to compare his vision with the cost of a 747, so why not compare the number of engines and overall security increase?

Offline douglas100

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #8 on: 10/08/2012 08:09 pm »
Soyuz-U, the worlds most reliable booster, has multiple engines.

It has many more nozzles than engines. It cannot recover from loss of an engine.
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Offline strangequark

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #9 on: 10/08/2012 08:11 pm »
Soyuz-U, the worlds most reliable booster, has multiple engines.

N-1, arguably the world's least reliable booster, also had many multiple engines. You cannot make a simple argument based on one vehicle (that only has five primary engines).

Aircrafts have multiple engines. Elon loves to compare his vision with the cost of a 747, so why not compare the number of engines and overall security increase?

Quote
<snip>
Engine redundancy works well for aircraft because uncontained failures are very rare. Rockets are higher pressure systems with lower margins.

DIRECT had a point with engine out for RL-10, which is a low pressure expander cycle engine (simple, and benign cycle) that they were going to use for EDS. I've never been as persuaded for a high-pressure gas generator first stage engine.

It's not necessarily that Elon is wrong (the jury is out), but comparing to aircraft turbofans is an extremely misleading simplification.
« Last Edit: 10/08/2012 08:16 pm by strangequark »

Offline savuporo

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #10 on: 10/08/2012 08:17 pm »
Soyuz-U, the worlds most reliable booster, has multiple engines.

N-1, arguably the world's least reliable booster, also had many multiple engines. You cannot make a simple argument based on one vehicle (that only has five primary engines).


Yes, statistically insignificant number of attempts like N-1 dont really contribute to the argument, but a booster that has flown more than 700 flights, with the highest ever achieved launch rate actually sort of does.

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Offline douglas100

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #11 on: 10/08/2012 08:17 pm »
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.
Douglas Clark

Offline strangequark

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #12 on: 10/08/2012 08:48 pm »
Soyuz-U, the worlds most reliable booster, has multiple engines.

N-1, arguably the world's least reliable booster, also had many multiple engines. You cannot make a simple argument based on one vehicle (that only has five primary engines).


Yes, statistically insignificant number of attempts like N-1 dont really contribute to the argument, but a booster that has flown more than 700 flights, with the highest ever achieved launch rate actually sort of does.



Really has no bearing on Falcon 9, and doesn't even inform the broader discussion. The success of one launch vehicle, with 5 engines tells you nothing about the general trend, especially if you don't delve into the particulars of those failures. Otherwise, you could equally argue that the Proton K, with 300+ launches (statistically significant, yes?), and a ~15% failure rate for a 6 engine vehicle argues against larger numbers of engines. Like I said before, what you really need to make a valid assessment is to look at how likely it is that failure of one engine will be such that it can result in fratricide of neighboring engines. So, in answer to the original poster:

Multiple engines on the first stage, good or bad idea?
Can be a good idea, but there is a tip over point where redundancy reduces reliability, and that is vehicle and engine dependent. That tip-over point is likely to be smaller, rather than larger, because first-stage rocket engines are high pressure, low margin devices, which increases the chances of an uncontained failure.

Offline Robotbeat

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #13 on: 10/08/2012 08:54 pm »
Soyuz-U, the worlds most reliable booster, has multiple engines.

N-1, arguably the world's least reliable booster, also had many multiple engines. You cannot make a simple argument based on one vehicle (that only has five primary engines).


Yes, statistically insignificant number of attempts like N-1 dont really contribute to the argument, but a booster that has flown more than 700 flights, with the highest ever achieved launch rate actually sort of does.



Really has no bearing on Falcon 9, and doesn't even inform the broader discussion. The success of one launch vehicle, with 5 engines tells you nothing about the general trend, especially if you don't delve into the particulars of those failures. Otherwise, you could equally argue that the Proton K, with 300+ launches (statistically significant, yes?), and a ~15% failure rate for a 6 engine vehicle argues against larger numbers of engines. Like I said before, what you really need to make a valid assessment is to look at how likely it is that failure of one engine will be such that it can result in fratricide of neighboring engines. So, in answer to the original poster:

Multiple engines on the first stage, good or bad idea?
Can be a good idea, but there is a tip over point where redundancy reduces reliability, and that is vehicle and engine dependent. That tip-over point is likely to be smaller, rather than larger, because first-stage rocket engines are high pressure, low margin devices, which increases the chances of an uncontained failure.
Soyuz has five stages, but no engine-out capability, thus has no redundancy.

There are more things going on than just number of engines. One big thing to take into account is /how many total engines are made and launched each year/? If I'm making and launching 2 engines a year, it's not likely I'll get very good at it for a reasonable cost. If I make 100, then I probably will get good at it. And If I have redundancy, then my reliability will be improved even more.
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Offline giggleherz

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #14 on: 10/08/2012 09:41 pm »
From a back seat point of view, it might be a good idea to run these engines to the breaking point to see just what that is, do they even know? I did have a thought about this launch insomuch as it seemed that the engine was on the outside corner. I could be wrong but what if it was the central engine? They may have just barely dodged a bullet and we will never know. I have always thought it would be extremely helpful to have a chase plane or a larger plane with a big camera and also allow that plane to follow a lot closer than the two or three mile limit. There must be a way that's cost effective to somehow have multiple cameras following a liftoff from close range.

Offline Lars_J

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #15 on: 10/08/2012 09:49 pm »
From a back seat point of view, it might be a good idea to run these engines to the breaking point to see just what that is, do they even know?

Of course they know. They have run Merlin engines in the test stands in burns *much* longer than a normal mission. They have even run engines to the point of failure. They know their engines better than anyone.

Now all engines aren't run that extreme, of course. It would be pointless to test all engines to destruction - you would have no engines left to actually fly. I'm sure that they are looking at all the telemetry from that engine to figure out what happened, and how they can improve qualification tests.

Offline Go4TLI

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #16 on: 10/08/2012 09:55 pm »
It's very much has to do with the design of the engine and the rocket.  Follow that up with a host of trades relative to technical decisions and cost and production targets that one wants to meet and you find it is as simple, and complicated, as that.

So, the answer is it really depends and no definitive answer will be derived here.

Offline mgfitter

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #17 on: 10/08/2012 10:23 pm »
From a back seat point of view, it might be a good idea to run these engines to the breaking point to see just what that is, do they even know?

By all accounts, they've been routinely doing at least 2 engine tests every single day for years now. That amount of testing results in a massive library of data on these engines by this point, not to mention that every time the F9 flies they gather 10 engines worth of flight data too.

This "anomaly" is simply their first hard data point for what happens during a real engine failure during the flight of a clustered first stage configuration. Given that the vehicle continued to fly and continued to transmit its data back to the ground after the anomaly, they are likely to learn a great deal from this event, knowledge that they ought to be able to use to help improve the reliability of future engines and prevent this failure type from occurring again.

What I would be interested in knowing, is when they determine the root cause of this failure, whether they can replicate the issue on the test stand in order to study it in greater detail.

Ultimately, this is going to be much the same process as the Delta-IV/PRW team are also going through, after their problems just a few days earlier.

Both teams have had failures in the past and are all-but guaranteed to have more failures in the future. They each have work ahead of them in order to remove these particular failure modes though. And they will.

-MG.

Offline Star One

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #18 on: 10/08/2012 10:29 pm »
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.

Be interesting to see how the Falcon Heavy gets on with its test flight as there you have a vehicle with twenty-seven engines that will be undergoing even greater stresses and strains than this launch.

Offline Joel

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #19 on: 10/08/2012 10:32 pm »
One thing I was pondering after today is whether multiple engines might increase the the chances of getting to an orbit, but might decrease the chances of getting to the intended orbit.

If this is the case, maybe it is fair to think of developing capability of rendezvous with and reboost satellites in wrong orbits.

Just a thought.

Offline thydusk666

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #20 on: 10/08/2012 10:46 pm »
I think it's definitely a Good idea:
1. Tactical: When you make 1000 engines/year instead of 50-100, it makes more sense for a bigger % of the rocket to be produced by assembly line (with many robots ;)).
2. Quality: When producing in heavy numbers, a fast, cheap QoC process needs to be implemented. By analogy with consumer products, that would inevitably raise the reliability line.
3. Economically: Having an efficient assembly line with a big output means cheaper products.
4. Redundancy: More engines means less risks for the mission outcome. That was shown today.

Offline Star One

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #21 on: 10/08/2012 10:53 pm »
What redundancy improvements will the V1.1 Falcon see?

Offline Jim

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #22 on: 10/08/2012 10:57 pm »

4. Redundancy: More engines means less risks for the mission outcome. That was shown today.

That wasn't shown.  Ask Obcomm.

Offline Robotbeat

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #23 on: 10/08/2012 11:06 pm »
I don't think redundant engines have proved themselves, yet. They'll have to fly a lot more times to do that.

But we DO have a verification that SpaceX has an engine-out capability which is more than just marketing spin.
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Offline garidan

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #24 on: 10/08/2012 11:10 pm »
 V1.1 Falcon will have redundancy improvements because Merlin 1D can throttle down instead of switching off 2 engines as for now.
What happened yesterday would have not bring consequences even to Orbcomm if it were a Falcon 1.1, because no engine were cut off and you could have brought (say) from 80% times 9 engines to 90% times 8 engines.


Offline Robotbeat

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #25 on: 10/08/2012 11:14 pm »
V1.1 Falcon will have redundancy improvements because Merlin 1D can throttle down instead of switching off 2 engines as for now.
What happened yesterday would have not bring consequences even to Orbcomm if it were a Falcon 1.1, because no engine were cut off and you could have brought (say) from 80% times 9 engines to 90% times 8 engines.


There might still have been consequences because the orbital elements would've been different.

That said, Falcon 9 v1.1 should have PLENTY of performance for launching cargo Dragon plus a slew of Orbcomm birds, even allowing some performance margin for engine-out. Merlin 1D is a huge improvement on the Merlin 1C, and Falcon 9 v1.1 makes several weight improvements, as well.
« Last Edit: 10/08/2012 11:15 pm by Robotbeat »
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Offline clongton

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #26 on: 10/08/2012 11:37 pm »
To me it seems as if the long shadow that the N1 and its multiple failures casts is one that has maybe set some against the idea of rockets with multiple engines on the first stage.

N1 is not a good example to put up as an example of multi-engine failure modes. N1 was forced to fly by politicians looking to beat the Americans to the moon who had zero knowledge of what they were demanding. The engines were not ready to fly and the N1 design itself was not complete. What was forced to fly was a hogepoge of incomplete designed sections and unqualified and untested engines. A beautiful example of why politicians should NOT EVER be involved in the decision making of launch vehicle design and flight schedules.
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Offline Sotar

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #27 on: 10/08/2012 11:42 pm »
True Engine out reliability.

So Elon likes to compare F9 to commercial aircraft most often the 747, I’m going to use the 777 to make things simpler.    It is my understand that with any twin engine jet  777, 737, 757 etc…  each engine needs to be able to produce 2x the thrust that is needed for take off.  This is done in order to ensure the ability to take off and conclude the flight should an engine out occur at any point during the flight, including the critical time beyond the point of no return on take off.

So given that it if he really wants to compare to a commercial jet would seem that a launch vehicle should have a minimum of 2 engines each capable of independently lifting the vehicle from the pad to orbit.  Assume that the engines would be throttled each at some increment between 50% to 75%, then use timing etc… to control max velocity etc.. .

Realistically that may or may not be possible.  You would have to take into consideration cost to develop, cost to manufacture, size of engine, bell etc.. vs. size of base of the launch vehicle, ability to throttle to 50% and up to 100%, ability to do it quick enough during a mission, efficiency of a smaller engine vs. that of a really big engine… I’m sure there is a bunch of things I’m leaving out and work arounds for many of these issues as well.…

To determine the optimal and or the maximum number of engines in theory you could do a decision tree with a cost/benefit type analysis to determine the number of engines.  You would need to compare the probability of an engine failure that would result in fratricide compared to the number of motors you add.  It would seem that the more motors the higher the probability of a failure and the more launches the higher probability of failure etc…   Of course you could take into considerations additional things like offset by barriers etc.. to prevent  fragments from a RUD event…

So from my non-rocket engineer point of view:

Minimum # of engines 2 – gives you 1 engine out reliability

Maximum # of engines TBD optimized based on probability of failure of that particular engine and mitigation factors that would prevent LOV from such a failure, assuming the remaining engines could produce the thrust needed at any point to ensure mission requirements are fully met.

Anyway that is just my logic, I’m not an engineer or expert on anything related to rocket science just an observer. 
1% for Space

Offline clongton

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #28 on: 10/08/2012 11:49 pm »
DIRECT had a point with engine out for RL-10, which is a low pressure expander cycle engine (simple, and benign cycle) that they were going to use for EDS.

We also had engine out capability with the core stage, able to loose a single engine after 31 seconds of flight and still make orbit.

I was then, and remain today, an avid supporter of multi-engine MPS for launch vehicles - to a point. I don't know where that point is yet but imho a launch vehicle should be able to loose 1 or 2 main engines, depending on the number in the cluster, and still make orbit. A crew should never ever have to abort a mission because of a single engine failure and a non-crewed mission should never ever be lost because of a single engine failure. Multi-engine redundancy is critical to mission success. I really don't care what the statisticians say about the number of increased potential failures because of an increase in engine count. Engines are not perfect and no matter how careful the technicians are, we WILL see engine failures in flight here and there. So build the engines big enough to be useful by themselves and reliable enough so that when clustered a mission can still be successful when 1 or 2 of them fail in flight - AS SOME OF THEM ABSOLUTELY WILL! We saw one yesterday, where a tested and certified engine failed in flight and had to be shut down. Yet the mission carried on and the spacecraft was delivered to the proper orbit on time.
« Last Edit: 10/09/2012 12:22 am by clongton »
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Offline QuantumG

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #29 on: 10/09/2012 12:21 am »

4. Redundancy: More engines means less risks for the mission outcome. That was shown today.

That wasn't shown.  Ask Obcomm.

The alternate outcome to this launch wasn't a perfect orbital insertion for Orbcomm, it was a smoking hole in the ocean (ala Taurus last year).

Human spaceflight is basically just LARPing now.

Offline FuseUpHereAlone

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #30 on: 10/09/2012 12:34 am »
Correct if I'm wrong here, but I don't think any launch vehicle has ever been designed engine-out capability as a requirement; it just happens to be a by-product of clustering smaller engines as a means of achieving higher thrust.  Case in point: the Soviet Union in the 60's decided it was more feasible to cluster NK-33 engines rather than dealing with combustion stability challenges associated with  a F-1 sized thrust chamber.  Fast forward 40 years later: the Falcon 9 was built as a scaled up version of Falcon 1, utilizing as much commonality as possible (including Merlin engines).   

Offline Go4TLI

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #31 on: 10/09/2012 12:57 am »
There is a piece here I have not seen anyone really comment on that is integral to the story.  That is reliability.

One vehicle could have all kinds of engines but if reliability is not there and one or two fail consistently or somewhat routinely, ultimately that will hurt business because of perception and perhaps with more challenging mission requirements where performance is critical.

There is nothing wrong with the choice SpaceX made, it works from a design perspective and their business model.  Less engines work for other rockets because it works from their design trades and their business model. 

Overall vehicle reliability must be an ultimate factor. 

Offline Ronsmytheiii

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #32 on: 10/09/2012 01:16 am »
The alternate outcome to this launch wasn't a perfect orbital insertion for Orbcomm, it was a smoking hole in the ocean (ala Taurus last year).

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

A point to remember though, there is a certain flight regime where ANY loss of engines will result in LOM and falling out of the sky, main engine numbers are not redundant during the first few seconds of flight.

Not that more than one engine is a bad thing, shuttle aborted to orbit and this launch are perfect examples. It is just a trade-off as multi-engine cores have weaknesses compared to single cores in certain areas.

Offline beancounter

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #33 on: 10/09/2012 03:10 am »
Good idea.  If you'd only the one or two, your mission's toast.
Believe the Fultron study into vehicle failures demonstrated increased reliability of the F9 due to design.
Beancounter from DownUnder

Offline strangequark

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #34 on: 10/09/2012 04:30 am »
DIRECT had a point with engine out for RL-10, which is a low pressure expander cycle engine (simple, and benign cycle) that they were going to use for EDS.

We also had engine out capability with the core stage, able to loose a single engine after 31 seconds of flight and still make orbit.

I was then, and remain today, an avid supporter of multi-engine MPS for launch vehicles - to a point. I don't know where that point is yet but imho a launch vehicle should be able to loose 1 or 2 main engines, depending on the number in the cluster, and still make orbit.

Indeed, and I certainly don't think that 1 engine is optimal for first stage either. However, there is a tipover point, and I'm biased toward thinking it's lower with first stages. Falcon 9 did demonstrate engine out this time, but there was a lot of debris flying around those other engines. Nine is fair number of engines (27 is huge if they ever fly it), and the proof will only come with flight rate. The positive is that they sure as hell produce a lot of data  points every flight :).

Offline Joffan

Correct if I'm wrong here, but I don't think any launch vehicle has ever been designed engine-out capability as a requirement; it just happens to be a by-product of clustering smaller engines as a means of achieving higher thrust.  Case in point: the Soviet Union in the 60's decided it was more feasible to cluster NK-33 engines rather than dealing with combustion stability challenges associated with  a F-1 sized thrust chamber.  Fast forward 40 years later: the Falcon 9 was built as a scaled up version of Falcon 1, utilizing as much commonality as possible (including Merlin engines).   

Yeah, you're wrong. Falcon-9 was indeed designed to survive engine out - that's why the on-board computer was ready and able to recompute how to use the remaining engines to meet its principle orbit insertion requirements.

The philosophical wrangling of whether a high-multiple engine design came about through one thought process or another is a pointless debate. All such decisions rest on multiple considerations.
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Offline A_M_Swallow

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #36 on: 10/09/2012 04:59 am »
Indeed, and I certainly don't think that 1 engine is optimal for first stage either. However, there is a tipover point, and I'm biased toward thinking it's lower with first stages. Falcon 9 did demonstrate engine out this time, but there was a lot of debris flying around those other engines. Nine is fair number of engines (27 is huge if they ever fly it), and the proof will only come with flight rate. The positive is that they sure as hell produce a lot of data  points every flight :).

2 * 27 = 54 engines

E(engine failure every) = 54/40 = 1.35 flights

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.
« Last Edit: 10/09/2012 05:00 am by A_M_Swallow »

Offline beancounter

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #37 on: 10/09/2012 05:20 am »
Indeed, and I certainly don't think that 1 engine is optimal for first stage either. However, there is a tipover point, and I'm biased toward thinking it's lower with first stages. Falcon 9 did demonstrate engine out this time, but there was a lot of debris flying around those other engines. Nine is fair number of engines (27 is huge if they ever fly it), and the proof will only come with flight rate. The positive is that they sure as hell produce a lot of data  points every flight :).


2 * 27 = 54 engines

E(engine failure every) = 54/40 = 1.35 flights

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.

On the face of it, yes.  But we don't know the root cause and whether it was statistically valid and can be applied from here on to all future engines or whether it can be corrected in future engines in which case no statistical validity.
If the former then a new engine design is needed, but I reckon they'll identify the root cause and fix it in which case this event will not affect the future reliability of the engine.
« Last Edit: 10/09/2012 05:24 am by beancounter »
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Offline ugordan

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #38 on: 10/09/2012 08:52 am »
What happened yesterday would have not bring consequences even to Orbcomm if it were a Falcon 1.1, because no engine were cut off and you could have brought (say) from 80% times 9 engines to 90% times 8 engines.

That is not true. All engines will burn at maximum thrust for almost the entire first stage burn to maximize performance. They will only throttle down to keep 5 G limits near the end of the burn. There is no option of kicking the engines from 80% to 90% if one engine fails because during most of the burn (all but the last 15 seconds or so) they will have already been running at 100%. You are confusing throttle-limit cutoff with engine failure-related cutoff.

Offline FinalFrontier

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #39 on: 10/09/2012 09:08 am »
After today's events can we form any kind of view whether it is better to have multiple engines on the first stage of a modern rocket or if it is best to have just one or two?

To me having multiple engines equals more things that could go wrong but also means on the other hand more redundancy if something does go wrong.


Good. Without multiple engines this would have most likely destroyed the vehicle or simply resulted in LOM.

Multiple redundancy does come with increased complexity but there is an ideal balance and this vehicle seems quite balanced. The fact that it withstood such a failure and kept flying is testament to the multi-engine-out design actually working.
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Offline FinalFrontier

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #40 on: 10/09/2012 09:10 am »
V1.1 Falcon will have redundancy improvements because Merlin 1D can throttle down instead of switching off 2 engines as for now.
What happened yesterday would have not bring consequences even to Orbcomm if it were a Falcon 1.1, because no engine were cut off and you could have brought (say) from 80% times 9 engines to 90% times 8 engines.



Wrong the shutdown would still have occurred the damage was too severe for that engine to continue operating.
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Offline Proponent

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #41 on: 10/09/2012 09:11 am »
To me it seems as if the long shadow that the N1 and its multiple failures casts is one that has maybe set some against the idea of rockets with multiple engines on the first stage. Though to me this is somewhat unfair considering the time and circumstances that contributed to the issues of the N1.

Seems to me the track record the Saturn I/IB is a pretty effective counter argument: 19 eight-engine successes in 19 attempts, despite one engine failure.  And it's especially impressive when you consider that the Saturn's track record includes the early years of the space age, when spectacular rocket failures were common.

Offline Proponent

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #42 on: 10/09/2012 09:19 am »
A point to remember though, there is a certain flight regime where ANY loss of engines will result in LOM and falling out of the sky, main engine numbers are not redundant during the first few seconds of flight.

Not necessarily.  Look at old Saturn documents, and you'll see that it was designed on the assumption that one of its eight engines would fail at lift-off and another half way through first-stage flight.

The documents also show performance curves (payload as a function of orbital altitude) for both one and two engines lost during ascent.  That is to say that engine-out tolerance depends on the margins allowed.  Insist on squeezing every bit of performance out of the launch vehicle, and there will be no tolerance at all.

Offline Proponent

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #43 on: 10/09/2012 09:25 am »
The philosophical wrangling of whether a high-multiple engine design came about through one thought process or another is a pointless debate. All such decisions rest on multiple considerations.

Yeah, consider that the Saturn I's first stage was originally to have been powered by four E-1 engines rather than eight H-1s.  The change was proposed by ARPA to save the cost of developing the E-1.  (This has always struck me as a little weird, since von Braun's designs in the 1950s featured massive clustering of engines, so I'd have thought he'd naturally have gone in that direction.)

The N-1's first stage had so many engines, because politics among the Soviet design bureaus prevent Korolev from getting the larger engines he wanted.

The Falcon 9 was originally to have been merely the Falcon 5 -- which likely would have tolerated loss of an engine over a smaller portion of the flight envelope -- but the rocket grew for reasons other than a desire to increase engine-out tolerance.

Offline rklaehn

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #44 on: 10/09/2012 09:34 am »
Definitely good idea.

Imagine the falcon 9 had only one engine. The mission would have been a total failure, and perhaps more importantly: they would only have found out about the design or QA error in the engine after the ~40th flight, which might have been a flight with a much more valuable payload.

Using many engines is a prerequisite to getting high reliability per engine. And with the current extremely low launch rates (compared to any other mode of transportation), having multiple identical engines per stage is a good way to increase the production rate, with all the benefits that brings.

Offline Proponent

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #45 on: 10/09/2012 09:40 am »
Minimum # of engines 2 – gives you 1 engine out reliability

I'd say two is the worst number, because unless the vehicle has enormous performance margins, it will not be able to complete its mission following the loss of half its thrust, unless maybe the loss occurs in the last few seconds of operation.

An exception might be upper stages of crew-carrying vehicles.  In that case, two engines could be better than one if even with one engine out the vehicle can reach some kind of contingency orbit or better abort scenario than would be possible with no thrust at all.

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #46 on: 10/09/2012 11:28 am »
Correct if I'm wrong here, but I don't think any launch vehicle has ever been designed engine-out capability as a requirement; it just happens to be a by-product of clustering smaller engines as a means of achieving higher thrust.  Case in point: the Soviet Union in the 60's decided it was more feasible to cluster NK-33 engines rather than dealing with combustion stability challenges associated with  a F-1 sized thrust chamber.  Fast forward 40 years later: the Falcon 9 was built as a scaled up version of Falcon 1, utilizing as much commonality as possible (including Merlin engines).   

Yeah, you're wrong. Falcon-9 was indeed designed to survive engine out - that's why the on-board computer was ready and able to recompute how to use the remaining engines to meet its principle orbit insertion requirements.

The philosophical wrangling of whether a high-multiple engine design came about through one thought process or another is a pointless debate. All such decisions rest on multiple considerations.

Nope, you are wrong.  It was not a design requirement but a fallout

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #47 on: 10/09/2012 01:10 pm »
True Engine out reliability.

So Elon likes to compare F9 to commercial aircraft most often the 747, I’m going to use the 777 to make things simpler.    It is my understand that with any twin engine jet  777, 737, 757 etc…  each engine needs to be able to produce 2x the thrust that is needed for take off.  This is done in order to ensure the ability to take off and conclude the flight should an engine out occur at any point during the flight, including the critical time beyond the point of no return on take off.


remember 1 bird strike can bring engine failure ......a couple of bird strikes can bring a jet down.
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Offline beancounter

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #48 on: 10/09/2012 02:06 pm »
Correct if I'm wrong here, but I don't think any launch vehicle has ever been designed engine-out capability as a requirement; it just happens to be a by-product of clustering smaller engines as a means of achieving higher thrust.  Case in point: the Soviet Union in the 60's decided it was more feasible to cluster NK-33 engines rather than dealing with combustion stability challenges associated with  a F-1 sized thrust chamber.  Fast forward 40 years later: the Falcon 9 was built as a scaled up version of Falcon 1, utilizing as much commonality as possible (including Merlin engines).   

Yeah, you're wrong. Falcon-9 was indeed designed to survive engine out - that's why the on-board computer was ready and able to recompute how to use the remaining engines to meet its principle orbit insertion requirements.

The philosophical wrangling of whether a high-multiple engine design came about through one thought process or another is a pointless debate. All such decisions rest on multiple considerations.

Nope, you are wrong.  It was not a design requirement but a fallout

Oh, interesting.  Didn't know that.  Source please?
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Offline Jim

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #49 on: 10/09/2012 03:42 pm »
Correct if I'm wrong here, but I don't think any launch vehicle has ever been designed engine-out capability as a requirement; it just happens to be a by-product of clustering smaller engines as a means of achieving higher thrust.  Case in point: the Soviet Union in the 60's decided it was more feasible to cluster NK-33 engines rather than dealing with combustion stability challenges associated with  a F-1 sized thrust chamber.  Fast forward 40 years later: the Falcon 9 was built as a scaled up version of Falcon 1, utilizing as much commonality as possible (including Merlin engines).   

Yeah, you're wrong. Falcon-9 was indeed designed to survive engine out - that's why the on-board computer was ready and able to recompute how to use the remaining engines to meet its principle orbit insertion requirements.

The philosophical wrangling of whether a high-multiple engine design came about through one thought process or another is a pointless debate. All such decisions rest on multiple considerations.

Nope, you are wrong.  It was not a design requirement but a fallout

Oh, interesting.  Didn't know that.  Source please?

The size of the existing Merlin engine dictated that 9 would be needed to meet mission requirements.  When you have multiple engines, you can have engine out capability.  If Spacex went to Merlin 2 right away, it would not have the ability to have engine out capability.

Spacex mantra for Falcon 1 was one engine for reliability.  They had to modify it for Falcon 9.

Offline Star One

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #50 on: 10/09/2012 07:27 pm »
Correct if I'm wrong here, but I don't think any launch vehicle has ever been designed engine-out capability as a requirement; it just happens to be a by-product of clustering smaller engines as a means of achieving higher thrust.  Case in point: the Soviet Union in the 60's decided it was more feasible to cluster NK-33 engines rather than dealing with combustion stability challenges associated with  a F-1 sized thrust chamber.  Fast forward 40 years later: the Falcon 9 was built as a scaled up version of Falcon 1, utilizing as much commonality as possible (including Merlin engines).   

Yeah, you're wrong. Falcon-9 was indeed designed to survive engine out - that's why the on-board computer was ready and able to recompute how to use the remaining engines to meet its principle orbit insertion requirements.

The philosophical wrangling of whether a high-multiple engine design came about through one thought process or another is a pointless debate. All such decisions rest on multiple considerations.

Nope, you are wrong.  It was not a design requirement but a fallout

Oh, interesting.  Didn't know that.  Source please?

The size of the existing Merlin engine dictated that 9 would be needed to meet mission requirements.  When you have multiple engines, you can have engine out capability.  If Spacex went to Merlin 2 right away, it would not have the ability to have engine out capability.

Spacex mantra for Falcon 1 was one engine for reliability.  They had to modify it for Falcon 9.

If there is an idea to move towards fewer engines in future maybe this incident will give pause for thought and perhaps consideration that it is better to stick with a greater number of engines.

Offline GalacticIntruder

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #51 on: 10/09/2012 07:32 pm »
I think so, for a 2 stage rocket. Things always fail at some point. If you can manage it, good.

It is safe to assume 9 engines per core is probably the max you would want to try clustering. SpX would prefer fewer if the cost worked out to build more powerful variants. Falcon 3 [engine] would be nice. They think HVM on smaller engines can lower the cost, and increase reliability.  I don't like single engines per core, but don't like more than 4 per core either. 3 core 27 engine Falcon Heavy is super crazy. Hope it works.

It seems like most failures world wide, now, are not boost stage but upper stage failures. If anything needs improving in the industry, it is that.

I don't think SpX is developing a 1 million pound class super engine, so clustering will be there for any super heavy design.
« Last Edit: 10/09/2012 07:33 pm by GalacticIntruder »
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Offline rklaehn

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #52 on: 10/09/2012 07:44 pm »
Spacex mantra for Falcon 1 was one engine for reliability.  They had to modify it for Falcon 9.

I seem to remember that their analysis was that you should have 1 for simplicity or at least 5 for redundancy. Hence the falcon 5. The increase to 9 engines was to capture the medium payload market without having to develop a new engine.

Offline cambrianera

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #53 on: 10/09/2012 07:50 pm »
Definitely good idea.

Imagine the falcon 9 had only one engine. The mission would have been a total failure, and perhaps more importantly: they would only have found out about the design or QA error in the engine after the ~40th flight, which might have been a flight with a much more valuable payload.

Using many engines is a prerequisite to getting high reliability per engine. And with the current extremely low launch rates (compared to any other mode of transportation), having multiple identical engines per stage is a good way to increase the production rate, with all the benefits that brings.
I agree with you.
There are other design and manufacturing advantages over those you said:
- multiple engines means distributed loads, hence simpler thrust structure (SpaceX will probably use this advantage in v1.1)
- mid size engines means cheaper machine tools being used
- multiple smaller engines means shorter engine bay

I will add that the complexity of managing multiple engines is greatly reduced by modern FADEC (or FADEC like) systems; it really means you must give to the engine only basic commands and settings and expect fast cut off for off nominal conditions (like in CRS-1)
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Offline rklaehn

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #54 on: 10/09/2012 08:03 pm »
I agree with you.
There are other design and manufacturing advantages over those you said:
- multiple engines means distributed loads, hence simpler thrust structure (SpaceX will probably use this advantage in v1.1)
- mid size engines means cheaper machine tools being used
- multiple smaller engines means shorter engine bay

There are even more advantages. The merlin engines can be worked on by placing them on a simple cart, similar to what you would do with a large truck engine.

They can be rolled around the factory floor. Compare that with something the size of an F-1 or RD-170 where every single fabrication step needs a specialized fixture or an oversized crane.

All these are small improvements, but they do add up.

Once they do something larger than falcon heavy, they might have to upsize the engines. But the falcon heavy has a lot of potential by improving the upper stage. So I hope they stick with the relatively small engine as long as possible.

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #55 on: 10/09/2012 08:07 pm »
I agree with you.
There are other design and manufacturing advantages over those you said:
- multiple engines means distributed loads, hence simpler thrust structure (SpaceX will probably use this advantage in v1.1)
- mid size engines means cheaper machine tools being used
- multiple smaller engines means shorter engine bay

There are even more advantages. The merlin engines can be worked on by placing them on a simple cart, similar to what you would do with a large truck engine.

They can be rolled around the factory floor. Compare that with something the size of an F-1 or RD-170 where every single fabrication step needs a specialized fixture or an oversized crane.

All these are small improvements, but they do add up.

Once they do something larger than falcon heavy, they might have to upsize the engines. But the falcon heavy has a lot of potential by improving the upper stage. So I hope they stick with the relatively small engine as long as possible.
They're very, very likely to get an engine-out on Falcon Heavy. They might even have to assume engine-out as essentially a nominal condition on a large fraction of flights and engineer around it. Which will be more complicated with a vehicle like Falcon Heavy, but crossfeed gives you more options.
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Offline Joffan

Correct if I'm wrong here, but I don't think any launch vehicle has ever been designed engine-out capability as a requirement; it just happens to be a by-product of clustering smaller engines as a means of achieving higher thrust.  Case in point: the Soviet Union in the 60's decided it was more feasible to cluster NK-33 engines rather than dealing with combustion stability challenges associated with  a F-1 sized thrust chamber.  Fast forward 40 years later: the Falcon 9 was built as a scaled up version of Falcon 1, utilizing as much commonality as possible (including Merlin engines).   

Yeah, you're wrong. Falcon-9 was indeed designed to survive engine out - that's why the on-board computer was ready and able to recompute how to use the remaining engines to meet its principle orbit insertion requirements.

The philosophical wrangling of whether a high-multiple engine design came about through one thought process or another is a pointless debate. All such decisions rest on multiple considerations.

Nope, you are wrong.  It was not a design requirement but a fallout

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".
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Offline guckyfan

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #57 on: 10/09/2012 09:15 pm »
The low thrust of a single engine will make things a lot easier for them if they can achieve powered landing of the first stage. One of the 9 engines has already very high thrust for landing at the weight of the empty first stage even when throttled. More thrust will make it much harder.

Let's hope they learn from the failure modes and make the engine more reliable with time. Merlin 1D is supposed to be a step in that direction.


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Re: Multiple engines on the first stage, good or bad idea?
« Reply #58 on: 10/09/2012 09:49 pm »
The distinction between an original specification point and the choice of meeting that specification with redundancy and engine-out ability

Wrong again.  Engine out capability is bug and not a designed in feature

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #59 on: 10/09/2012 10:11 pm »
The distinction between an original specification point and the choice of meeting that specification with redundancy and engine-out ability

Wrong again.  Engine out capability is bug and not a designed in feature
Depends on how you look at it. If you take the view point that any redundant system is mass that you cannot get to orbit then yes. If you take the realistic view point that the cost of being 100% perfect is less then the cost of redundancy then no it isn't.
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Re: Multiple engines on the first stage, good or bad idea?
« Reply #60 on: 10/09/2012 11:10 pm »
I have complete faith in SpaceX that they will iron out the problem but you have to believe Elon likely hasn't had much sleep mulling this over. My gut is telling me that they really need to ramp up these engines on the test bed and blow a few up. The idea of a test burn lasting exactly eight minutes and twenty three seconds because the real burn is twenty one seconds ain"t gonna cut it any more.
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Offline Mader Levap

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #61 on: 10/09/2012 11:15 pm »
My gut is telling me that they really need to ramp up these engines on the test bed and blow a few up.
They did what you want long ago. They use their test stands quite often. They do single, two, three, five, all nine. They do shorter and longer burns. They do multiple burns of same engines in row (you know, reuseability kinda requires engine capable of being used again). They had also their share of RUDs.
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Offline Oberon_Command

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #62 on: 10/09/2012 11:25 pm »
My gut is telling me that they really need to ramp up these engines on the test bed and blow a few up.
They did what you want long ago. They use their test stands quite often. They do single, two, three, five, all nine. They do shorter and longer burns. They do multiple burns of same engines in row (you know, reuseability kinda requires engine capable of being used again). They had also their share of RUDs.

But did they ever blow one up intentionally to see what would happen to the others?

Offline Lars_J

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #63 on: 10/09/2012 11:44 pm »
The distinction between an original specification point and the choice of meeting that specification with redundancy and engine-out ability

Wrong again.  Engine out capability is bug and not a designed in feature

It may not have been their original reason to go with 9 engines - but that statement is simply wrong, and even a layman can see that. Once you have a need for multiple engines, you have a deliberate decision to make if you need or want to support engine out capability.

You may see the need for more than one engine as a "bug" - but it is quite clear that a deliberate effort from SpaceX with regards to hardware and software design to support engine out capability paid off significantly in this last flight.

(I suppose I could have simply written "wrong" like you would have, so you'll have to excuse me)

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #64 on: 10/09/2012 11:59 pm »
My gut is telling me that they really need to ramp up these engines on the test bed and blow a few up.
They did what you want long ago. They use their test stands quite often. They do single, two, three, five, all nine. They do shorter and longer burns. They do multiple burns of same engines in row (you know, reuseability kinda requires engine capable of being used again). They had also their share of RUDs.

how many tested when at supersonic speeds?  Only an agressive launch schedule can do this test I believe.
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Offline beancounter

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #65 on: 10/10/2012 01:48 am »
The distinction between an original specification point and the choice of meeting that specification with redundancy and engine-out ability

Wrong again.  Engine out capability is bug and not a designed in feature

It may not have been their original reason to go with 9 engines - but that statement is simply wrong, and even a layman can see that. Once you have a need for multiple engines, you have a deliberate decision to make if you need or want to support engine out capability.

You may see the need for more than one engine as a "bug" - but it is quite clear that a deliberate effort from SpaceX with regards to hardware and software design to support engine out capability paid off significantly in this last flight.

(I suppose I could have simply written "wrong" like you would have, so you'll have to excuse me)

No I agree with Jim here.  You don't design in an engine failure.  No commercial aircraft is designed just in case they have an engine failure.  That's an off-nominal event and SpaceX shouldn't have considered that as part of the design criteria.  They would have built their engines not to fail.  The fact that they lv can still make orbit under 8 engines is, I think, a by-product, not a deliberate design criteria.
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Offline Lars_J

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #66 on: 10/10/2012 01:59 am »
No I agree with Jim here.  You don't design in an engine failure. No commercial aircraft is designed just in case they have an engine failure.  That's an off-nominal event and SpaceX shouldn't have considered that as part of the design criteria.

Wrong. So very wrong.

Every commercial airliner is designed to be able to take off with one engine out. Even the large twin-engine airliners such as A330, 777, 787 are *required* to be able to take off and climb on one engine only - in the event of an engine failure past the point of no return during take-off. And that's just scraping at the top of this topic. Google ETOPS, it might be a useful starting point for you.

Offline dbhyslop

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #67 on: 10/10/2012 02:00 am »

No I agree with Jim here.  You don't design in an engine failure.  No commercial aircraft is designed just in case they have an engine failure.  That's an off-nominal event and SpaceX shouldn't have considered that as part of the design criteria.  They would have built their engines not to fail.  The fact that they lv can still make orbit under 8 engines is, I think, a by-product, not a deliberate design criteria.

Isn't every commercial aircraft designed to survive just in case they have an engine failure?

Offline FuseUpHereAlone

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #68 on: 10/10/2012 02:01 am »
The distinction between an original specification point and the choice of meeting that specification with redundancy and engine-out ability

Wrong again.  Engine out capability is bug and not a designed in feature

It may not have been their original reason to go with 9 engines - but that statement is simply wrong, and even a layman can see that. Once you have a need for multiple engines, you have a deliberate decision to make if you need or want to support engine out capability.

You may see the need for more than one engine as a "bug" - but it is quite clear that a deliberate effort from SpaceX with regards to hardware and software design to support engine out capability paid off significantly in this last flight.

(I suppose I could have simply written "wrong" like you would have, so you'll have to excuse me)

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.

Offline joek

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #69 on: 10/10/2012 02:04 am »
No I agree with Jim here.  You don't design in an engine failure.  No commercial aircraft is designed just in case they have an engine failure.

Designing "in an engine failure" is very different than designing "for an engine failure".  Is there any evidence to suggest SpaceX is doing the former and not the latter?  Commercial aircraft are designed "in case they have an engine failure" because even given the best engineering, QA, manufacturing and maintainence, failures  happen.

Offline Lars_J

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #70 on: 10/10/2012 02:05 am »
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.

Offline Oberon_Command

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #71 on: 10/10/2012 02:33 am »
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?

Offline joek

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #72 on: 10/10/2012 02:39 am »
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...

Offline ArbitraryConstant

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #73 on: 10/10/2012 03:38 am »
SpaceX probably sized the Merlin engine to be about right for one of them to be able to launch the smallest rocket they decided was commercially feasible, at the time a Falcon 1.

The 9 engine design was the quickest, cheapest way to get something in the EELV size range. A bigger engine would have been nice but they didn't have one, and they didn't have the money or time to make one. That's life. They made the most of it. They did engine out because they could and because they kinda had to with that many engines. There's pluses and minuses, probably mostly minuses, but it's what was doable at the time. I think ultimately Musk would rather have the rocket work with an eccentric engine configuration to support than to have gone out of business trying to build a new engine.

Offline neilh

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #74 on: 10/10/2012 05:17 am »
For reference, here's a quote from SpaceX's website in 2003 (they were incorporated in 2002):

(bolding mine)

http://web.archive.org/web/20030421122216/http://spacex.com/
Quote
Why do we believe Falcon is a high reliability design?
The vast majority of launch vehicle failures in the past two decades can be attributed to three causes: engine, stage separation and, to a much lesser degree, avionics failures. An analysis of launch failure history between 1980 and 1999 by Aerospace Corporation showed that 91% of known failures can be attributed to those subsystems.

Engine Reliability
It was with this in mind that we designed Falcon I to have the minimum number of engines. As a result, there is only one engine per stage and only one stage separation event – the minimum pragmatically possible number.

In the case of Falcon V, there are five first stage engines, but the vehicle is capable of sustaining an engine failure at any point in flight and still successfully completing its mission. This actually results in an even higher level of reliability than a single engine stage. The SpaceX five engine architecture is an improved version of that employed by the Saturn V Moon rocket, which had a flawless flight record despite losing an engine on two of its missions.
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Offline Maciej Olesinski

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #75 on: 10/10/2012 08:11 am »
It is obvious that multiple engines of the first stage is bad idea! I can not imagine how someone can be so ridiculous to deliver payload after rocket failure. Shame on You SpaceX! Shame on You!  :)

You forgot the smiley-face...
Fixed!
« Last Edit: 10/10/2012 10:14 am by Maciej Olesinski »

Offline woods170

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #76 on: 10/10/2012 10:03 am »
You forgot the smiley-face...

Offline Mader Levap

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #77 on: 10/10/2012 10:12 am »
But did they ever blow one up intentionally to see what would happen to the others?
I do not know, but I would bet they did. If they needed it at all, considering they had unintended (so presumably more realistic) RUDs to analyze.

how many tested when at supersonic speeds?
I do not know. It is at all possible to test thing like that on ground?

They would have built their engines not to fail.
Do you know that you demand impossible? There is no such thing that "do not fail". Entropy is harsh mistress and all of that.

You forgot the smiley-face...
I think his sarcasm was blindingly obvious...
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Offline alexterrell

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #78 on: 10/10/2012 11:21 am »
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?


Offline hrissan

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #79 on: 10/10/2012 11:50 am »
If the engine is 100% reliable, then a single engine would be best.
There is another reason the 9:1 engines configuration could be better even in case nothing fails: if you have large commonality between first and second stage engines. In that case (Merlin and MVac) you save money designing testing and producing the engines and their components.

Offline JBF

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #80 on: 10/10/2012 12:06 pm »
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.
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Offline peter-b

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #81 on: 10/10/2012 12:56 pm »
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.
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Offline JohnFornaro

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #82 on: 10/10/2012 02:39 pm »
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!"

« Last Edit: 10/10/2012 02:44 pm by JohnFornaro »
Sometimes I just flat out don't get it.

Offline Proponent

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #83 on: 10/10/2012 02:46 pm »
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"
« Last Edit: 10/10/2012 07:06 pm by Proponent »

Offline FuseUpHereAlone

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #84 on: 10/10/2012 03:21 pm »
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.

Offline FuseUpHereAlone

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #85 on: 10/10/2012 03:34 pm »
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).

Offline FuseUpHereAlone

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #86 on: 10/10/2012 03:37 pm »
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.

Offline Lars_J

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #87 on: 10/10/2012 03:54 pm »
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?

Offline Rik ISS-fan

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #88 on: 10/10/2012 04:32 pm »

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. 

Offline cambrianera

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #89 on: 10/10/2012 06:36 pm »
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 !
Oh to be young again. . .

Offline Downix

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #90 on: 10/10/2012 06:42 pm »
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.
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Offline GalacticIntruder

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #91 on: 10/10/2012 07:04 pm »
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?
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Offline Jim

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #92 on: 10/10/2012 07:27 pm »
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

Offline mmeijeri

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #93 on: 10/10/2012 07:29 pm »
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".
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Offline 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

Offline giggleherz

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #95 on: 10/10/2012 09:06 pm »
Awesome link, feels strange going back ten years reading that stuff.

Offline A_M_Swallow

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #96 on: 10/10/2012 09:48 pm »
{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 rateLaunch anomaly Rate
1:27 Every launch
1:2701:10
1:27001:100

Offline giggleherz

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #97 on: 10/11/2012 12:10 am »
Lets call it "um", for unassuming malfunction.

Offline beancounter

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #98 on: 10/11/2012 01:55 am »
{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 rateLaunch anomaly Rate
1:27 Every launch
1:2701:10
1:27001:100

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.
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Offline sdsds

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #99 on: 10/11/2012 06:50 am »
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?
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Offline alexterrell

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #100 on: 10/11/2012 07:09 am »
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.
In which case, under ordinary circumstances, they would have tried it? They'd have a choice between try it and maybe fail, or not try it and definitely fail.

It's a shame they weren't allowed to try it, because they'd then have excellent data on the fuel consumption of the vehicle and in future work out precisely what cargo they can carry and still succeed with one, or two, engines out.

Equally, can they run their engines at >100% of thrust, whilst increasing the engine failure risk? So if they're carrying a maximum cargo and one engine fails, can the others go to 110% and what does this do for failure risk?

With enough data it becomes less like rocket science and more like complicated statistics.

Offline A_M_Swallow

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #101 on: 10/11/2012 07:51 am »
{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 rateLaunch anomaly Rate
1:27 Every launch
1:2701:10
1:27001:100

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.

It is also a way of calculating what the MTBF needs to be.  Possibly written as MLBF (Mean Launches Between Failure).

As for the Merlin 1C fault, having taken part is fault investigations, the fault will be either a design error or a manufacturing process error.  Find the fault and remove it.  The failure rate will improve.

Offline rklaehn

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #102 on: 10/11/2012 07:55 am »
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.
In which case, under ordinary circumstances, they would have tried it? They'd have a choice between try it and maybe fail, or not try it and definitely fail.

I think without the NASA safety constraints they would have definitely tried.

Even a slightly higher orbit would have been a win for the orbcomm spacecraft, since a) they might be able to reach their destination orbit with their own propulsion system or b) they would at least have had more time to check out and evaluate the satellite before reentry.

Quote
With enough data it becomes less like rocket science and more like complicated statistics.

Complicated statistics is one aspect of rocket science. At least if you want a reliable rocket...
« Last Edit: 10/11/2012 07:55 am by rklaehn »

Offline rklaehn

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #103 on: 10/11/2012 08:18 am »
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".

Agree. Have heard that term used a lot on arocket, not referring to spacex but to some amateur liquid engines.

Offline clongton

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #104 on: 10/11/2012 01:06 pm »
In which case, under ordinary circumstances, they would have tried it? They'd have a choice between try it and maybe fail, or not try it and definitely fail.

Never jepordize the primary mission in order to "try" to complete the secondary.
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Offline Proponent

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #105 on: 10/11/2012 02:05 pm »
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. Engine redundancy works well for aircraft because uncontained failures are very rare. Rockets are higher pressure systems with lower margins.

Attached is a plot showing the probability of failure for a stages of various numbers of engines as a function of the fraction of lost thrust that can be tolerated*.  For example, if a 5-engine stage can afford to lose one engine, then the tolerable fractional thrust loss is 0.2.  The plotting symbols indicate the number of engines, except for '*', which denotes 27 engines.

I've arbitrarily assumed that each engine has a probability of 0.01 of failing benignly (i.e., shutting down without damaging its neighbors) and 0.001 of failing catastrophically (essentially destroying the stage).

Note that for large numbers of engines (9 or 27), the reliability asymptotically approaches a rather large value as the tolerance of thrust loss increases.  What's happening here is that large numbers of benign failures become extremely improbable and the risk is dominated by catastrophic failures.  SpaceX must be believe it can reduce the probability of catastrophic Merlin 1D failure to less than 1 in 10,000 or so.

*The concept of the tolerable thrust loss is a bit fuzzy, of course, since it actually depends on when the thrust is lost.  This plot is meant to be merely indicative.
« Last Edit: 10/11/2012 02:33 pm by Proponent »

Offline Robotbeat

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #106 on: 10/11/2012 06:28 pm »
Even if you have fratricide, it may only be one other engine that is damaged, which may mean you can still make orbit.

Btw, it is obvious that in order for engine-out on falcon 9 to make sense, the risk of fratricide must be LESS THAN an order of magnitude less than typical failure. In your calculation, proponent, you arbitrarily made the assumption that fratricide was only an order of magnitude less likely than regular engine failure, which unsurprisingly leads to the conclusion that the reliability is no better than a single engine. (not taking into account increase in reliability due to higher production volume) if, say, fratricide only occurs for 5% of failures, then 9 engines wins.
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Offline cambrianera

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #107 on: 10/11/2012 07:05 pm »
@Proponent
Your plot is interesting, and strongly supports strangequark's words.
But I think Robotbeat sets another interesting point; fratricide failures should be controlled by continuously monitored functions of the engine and the protection shields.
I must add that I don't agree with the reaction of many people about engine 1 failure on CRS-1; it wasn't much more energetic than the normal performance of the engine.
Obviously with the kind of energy involved you can't save some parts of the engine, it's like pretending not having some body damage in a low speed car crash.
I really think that CRS-1 will be the opportunity to transform a good idea in a better one.
Oh to be young again. . .

Offline upjin

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #108 on: 10/11/2012 07:05 pm »
I wonder if the question should be more about 9 engines versus just 6 more powerful engines and still having engine-out?

9 engines works with the amount of thrust they are getting and to still have the engine-out capability to make orbit. 

Would it be more economically beneficial for them, if they had a slightly more powerful engine and could still do engine-out?

When Merlin 2s are considered, they are often drawn up as having so much thrust that you lose engine-out (by only having 1 or 2 of them).  So could a Merlin 1E or less powerful Merlin 2, be the right answer?

« Last Edit: 10/11/2012 07:33 pm by upjin »

Offline JohnFornaro

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #109 on: 10/11/2012 07:30 pm »
Would it be more economically beneficial for them, if they had a slightly more powerful engine and could still do engine-out?

Not at the moment, since that engine is not on the shelf, ready to use.  Remember too, that a different number of engines also means a different thrust structure, different plumbing, and different flight software.  All of that would have to be designed, built, and tested.

And Martijn:  You owe me a cup of green tea.   After reading "engine rich exhaust", too many giggles and snorts, while holding the cup.
Sometimes I just flat out don't get it.

Offline upjin

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #110 on: 10/11/2012 07:44 pm »
Would it be more economically beneficial for them, if they had a slightly more powerful engine and could still do engine-out?

Not at the moment, since that engine is not on the shelf, ready to use.  Remember too, that a different number of engines also means a different thrust structure, different plumbing, and different flight software.  All of that would have to be designed, built, and tested.

And Martijn:  You owe me a cup of green tea.   After reading "engine rich exhaust", too many giggles and snorts, while holding the cup.

So is the Merlin 1D the end of that series?  I'm wondering if they are not planning to upgrade again, like from 1C to 1D, to 1E?  It is hard to see what else could they get out of the design past 1D, but who knows, maybe they can. In any case, that would likely still be 9 engines.

However, if SpaceX makes any significant upgrades to engine thrust for the 1 series or gets the Merlin 2 out, they would have to rethink the 9 engine configuration and likely go to fewer engines.

I'm also wondering about the 9 engine configuration from an economic stand point.  Let's suppose they already had that 1E or Merlin 2 engine, how many engines would still give them engine-out capability AND be cheap to build?  Seems like 5 or 6 is that magic number, in theory.

Offline JohnFornaro

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #111 on: 10/11/2012 07:55 pm »
From:

http://www.orbcomm.com/Collateral/Documents/English-US/OG2%20Prototype.pdf

Quote
The Company has filed a notice of claim under its launch insurance policy for a total loss of the OG2 prototype.  The maximum amount covered by the policy is $10 million, which would largely offset the expected cost of the OG2 prototype and associated launch services and launch insurance.

Some consolation in the offing.
« Last Edit: 10/11/2012 07:58 pm by JohnFornaro »
Sometimes I just flat out don't get it.

Offline upjin

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #112 on: 10/11/2012 08:26 pm »
Well, at least they got some testing done on the prototype.

"Notwithstanding the shortened life of the OG2 prototype, the OG2 program engineering teams from ORBCOMM, Sierra Nevada Corporation and Boeing made significant strides in testing various hardware components. After telemetry and command capability was established, several critical
system verifications were performed. The solar array and communications payload antenna deployments were successful, along with verifying the performance of various components of both the OG2 satellite bus and the communications payload. The OG2 satellite bus systems including
power, attitude control, thermal and data handling were also tested to verify proper operation. The unique communications payload, which incorporates a highly reprogrammable software radio with common hardware for both gateway and subscriber messaging, also functioned as expected.

These verification successes achieved from the single prototype satellite validate that the innovative OG2 satellite technology operates as designed before launching the full constellation of OG2 satellites. With this verification data, ORBCOMM can focus on completing and launching the OG2
satellites as the primary mission payloads on two planned Falcon 9 launches, the first in mid-2013 and the second in 2014, directly into their operational orbit."

Offline KingAlbert

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #113 on: 10/11/2012 08:35 pm »
Using standard reliability analysis we can calculate what the reliability of a multi-cluster engine relative to one big engine.

For simplicity’s sake let us assume the following:
1)   In a multi-engine design the individual engines can fail without causing an automatic failure of the entire stage.
2)   Engine failures are independent of each other, ie the failure of any one engine has no effect on the failure probability of any other engine. This is probably not totally realistic as the failure of an engine would probably stress the remaining engine more, but it’s probably close enough for these calcs.
3)   The timing of the failure is treated as irrelevant, failure in the 1st second of burn is as bad as failure during the last second of burn. (Not true, but close enough).
4)   The failure rate for 1 big engine is the same as the individual failure rates of small engines. (Not true, but close enough).
5)   For a 9 engine cluster we will assume that the failure of any 1 engine won’t cause a launch failure and the failure of 2 engines might not if they are on opposite sides of the cluster. Assume the engines are laid out: 

      1  2  3
      4  5  6
      7  8  9

The failure of the engine pairs 1-9, 2-8, 3-7 and 4-6 won’t cause a launch failure.

Define probability of engine failure == Pf
Probability of engine not failing == Ps = 1-Pf
Probability of launch failure for the cluster of nine engines == P9

P9 = (Number of different ways 2 engine failures can cause launch failure)*Pf^2*Ps^7 +
   (Number of different ways 3 engine failures can cause launch failure)*Pf^3*Ps^6 + …

P9 = {(9!/7!2!)-4)}*Pf^2*Ps^7 + (9!/6!3!)*Pf^3*Ps^6 + (9!/5!4!)*Pf^4*Ps^5 + (9!/5!4!)*Pf^5*Ps^4 + (9!/6!3!)*Pf^6*Ps^3 + (9!/2!7!)*Pf^7*Ps^2 +(9!/8!)*Pf^8*Ps + Pf^9


     Pf            P9
   =====   =====
         0         0
    0.0050    0.0008
    0.0100    0.0031
    0.0150    0.0067
    0.0200    0.0117
    0.0250    0.0179
    0.0300    0.0252
    0.0350    0.0336
    0.0400    0.0430
    0.0450    0.0532
    0.0500    0.0642
    0.0550    0.0760
    0.0600    0.0885

Around an engine failure rate of 3.5% the 9-cluster and the single engine both have a launch failure rate of ~3.5%. Below an engine failure rate of 3.5% the 9-cluster becomes more reliable than a single engine and if the engine failure rate is above 3.5% the 9-cluster is less reliable than a single engine.


PS: in matlab

P9=(nchoosek(9,2)-4).*Pf.^2.*Ps.^7 + nchoosek(9,3).*Pf.^3.*Ps.^6 + nchoosek(9,4).*Pf.^4.*Ps.^5 + nchoosek(9,5).*Pf.^5.*Ps.^4 + nchoosek(9,6).*Pf.^6.*Ps.^3 + nchoosek(9,7).*Pf.^7.*Ps.^2 + nchoosek(9,8).*Pf.^8.*Ps + Pf.^9


Offline Proponent

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #114 on: 10/11/2012 09:14 pm »
Even if you have fratricide, it may only be one other engine that is damaged, which may mean you can still make orbit.

Btw, it is obvious that in order for engine-out on falcon 9 to make sense, the risk of fratricide must be LESS THAN an order of magnitude less than typical failure. In your calculation, proponent, you arbitrarily made the assumption that fratricide was only an order of magnitude less likely than regular engine failure, which unsurprisingly leads to the conclusion that the reliability is no better than a single engine. (not taking into account increase in reliability due to higher production volume) if, say, fratricide only occurs for 5% of failures, then 9 engines wins.

I think that even with the assumptions I made, a 9-engine cluster looks pretty good, bearing in mind that the whole idea of a fixed tolerable thrust loss is a simplification.  There's a good chance that independent benign failures will occur at significantly different times.  Given a little time between failures, a 9-engine cluster could probably tolerate two engines out with less loss of performance than a 5-engine cluster with a single engine out.  And allowing for the possibility that some catastrophic failures might be survivable makes things look even better, not to mention the point that others have made about the 9-engine vehicle clocking up flight time on the engines faster than smaller clusters.

For the fun of it, here's the same calculation with a catastrophic failure probability of 0.0005, i.e., half of the previous value, though strangequark may be skeptical that it could be so low.

Offline cordor

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #115 on: 10/11/2012 09:24 pm »

4)   The failure rate for 1 big engine is the same as the individual failure rates of small engines. (Not true, but close enough).


With the same budget and time, there is no way to design, develop, produce, test 1 big engine for first stage and 1 small engine for upper stage, yet have the same failure rates as merlin engine in 9+1 falcon 9 configuration.

Offline giggleherz

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #116 on: 10/11/2012 09:24 pm »
We don't know yet what happened but if these engines take a beating and wear out fast, is it really worth it to spend so much time and money to make them reusable.

Offline starsilk

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #117 on: 10/11/2012 09:37 pm »
Well, at least they got some testing done on the prototype.

"Notwithstanding the shortened life of the OG2 prototype, the OG2 program engineering teams from ORBCOMM, Sierra Nevada Corporation and Boeing made significant strides in testing various hardware components. After telemetry and command capability was established, several critical
system verifications were performed. The solar array and communications payload antenna deployments were successful, along with verifying the performance of various components of both the OG2 satellite bus and the communications payload. The OG2 satellite bus systems including
power, attitude control, thermal and data handling were also tested to verify proper operation. The unique communications payload, which incorporates a highly reprogrammable software radio with common hardware for both gateway and subscriber messaging, also functioned as expected.

These verification successes achieved from the single prototype satellite validate that the innovative OG2 satellite technology operates as designed before launching the full constellation of OG2 satellites. With this verification data, ORBCOMM can focus on completing and launching the OG2
satellites as the primary mission payloads on two planned Falcon 9 launches, the first in mid-2013 and the second in 2014, directly into their operational orbit."

why would an insurance policy pay out to the full amount, when they clearly got a lot (or enough) value out of the satellite to mean they don't need to fly another one?

seems to me like Orbcomm should be wringing their hands at the moment, complaining bitterly about how this is going to affect their bottom line, and how they may need to launch another test satellite..

Offline douglas100

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #118 on: 10/11/2012 09:50 pm »
We don't know yet what happened but if these engines take a beating and wear out fast, is it really worth it to spend so much time and money to make them reusable.

SpaceX appear to think re-usability is worth it. As far as spending "so much time and money" on it, they seem to think it is. It's their money after all and we don't know how much they are spending.

The engines are extensively tested on the ground. They will have a good idea of the lifetime of critical components. I very much doubt they "wear out fast." When the cause of the current failure is identified they will certainly make changes to make them even more robust.

Also, their approach to re-usability depends on multiple engines on the first stage. So this is another point in its favor.
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Offline neilh

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #119 on: 10/11/2012 09:55 pm »
We don't know yet what happened but if these engines take a beating and wear out fast, is it really worth it to spend so much time and money to make them reusable.

Are you talking about the 1C or 1D?
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Offline cordor

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #120 on: 10/11/2012 10:45 pm »
We don't know yet what happened but if these engines take a beating and wear out fast, is it really worth it to spend so much time and money to make them reusable.

You wish you can use engine 5 landing back and check what went wrong on engine 1. no guessing :)

vertical landing not only allow reusable rocket, also they can build mars lander for NASA if ever need one. kinda kill 2 birds with 1 stone. It's all about business strategy.

Offline upjin

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #121 on: 10/11/2012 11:22 pm »
Well, at least they got some testing done on the prototype.

"Notwithstanding the shortened life of the OG2 prototype, the OG2 program engineering teams from ORBCOMM, Sierra Nevada Corporation and Boeing made significant strides in testing various hardware components. After telemetry and command capability was established, several critical
system verifications were performed. The solar array and communications payload antenna deployments were successful, along with verifying the performance of various components of both the OG2 satellite bus and the communications payload. The OG2 satellite bus systems including
power, attitude control, thermal and data handling were also tested to verify proper operation. The unique communications payload, which incorporates a highly reprogrammable software radio with common hardware for both gateway and subscriber messaging, also functioned as expected.

These verification successes achieved from the single prototype satellite validate that the innovative OG2 satellite technology operates as designed before launching the full constellation of OG2 satellites. With this verification data, ORBCOMM can focus on completing and launching the OG2
satellites as the primary mission payloads on two planned Falcon 9 launches, the first in mid-2013 and the second in 2014, directly into their operational orbit."

why would an insurance policy pay out to the full amount, when they clearly got a lot (or enough) value out of the satellite to mean they don't need to fly another one?

seems to me like Orbcomm should be wringing their hands at the moment, complaining bitterly about how this is going to affect their bottom line, and how they may need to launch another test satellite..


I'm thinking the same thing, but then again, it could be another way to increase cash flow until they launch their other satellites.  Positive news, is good for investors.  And if the insurance company pays, than that is +2.


Offline KingAlbert

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #122 on: 10/11/2012 11:34 pm »

4)   The failure rate for 1 big engine is the same as the individual failure rates of small engines. (Not true, but close enough).


With the same budget and time, there is no way to design, develop, produce, test 1 big engine for first stage and 1 small engine for upper stage, yet have the same failure rates as merlin engine in 9+1 falcon 9 configuration.


That's certainly true, but I was trying to do an apples2apples comparison.

If I throw in a 'catastrophic' failure prob = 1/10 the 'benign' failure case, the odds change quite a bit:

    Prob of   Prob of
    benign    stage
    engine    failure
    failure
    =====  =====
         0         0
    0.0050    0.0053
    0.0100    0.0120
    0.0150    0.0200
    0.0200    0.0292
    0.0250    0.0394
    0.0300    0.0506
    0.0350    0.0626
    0.0400    0.0753
    0.0450    0.0887
    0.0500    0.1027
    0.0550    0.1171
    0.0600    0.1319
 

Offline Jim

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #123 on: 10/12/2012 01:14 am »

vertical landing not only allow reusable rocket, also they can build mars lander for NASA if ever need one. kinda kill 2 birds with 1 stone. It's all about business strategy.

Just plain wrong.  Again.  Quit posting incorrect statements

Offline Jim

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #124 on: 10/12/2012 01:15 am »

4)   The failure rate for 1 big engine is the same as the individual failure rates of small engines. (Not true, but close enough).


With the same budget and time, there is no way to design, develop, produce, test 1 big engine for first stage and 1 small engine for upper stage, yet have the same failure rates as merlin engine in 9+1 falcon 9 configuration.


You have no proof of that

Offline cordor

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #125 on: 10/12/2012 05:27 am »

4)   The failure rate for 1 big engine is the same as the individual failure rates of small engines. (Not true, but close enough).


With the same budget and time, there is no way to design, develop, produce, test 1 big engine for first stage and 1 small engine for upper stage, yet have the same failure rates as merlin engine in 9+1 falcon 9 configuration.


You have no proof of that

You are more than welcome to proof me wrong, explain to me how you can use the same amount of resources to get the job done. teach me how specialize can be more cost effective than modularize especially at design and development phase. In fact, i really want to know. Im begging you.

To me, if i can reuse the same engine everything, that's perfect. Earlier, i even point out micro engine like this one. RCS use it, escape system use it, stage 1 stage 2 use it. With the quantity, we can do mass production and bring engine cost to dirt cheap. Of cause, plumbing becomes a problem, and john smith 19 pointed out it's not ready yet.

« Last Edit: 10/12/2012 07:37 am by cordor »

Offline A_M_Swallow

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #126 on: 10/12/2012 08:01 am »
{snip}

However, if SpaceX makes any significant upgrades to engine thrust for the 1 series or gets the Merlin 2 out, they would have to rethink the 9 engine configuration and likely go to fewer engines.

I'm also wondering about the 9 engine configuration from an economic stand point.  Let's suppose they already had that 1E or Merlin 2 engine, how many engines would still give them engine-out capability AND be cheap to build?  Seems like 5 or 6 is that magic number, in theory.

I predict that there will be a Merlin 1E, the 1D with reliability enhancements.  Since the 1D already exists the new version number will be needed for public relations purposes.

p.s.  2 engines give you engine-out capability.  This is viable for an upper stage.

A lower stage would normally have to launch with engines throttled to 50%.  So 3 engines would be more appropriate.

The current Falcon Heavy has 3 cores so an upgrade from 27 to 3 Merlin 2 engines may work.
« Last Edit: 10/12/2012 08:07 am by A_M_Swallow »

Offline Jim

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #127 on: 10/12/2012 10:57 am »

You are more than welcome to proof me wrong, explain to me how you can use the same amount of resources to get the job done. teach me how specialize can be more cost effective than modularize especially at design and development phase. In fact, i really want to know. Im begging you.

To me, if i can reuse the same engine everything, that's perfect. Earlier, i even point out micro engine like this one. RCS use it, escape system use it, stage 1 stage 2 use it.

That sheet of metal is not proof since it is not going be ever used on a launch vehicle.

MVAC is very different from the booster version

Anyways, the proof is on you, that was the point.
« Last Edit: 10/12/2012 10:58 am by Jim »

Offline Proponent

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #128 on: 10/12/2012 11:08 am »
Come to think of it, what specific cases in recent-ish history have there been of failing engines damaging other engines, or of having been likely to do so were it not for the fact that no other engines were present?

Offline JohnFornaro

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #129 on: 10/12/2012 01:03 pm »
Using standard reliability analysis we can calculate what the reliability of a multi-cluster engine relative to one big engine.

For simplicity’s sake let us assume the following:

1)   In a multi-engine design the individual engines can fail without causing an automatic failure of the entire stage.

You can't "assume" that, you have to engineer that.

Quote
2)   Engine failures are independent of each other, ie the failure of any one engine has no effect on the failure probability of any other engine. This is probably not totally realistic as the failure of an engine would probably stress the remaining engine more, but it’s probably close enough for these calcs.

Another element of engineering, not assumption, which you sorta ackowledge with the squishy term "probably".

Quote
3)   The timing of the failure is treated as irrelevant, failure in the 1st second of burn is as bad as failure during the last second of burn. (Not true, but close enough).

Not anywhere near close enough without a more complete analysis.

Quote
4)   The failure rate for 1 big engine is the same as the individual failure rates of small engines. (Not true, but close enough).

You need to give up this line of reasoning.  "Not true" is your hint, particularly when making assumptions about one engine with thrust capability of the nine.  "Close enough" is where your mistaken assumptions can be found, if you dig deeper.  You know about GIGO; you can't use standard reliability analysis without appropriate assumptions.

Quote
5)   For a 9 engine cluster we will assume that the failure of any 1 engine won’t cause a launch failure and the failure of 2 engines might not if they are on opposite sides of the cluster. Assume the engines are laid out:

Well, since the nine engine cluster is admittedly engineered to that specification, ok.  So, you're at one out of five, which is consistent with the reported engineering of the F-5.

why would an insurance policy pay out to the full amount, when they clearly got a lot (or enough) value out of the satellite to mean they don't need to fly another one?

"Clearly"? And how do you know this?  As to "why" would the insurance company pay, and to what extent, are you alluding to certain knowledge of the specific terms of that policy?

************************

It must have been nerve wracking to watch the pieces fall off of the LV in real time.  Who knows how close they came to having one more piece fall off, which would have caused catastrophic failure.  There is clearly some pretty good design going on there for the LV to have succeeded as well as it did.
« Last Edit: 10/12/2012 01:04 pm by JohnFornaro »
Sometimes I just flat out don't get it.

Offline peter-b

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #130 on: 10/12/2012 01:27 pm »
You need to give up this line of reasoning.  "Not true" is your hint, particularly when making assumptions about one engine with thrust capability of the nine.  "Close enough" is where your mistaken assumptions can be found, if you dig deeper.  You know about GIGO; you can't use standard reliability analysis without appropriate assumptions.

It's called Fermi estimation, and I found his modelling interesting. I particularly liked that he stated his assumptions and where shortcomings might be found. Since your tone is that of someone who thinks they know what they are talking about, some constructive criticism (i.e. suggesting improved models and some possible sources of data) would perhaps be in order, rather than just saying "your assumptions are bad and you should feel bad". I wish there were more NSF forums who put as much thought into their posts as KingAlbert. We should encourage them, rather than driving them away with unnecessary negativity and confrontational attitude.
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Offline KingAlbert

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #131 on: 10/12/2012 02:23 pm »
Using standard reliability analysis we can calculate what the reliability of a multi-cluster engine relative to one big engine.

For simplicity’s sake let us assume the following:

1)   In a multi-engine design the individual engines can fail without causing an automatic failure of the entire stage.

You can't "assume" that, you have to engineer that.

You seem to have a misunderstanding about what my model is for. It's to get a rough sense of the reliability implications of 9 (with a 1 engine out benign failure) vs 1 engine. I don't work for spacex so I don't know the exact details of its' design, I have to "assume" things just to get started. I'm the one making the assumptions, and being open about it, I'm can't be sure about what spacex has actually built. I don't even pretend to know the engine reliability numbers (an incredibly important number), which is why the calcs cover a range of reliability numbers for the engines.

In the case where the F9 cluster doesn't have engine-out capability then the calcs become pretty trivial and we can say for sure it's much less reliable to have more engines. Not very interesting.

With regards to F9 is particular, they've stated that have engine-out capability and they seem to have, at least partially, displayed it, so I decided to include that. If you're sure that's not true then refer to my previous paragraph.
 
I did post a 2nd set of calcs that also included the probability of a catasrophic failure from a single engine out with an assuming 10:1 ratio of benign:catastrophic failure.

2)   Engine failures are independent of each other, ie the failure of any one engine has no effect on the failure probability of any other engine. This is probably not totally realistic as the failure of an engine would probably stress the remaining engine more, but it’s probably close enough for these calcs.
Another element of engineering, not assumption, which you sorta ackowledge with the squishy term "probably".

Given that the engines are designed to burn for much longer than they're actually used, I doubt that the small increase in burn time required to compensate for an engine failure would significantly alter the failure rate of the remaining 8 engines. A small increase in failure rate for that case simply won't alter the final number much. If you have proof that there is a big change in failure rates, please provide it and I'd be happy to take it into account.


3)   The timing of the failure is treated as irrelevant, failure in the 1st second of burn is as bad as failure during the last second of burn. (Not true, but close enough).

Not anywhere near close enough without a more complete analysis.

I'm making a very pessimistic assumption. If timing did matter then that would mean that a 'late' failure might allow the 9-engine design to withstand 2 or 3 late engine failures, thus improving the 9-engine failure rate. As it is I just have the pessimistic case.

4)   The failure rate for 1 big engine is the same as the individual failure rates of small engines. (Not true, but close enough).

You need to give up this line of reasoning. 

Given that the "one big engine" for a F9 is purely hypothetical, good reliability numbers be hard to come by. In any event, it's completely trivial to see the effect of the big engine having different reliability numbers so I don't even see the point of including that.






Offline JohnFornaro

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #132 on: 10/12/2012 02:43 pm »
Shoot.  I wrote all this while King Albert was writing his rebuttal.  I'll be back in a few minutes...

You need to give up this line of reasoning.  "Not true" is your hint, particularly when making assumptions about one engine with thrust capability of the nine.  "Close enough" is where your mistaken assumptions can be found, if you dig deeper.  You know about GIGO; you can't use standard reliability analysis without appropriate assumptions.

It's called Fermi estimation, and I found his modelling interesting. I particularly liked that he stated his assumptions and where shortcomings might be found. Since your tone is that of someone who thinks they know what they are talking about, some constructive criticism (i.e. suggesting improved models and some possible sources of data) would perhaps be in order, rather than just saying "your assumptions are bad and you should feel bad". I wish there were more NSF forums who put as much thought into their posts as KingAlbert. We should encourage them, rather than driving them away with unnecessary negativity and confrontational attitude.

Well, I certainly can take it, and I do know how to dish it out. Let's be very clear that I assert that his assumptions are bad, and say nothing about his "feelings".  Let the analysis stand and let the emotions go.  True, it is possible to theoretically discuss any number of things.  Although I am partial to your advice to encourage thought and analysis by posters, rather than "driving them away with unnecessary negativity and confrontational attitude", I am not at all in agreement that an untenable idea should be pursued very far at all.

I know about Fermi estimation, which is not an assertion that I know all there is to know about it.  My biggest analytical shortcoming is SME's: Stupid Math Errors.  Note that I do not criticize the correctness of his math; as to my tone, sugarcoat the garbage in any way you want to, but it is still garbage.

The main failure point of his argument is that there is no single engine made by SpaceX offering 5.88 MN (1,320,000 lbf) of thrust.  True, it is possible to theoretically discuss any number of things.  Why?  It cannot be only that this is a discussion of math skills.  I'm mostly interested in pragmatic applications, and less interested in too many theoretical speculations.

The obvious comparison would be the F-1 engine, RP1/Lox propellant, and capable of 1,522,000 lbf (6.77 MN).  To the best of my knowledge, this engine did not fail in flight.  At the same time, Saturn V used, well, V of them, and did not have engine out capability.  A short half hour of googoling didn't reveal any info on the failure rate of this engine.   

The RD-180 has 860,568 lbf (3.83 MN) of thrust.  It's a different engine design, sorta the same size, and "reliable" too, per the googol:

http://www.google.com/#sclient=psy-ab&hl=en&site=&source=hp&q=reliability+of+rd-180&oq=reliability+of+rd-180&gs_l=hp.3...1422.7672.0.8516.21.17.0.2.2.1.282.2516.5j8j4.17.0.les%3B..0.0...1c.1.UXNay1awRDg&pbx=1&bav=on.2,or.r_gc.r_pw.&fp=c0fa44e838a0f9f3&bpcl=35243188&biw=1077&bih=480

So where does a 3.5% failure rate come from?  In other words, where does Pf come from?  You can't just "assume" it.  As to the five points he raises; there's too many loopchasms in the assumptions.

Quote
Since your tone is that of someone who thinks they know what they are talking about, some constructive criticism...

So forget the subjectivity of the tone; instead, provide numerical support for the five points he raised.

***************************

Some interesting stuff about RD-180:

http://www.pwrengineering.com/dataresources/rd-180-pres-100101.pdf
 http://www.nasa.gov/pdf/427652main_PMC_2010_Pech_Russian.pdf

RD-180 reliability assertions, without numbers, here on the forum:

http://forum.nasaspaceflight.com/index.php?topic=29130.1080

Sidetrack dissertation about reliabilty and its calculation, which I did not read:

http://www.ssdl.gatech.edu/papers/phdTheses/YoungD-Thesis.pdf
Sometimes I just flat out don't get it.

Offline Prober

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #133 on: 10/12/2012 03:02 pm »

4)   The failure rate for 1 big engine is the same as the individual failure rates of small engines. (Not true, but close enough).


With the same budget and time, there is no way to design, develop, produce, test 1 big engine for first stage and 1 small engine for upper stage, yet have the same failure rates as merlin engine in 9+1 falcon 9 configuration.


You have no proof of that

You are more than welcome to proof me wrong, explain to me how you can use the same amount of resources to get the job done. teach me how specialize can be more cost effective than modularize especially at design and development phase. In fact, i really want to know. Im begging you.

To me, if i can reuse the same engine everything, that's perfect. Earlier, i even point out micro engine like this one. RCS use it, escape system use it, stage 1 stage 2 use it. With the quantity, we can do mass production and bring engine cost to dirt cheap. Of cause, plumbing becomes a problem, and john smith 19 pointed out it's not ready yet.



Let me try and help you out a bit here......maybe move these micro engines to another project?  See this thread..... http://forum.nasaspaceflight.com/index.php?topic=28369.30
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Offline JohnFornaro

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #134 on: 10/12/2012 03:12 pm »
You seem to have a misunderstanding about what my model is for. It's to get a rough sense of the reliability implications of 9 ... vs 1 engine. I don't work for spacex so I don't know the exact details of its' design, I have to "assume" things just to get started. I'm the one making the assumptions, and being open about it, I'm can't be sure about what spacex has actually built. I don't even pretend to know the engine reliability numbers (an incredibly important number), which is why the calcs cover a range of reliability numbers for the engines.

I realize that you hope to derive a rough sense of reliability, which is fine in principle.  Yes, you are "open" about your assumptions, but the assumptions are so crucial to the bottom line of your calculation, that I end up questioning the point of the exercise.

As I concluded in my previous post, you don't have the most important number at all, which is not your fault by any stretch; it is proprietary knowledge.  Without their reliabilty numbers, I continue to struggle with your results, and question them on the basis of "why bother?", when so much is unknown.

Quote
In the case where the F9 cluster doesn't have engine-out capability then the calcs become pretty trivial and we can say for sure it's much less reliable to have more engines. Not very interesting.

A case which is clearly, by design, not the case.  I'm engaging on this to try and understand why calculate about that which is not the case?

Quote
With regards to F9 is particular, they've stated that have engine-out capability and they seem to have, at least partially, displayed it, so I decided to include that. If you're sure that's not true then refer to my previous paragraph.

"Seem to have, at least partially", displayed engine out capability?  How can what happened be honestly described as a "seeming"?  The engine went out after receiving the shutoff command, the rocket went up.  Why can't what happened be what happened?   It only "seemed" to happen?  I really struggle with semantics like this, but hey:  This is a pet peeve of mine which you are free to ignore.
 
Quote
I did post a 2nd set of calcs that also included the probability of a catasrophic failure from a single engine out with an assuming 10:1 ratio of benign:catastrophic failure.

I saw that too.  Once you've created a working spreadsheet, any number of values can be plugged in.

Quote
2)   Engine failures are independent of each other, ie the failure of any one engine has no effect on the failure probability of any other engine. This is probably not totally realistic ... but it’s probably close enough for these calcs.
Another element of engineering, not assumption ...

Given that the engines are designed to burn for much longer than they're actually used, I doubt that the small increase in burn time ... would significantly alter the failure rate of the remaining 8 engines. A small increase in failure rate for that case simply won't alter the final number much. If you have proof that there is a big change in failure rates, please provide it and I'd be happy to take it into account.

Burn time is one part of the equation, and the engines are conservatively rated so that burn time does not influence the failure rate, but I didn't read your sentence the same way you did.  You haven't accounted for fratricide.  You will have to find out what that rate should be, and plug that it. I have no idea what would be a reasonable figure.

Quote
3)   The timing of the failure is treated as irrelevant, failure in the 1st second of burn is as bad as failure during the last second of burn. (Not true, but close enough).

Not anywhere near close enough without a more complete analysis.

I'm making a very pessimistic assumption. If timing did matter then that would mean that a 'late' failure might allow the 9-engine design to withstand 2 or 3 late engine failures, thus improving the 9-engine failure rate. As it is I just have the pessimistic case.

Granted for the moment. I accept the idea of counting the worst case.

Quote
4)   The failure rate for 1 big engine is the same as the individual failure rates of small engines. (Not true, but close enough).

You need to give up this line of reasoning. 

Given that the "one big engine" for a F9 is purely hypothetical, good reliability numbers be hard to come by. In any event, it's completely trivial to see the effect of the big engine having different reliability numbers so I don't even see the point of including that.

Then, what's the value of the comparison?
« Last Edit: 10/12/2012 03:21 pm by JohnFornaro »
Sometimes I just flat out don't get it.

Offline Lars_J

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #135 on: 10/12/2012 03:38 pm »
I predict that there will be a Merlin 1E, the 1D with reliability enhancements.  Since the 1D already exists the new version number will be needed for public relations purposes.

The 1D will already have reliability improvements over the 1C. (fewer parts, etc.)

A 1E variant may happen eventually, but it will *not* be due to PR issues from this flight.

Offline giggleherz

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #136 on: 10/13/2012 12:42 am »
One small step off topic because because. Please tell us curious wannabees what a Pintle looks like vs.something that looks like a shower head? I did get to peek inside the Saturn cones twenty five years ago but I cant remember what they looked like. I thought they all had a shower head design but someone mentioned a Pintle.
« Last Edit: 10/13/2012 09:52 pm by giggleherz »

Offline modemeagle

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« Last Edit: 10/13/2012 01:22 am by modemeagle »

Offline JohnFornaro

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #138 on: 10/13/2012 02:04 pm »
...pinnacle...

Pintle.  Or are you just being cute?
Sometimes I just flat out don't get it.

Offline KingAlbert

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #139 on: 10/14/2012 02:45 am »

I realize that you hope to derive a rough sense of reliability, which is fine in principle.  Yes, you are "open" about your assumptions, but the assumptions are so crucial to the bottom line of your calculation, that I end up questioning the point of the exercise.

As I concluded in my previous post, you don't have the most important number at all, which is not your fault by any stretch; it is proprietary knowledge.  Without their reliabilty numbers, I continue to struggle with your results, and question them on the basis of "why bother?", when so much is unknown.

Spacex is never going to release detailed information about engine reliability, as you yourself said. Never. People on the outside will always be engaging in speculation and educated guesses. I still find it interesting, but if you have to have hard, detailed, and very reliable numbers, I think you're out of luck.

Quote
In the case where the F9 cluster doesn't have engine-out capability then the calcs become pretty trivial and we can say for sure it's much less reliable to have more engines. Not very interesting.

A case which is clearly, by design, not the case.  I'm engaging on this to try and understand why calculate about that which is not the case?

I only did the calcs for engine out capability.

Burn time is one part of the equation, and the engines are conservatively rated so that burn time does not influence the failure rate, but I didn't read your sentence the same way you did.  You haven't accounted for fratricide.  You will have to find out what that rate should be, and plug that it. I have no idea what would be a reasonable figure.

The 2nd set of calcs had an example fratricide ratio.


Given that the "one big engine" for a F9 is purely hypothetical, good reliability numbers be hard to come by. In any event, it's completely trivial to see the effect of the big engine having different reliability numbers so I don't even see the point of including that.
Then, what's the value of the comparison?

The value of the model is to see the reliability implications of of 9 engine/1 out design, which is interesting in itself. Other people might be able to add more information in the future to improve the analysis.

Offline cordor

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #140 on: 10/14/2012 03:49 am »

Let me try and help you out a bit here......maybe move these micro engines to another project?  See this thread..... http://forum.nasaspaceflight.com/index.php?topic=28369.30


There is no need, im not going to discuss micro engine here. The reason i pointed out was modularization has it's advantage. I don't think spacex had the resources and time to develop 1 big engine for 1st stage and 1 small engine for second stage. So, it's not a matter of good idea or bad idea. I think that's the only way can be done.

Offline JohnFornaro

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #141 on: 10/14/2012 04:47 pm »
I realize that you hope to derive a rough sense of reliability, which is fine in principle.  Yes, you are "open" about your assumptions, but the assumptions are so crucial to the bottom line of your calculation, that I end up questioning the point of the exercise.

...  Without [SpaceX's] reliabilty numbers, I continue to struggle with your results, and question them on the basis of "why bother?", when so much is unknown.

Spacex is never going to release detailed information about engine reliability, as you yourself said. Never. People on the outside will always be engaging in speculation and educated guesses. I still find it interesting, but if you have to have hard, detailed, and very reliable numbers, I think you're out of luck.

I'm not out of luck.  I'm saying, why bother?

Given that the "one big engine" for a F9 is purely hypothetical, good reliability numbers be hard to come by. In any event, it's completely trivial to see the effect of the big engine having different reliability numbers so I don't even see the point of including that.
Then, what's the value of the comparison?

The value of the model is to see the reliability implications of of 9 engine/1 out design, which is interesting in itself. Other people might be able to add more information in the future to improve the analysis.

And you're saying, "for purposes of better understanding", which is fine. 

I'm developing a new appreciation of the value of your analysis.

If they can get the engine to have a reliability better than 3.5%, then a cluster design will be more reliable than a single engine design.  So, today, based on four flights, there's a demonstrated reliability record of one engine out in thirty-six, 2.8%?
Sometimes I just flat out don't get it.

Offline Robotbeat

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #142 on: 10/14/2012 06:21 pm »
One engine out in 39, actually. The last three flights of FalcOn 1 used merlin1c and they didn't have an engine out.

Edit: also, because of the small sample size, we can't say with certainty that the real reliability is 1/39. As far as we know, the real reliability of merlin 1c could be somewhere between 90% and 99.9%. So we don't have enough of a sample size to say whether theirs is a good design choice versus a single engine even assuming the rest of the analysis is correct.
« Last Edit: 10/14/2012 06:45 pm by Robotbeat »
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Offline guckyfan

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #143 on: 10/14/2012 07:00 pm »
One engine out in 39, actually. The last three flights of FalcOn 1 used merlin1c and they didn't have an engine out.

Edit: also, because of the small sample size, we can't say with certainty that the real reliability is 1/39. As far as we know, the real reliability of merlin 1c could be somewhere between 90% and 99.9%. So we don't have enough of a sample size to say whether theirs is a good design choice versus a single engine even assuming the rest of the analysis is correct.

It will hopefully be better than 1/40. Or become better with experience and implemented changes. 1/40 would mean on average one of 40 upper stage engines would fail. One would hope for a better reliability.

Edit: BTW I heard a commentator on chinese CCTV say the calculated failure rate of their manned rocket would be 3%, just for comparison.
« Last Edit: 10/14/2012 07:12 pm by guckyfan »

Offline KingAlbert

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #144 on: 10/17/2012 05:36 pm »
Unfortunately we haven't seen enough launches to get a good estimate on reliability. Spacex has their testbench data to help them out.

I've included below the latest version of my matlab code (should work with Octave/Scilab which are matlab near-clones) for stage 1.

If people are interested I can extend the calcs to cover all the stages for a Falcon 9 & Falcon Heavy.

Also, I could do a spreadsheet if people want to play with it.

% base vector of possible engine failure rates
Pf = [0.0 0.001 0.002 0.003 0.004 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 0.055 0.06];

% probability of engine success
Ps = 1-Pf

% probabilities of catastrophic engine failures/sucesses (ie, chance of taking out the whole rocket)
Pcf=Pf./20
Pcs=1-Pcf

% non-catastrophic probability of failure for 9 engines
P9=(nchoosek(9,2)-4).*Pf.^2.*Ps.^7 + nchoosek(9,3).*Pf.^3.*Ps.^6 + nchoosek(9,4).*Pf.^4.*Ps.^5 + nchoosek(9,5).*Pf.^5.*Ps.^4 + nchoosek(9,6).*Pf.^6.*Ps.^3 + nchoosek(9,7).*Pf.^7.*Ps.^2 + nchoosek(9,8).*Pf.^8.*Ps + Pf.^9

% catastrophic probability of failure for 9 engines
Pc9=9*Pcf.*Pcs.^8 + nchoosek(9,2).*Pcf.^2.*Pcs.^7 + nchoosek(9,3).*Pcf.^3.*Pcs.^6 + nchoosek(9,4).*Pcf.^4.*Pcs.^5 + nchoosek(9,5).*Pcf.^5.*Pcs.^4 + nchoosek(9,6).*Pcf.^6.*Pcs.^3 + nchoosek(9,7).*Pcf.^7.*Pcs.^2 + nchoosek(9,8).*Pcf.^8.*Pcs + Pcf.^9

% combined failure modes
Ptf= P9.*(1-Pc9) + (1-P9).*Pc9;


Offline giggleherz

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #145 on: 10/18/2012 02:07 am »
Just a thought about what is the exact definition of an engine. Someone mentioned Soyuz has only five engines on the first stage so I checked and yep five pumps and um-tine nozzles which begs the question. If its possible to have a rocket engine without a high pressure fuel pump albeit very low power and clearly its possible to have one pump fuel multiple nozzles, just exactly what is the definition of a rocket engine?

Offline guckyfan

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #146 on: 10/18/2012 06:34 am »
.................... just exactly what is the definition of a rocket engine?

That is a good question. It seems to me that the Sojuz engines are considerably more complex than other engines. That would put them in the same complexity range as the 9 engines of Falcon 9. With growing launch history I see no reason why they should not have a comparable reliability.

And Sojuz is very reliable.

Offline oldAtlas_Eguy

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #147 on: 10/18/2012 05:38 pm »
From the observed engine failure rate of 1:40 gives .025 which from your equation gives a stage failure rate of .03 which is a reliability rate of 97% but only as it relates to engines. Which means that it is basiclly a wash between 1 engine vs 9 engines whose single engine failure rate is the same.

Offline DonEsteban

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #148 on: 10/18/2012 08:10 pm »
There are two crucial parameters in the stage failure rate calculation:

1) the probability that an engine failure is catastrophic and destroys the whole stage

2) the number of benign engine failures that the stage can tolerate


In order to have good stage failure probability, you need to have both
1) probability of catastrophic engine failure much less then 1/#of engines(here, 1/20 vs 1/9, not significantly smaller)

and

2) the probability of too many benign failures (P9 in the code) should be much less than the probability of single engine failure. Here, it is also not the case - essentially because (9 choose 2)/40 is not much smaller than 1. This term would get much better (essentially (9 choose 3)/1600) ~ 1/20) if you can tolerate 2 failures out of 9 engines (which might be the case for late failures, although it is not the case for early ones).

In any case, stage failure rate vs engine failure rate gets much better as the engine reliability improves .. 1/40 is just not small enough.
« Last Edit: 10/18/2012 08:24 pm by DonEsteban »

Offline oldAtlas_Eguy

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #149 on: 10/18/2012 08:23 pm »
There are two crucial parameters in the stage failure rate calculation:

1) the probability that an engine failure is catastrophic and destroys the whole stage

2) the number of benign engine failures that the stage can tolerate


In order to have good stage failure probability, you need to have both
1) probability of catastrophic engine failure much less then 1/#of engines(here, 1/20 vs 1/9, not significantly smaller)

and

2) the probability of too many benign failures (P9 in the code) should be much less than the probability of single engine failure. Here, it is also not the case - essentially because 40 is not much smaller than (9 choose 2) /9. This term would get much better if you can tolerate 2 failures out of 9 engines (which might be the case for late failures, although it is not the case for early ones).

In any case, stage failure rate vs engine failure rate gets much better as the engine reliability improves .. 1/40 is not good enough.

Ok so a dual benign failure that causes a stage failure using the benign failure rate of .025 (1:40) would yeild a stage failure rate of  .005? A reliability value of 99.5%? Or is my math off?

Do you have an equation and a value for stage failure based on the obsered benign failure rate?

Offline DonEsteban

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #150 on: 10/18/2012 08:39 pm »
It is in the equations in the code mentioned above.

P9 is the probability of stage failure due to too many benign failures.

If the stage tolerates only one bening failure, the leading (largest, most important) term is (nchoosek(9,2)-4).*Pf.^2.*Ps.^7, which is close to (9 choose 2)*1/(40^2)=0,0225, i.e. about 1/44, which is almost the same as single engine failure probability. In other words, even if we assume the are no catastrophic engine failures, stage reliability would be only cca 98%.

On the other hand, if the stage tolerated 2 benign failures, the leading term in P9 would be approximately (9 choose 3)/40^3 ~ 0.0013, i.e. stage reliability (assuming no catastrophic failures) of cca 99.87%.

Alternatively, with only 5 engines and one engine out tolerance, the stage failure rate (again assuming 1/40 engine failure rate) would be about (5 choose 2)/1600 ~ 0,00625, i.e. stage reliability of 99.375%.


Offline oldAtlas_Eguy

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Re: Multiple engines on the first stage, good or bad idea?
« Reply #151 on: 10/18/2012 08:58 pm »
It is in the equations in the code mentioned above.

P9 is the probability of stage failure due to too many benign failures.

If the stage tolerates only one bening failure, the leading (largest, most important) term is (nchoosek(9,2)-4).*Pf.^2.*Ps.^7, which is close to (9 choose 2)*1/(40^2)=0,0225, i.e. about 1/44, which is almost the same as single engine failure probability. In other words, even if we assume the are no catastrophic engine failures, stage reliability would be only cca 98%.

On the other hand, if the stage tolerated 2 benign failures, the leading term in P9 would be approximately (9 choose 3)/40^3 ~ 0.0013, i.e. stage reliability (assuming no catastrophic failures) of cca 99.87%.

Alternatively, with only 5 engines and one engine out tolerance, the stage failure rate (again assuming 1/40 engine failure rate) would be about (5 choose 2)/1600 ~ 0,00625, i.e. stage reliability of 99.375%.



Thank you.

The real item here is the requirement of exess thrust in the amount of 20% in both the 5:1 engine and 9:2 engine case for delivery of the same payload, with the 9:2 engine case there is a margininal better reliability.

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