More powerful Merlin D?

[PTNLemay]

I was surprised to see this article:  http://www.theverge.com/2016/5/1/11549456/spacex-falcon-9-heavy-launch-capabilities-weight-mars

Long story short, they've re-spec-ed the Falcon 9 so that it can haul quite a bit more mass into low orbit.  22 metric tons versus the old 13 tons.  And the F9-Heavy will be able to lift even more onto Mars.  I know that most rockets operate with a good safety margin, but I'm a bit confused as to how they could have gotten this much extra out of it.  Elon Musk's tweets say that they haven't' substantially changed the machinery of the engines themselves.  I take this to mean that they're just pushing them harder, which would mean a higher thrust and maybe higher exhaust velocity?

Does anyone have more data on the old specs and the new specs?   On wikipedia it currently lists the Merlin engine as having 282 s at sea level.   No idea if this is the new one or the old one.

 

Edited May 2, 2016 by PTNLemay

[sojourner]

Elon tweet that the new numbers for Merlin are due to refinements in the margins they use. AS more real data has been collected they've been able to decrease the "safety" margins and push the engine to a higher thrust.

[PTNLemay]

So... the ISP hasn't changed and the benefits they're seeing would stem primarily from them burning harder lower in the gravity well and taking advantage of the Oberth effect?

Because that's a BIG bump in performance.

Edited May 2, 2016 by PTNLemay

[PB666]

15 minutes ago, PTNLemay said:

I was surprised to see this article:  http://www.theverge.com/2016/5/1/11549456/spacex-falcon-9-heavy-launch-capabilities-weight-mars

Long story short, they've re-spec-ed the Falcon 9 so that it can haul quite a bit more mass into orbit.  50,000 pounds versus the old 29,000 pounds.  And the F9-Heavy will be able to lift even more onto Mars.  I know that most rockets operate with a good safety margin, but I'm a bit confused as to how they could have gotten this much extra out of it.  Elon Musk's tweets say that they haven't' substantially changed the machinery of the engines themselves.  I take this to mean that they're just pushing them harder, which would mean a higher thrust and maybe higher exhaust velocity?

Does anyone have more data on the old specs and the new specs?   On wikipedia it currently lists the Merlin engine as having 282 s at sea level.   No idea if this is the new one or the old one.

 

There are a couplevof things withbthe merlin engines, they extnded the bell improving ISP and performance, the engines themselves from 1C has produced the 1D the 1D+ and the 1D vacuum in which the wiki states the engines as of yet are not at full capacity, possibly withbthe new engines they are gradually increasing output to see just how far they can push them. I look at the 1d vacuum in flight i would say it is already close tomits limit. 

This is what I think you want to read. 

https://en.m.wikipedia.org/wiki/Falcon_9_full_thrust

Note that the merlin 1D vacuum has an ISP of 348, the it can iperate from 39 % to a full,power of 934 kN. 

https://en.m.wikipedia.org/wiki/Merlin_(rocket_engine_family)#Merlin_1D_Vacuum

The peak launch thrust at msl is between 723 and 730 KN per engine and can reach 825 kN at the end of its flight. This means that falcon 9 FT can achieve 6570 kN. However CRS-8 launched with an extraploated TWR of 1.22 and a stated weight.  That is for the 282, howvever the stated thrust is 7204 kN indicating a higher ISP. Thus uncertainty in the wiki. The rockets weight is 543 KT and payload is 3.2 giving a total weight 546 kt gives a starting thrust of 13.55a and i calculated that equilibration to full thrust extrapolated back launch at 12.3 or 6715 kN. They appear to still not be using full thrust, they maybe holding back in case an engine fails in early flight they still have some capacity to compensate. If they lost an engine right after launch, with a TWR of 1.2 it would be devastating, they would not have enough fuel left to recover the launch, however at TWR of 1.35 they could lose one engine after 51 seconds and still have adequate thrust on 7 remaing, pairing down 1 additional engine to complete the flight. 

I don't know what the current ISP except for the 1D vacuum, it has an ISP of 348. 

 

 

 

[Streetwind]

Less Oberth effect, and more reducing gravity losses with a higher TWR off the pad, a shorter vertical flight and a flatter overall trajectory. Gravity drag is the second highest dV cost position, after orbital velocity, and is easily five times higher than atmospheric drag. As long as your rocket can still handle max-Q, pushing harder early-on can bring significant savings. (This is true even in KSP, where for example a straight-upward launch still gains efficiency even for increasing TWR beyond 6.)

Also note, though, that the harder early push and the flatter overall trajectory also increases the payload cost for booster recovery. While SpaceX originally spoke about nearly 30% for the Falcon 9 v1.1 going RTLS, they're now admitting closer to 40% for this "even fuller thrust" variant. Since F9 is probably never going to fly expendable, the actual performance improvements are not going to end up as drastic as those theoretical expendable numbers suggest.

But yeah, it's still darn impressive how they can keep uprating the Merlin 1D again and again without making physical changes to the engine and still maintaining a planned 10-flight lifetime for them. When it was introduced, the Merlin 1D already broke the record for the highest TWR of any rocket engine ever flown, at >150; at the end of this year, it's probably going to exceed 200. What an incredible motor.

Edited May 2, 2016 by Streetwind

[sevenperforce]

If they lose a periphery engine shortly after launch, can they gimbal the other engines to maintain COM/COT, or would they have to shut down the opposite engine entirely to balance?

If the latter, then I expect that the writing and plumbing associated with the upper parts of the engine and turbopump is probably done in pairs, so that if they lose two engines it is more likely to be an opposite pair. 

And as far as specific impulse is concerned, any improvements would be very minor, I think. I can't remember if it is possible to calculate the backpressure of the exhaust flow based on published ISP differences or not, but if so then I ought to be able to get a general idea of the increased pressure thrust associated with the higher flow. 

Also, unless I'm mistaken the uprated thrust should now come with a larger throttling capability as a percentage of total thrust. 

[KerbonautInTraining]

22 minutes ago, sevenperforce said:

If they lose a periphery engine shortly after launch, can they gimbal the other engines to maintain COM/COT, or would they have to shut down the opposite engine entirely to balance?

If the latter, then I expect that the writing and plumbing associated with the upper parts of the engine and turbopump is probably done in pairs, so that if they lose two engines it is more likely to be an opposite pair. 

On an early version of the F9 a corner engine died, it continued flying straight and true.

My guess is that engine failure = no stage recovery. (You don't know the extent of the damage) The fuel reserve for boostback/landing would far exceed the increased gravity losses. 

[sevenperforce]

Speculation on first stage uprated Merlin 1D thrust and specific impulse...

Elon stated that the F9 would be launching at 1.71 million pounds thrust with a vacuum thrust on the first stage of 1.9 million pounds. Doing the math, that comes to a whopping 845.6 kN launch thrust and 938.9 kN vacuum thrust on the first-stage Merlin 1D engines.

If you're tracking, that's a vacuum TWR of 204. Absolutely astounding.

Anyway, I think we should see Isp come up a bit due to increased pressure thrust, since the nozzle diameter is unchanged. In other words, the increased flow means the exhaust is now underexpanded at launch. Assuming no increase in actual exhaust velocity, we are looking at a mass flow increase. 846/642 = 132%, so a very broad estimate using the ideal gas law suggests a 32% increase in gas pressure.

We can figure out what Isp increase this represents by using the difference between published vacuum Isp and published SL Isp. Assume for simplicity that earlier SL thrust matches atmospheric pressure (even though it probably was already slightly underexpanded). 311 s from 282 s corresponds to a 10.2% increase in specific impulse due to pressure thrust. This suggests that going up by one atmosphere of pressure is a 10.2% increase in mass-specific impulse.

So, going up by 32/100ths of an atmosphere is probably going to be a 3.26% increase in specific impulse. That means a SL specific impulse of roughly 291.2 seconds and a vacuum specific impulse of roughly 320.3 seconds. Vacuum specific impulse of the Merlin 1D Vacuum will remain the same because it uses a better expansion ratio so its pressure thrust will be largely unaffected...maybe 0.5-1.5% increase at best.

[Streetwind]

58 minutes ago, sevenperforce said:

If they lose a periphery engine shortly after launch, can they gimbal the other engines to maintain COM/COT, or would they have to shut down the opposite engine entirely to balance?

If the latter, then I expect that the writing and plumbing associated with the upper parts of the engine and turbopump is probably done in pairs, so that if they lose two engines it is more likely to be an opposite pair. 

An engine-out event during first stage flight will result in the opposite engine being shut off as well, except for the case where the center engine drops out. At least, that's what I read whenever that topic is discussed. In the case of CRS flight 1, though, it continued on 8 engines instead of 7. Which were admittedly Merlin 1C's, not the current 1D's. But from the looks of it, both options are a real possibility.

It probably depends on in which context the engine failure happens. For missions that don't need the full power, which probably involves the majority of them nowadays (Dragon missions are underweight due to being designed for v1.0, GTO missions are underweight because they need extra dV, and so on), they probably throttle the engines back for max-Q and then just leave them throttled for the rest of the flight, since the critical early seconds where the vehicle fights gravity the most are long past... or even outright shut down two engines near the end of the first stage burn (v1.0 sometimes did this with its not-very-throttleable 1C's). Should an engine be lost in such a scenario, they should easily be able to afford shutting off the opposing engine and throttling the rest back up to full.

So in a way, the repeated thrust uprating of the Merlins makes the F9 even more resilient against engine losses than it ever was before

20 minutes ago, sevenperforce said:

So, going up by 32/100ths of an atmosphere is probably going to be a 3.26% increase in specific impulse. That means a SL specific impulse of roughly 291.2 seconds and a vacuum specific impulse of roughly 320.3 seconds. Vacuum specific impulse of the Merlin 1D Vacuum will remain the same because it uses a better expansion ratio so its pressure thrust will be largely unaffected...maybe 0.5-1.5% increase at best.

Interesting math, but I wonder - they'll have to feed extra fuel through the gas generator to spin the pumps faster. So that in turn reduces the Isp gains, because you're essentially throwing more fuel overboard than before. It's probably still higher than the original 311s at 720 kN vacuum thrust, but I wonder if it actually made it all the way to 320s.

 

[wumpus]

The SSME (Space Shuttle Main Engines) are famous for running at "110%" of rated power.  That rating was from the 1970s as designed, and either manufacturing improvements or testing showed that they could run at the new rating.  Also remember that Falcon9 (1.1 FT) takes off with a TWR of 1.3, a 10% increase of power would mean a TWR of 1.4 or a 33% increase in delta-v (at take-off, which would have to be toned down after awhile due to max-Q issues, but would at least get the thing going a lot faster).

[PB666]

1 hour ago, wumpus said:

The SSME (Space Shuttle Main Engines) are famous for running at "110%" of rated power.  That rating was from the 1970s as designed, and either manufacturing improvements or testing showed that they could run at the new rating.  Also remember that Falcon9 (1.1 FT) takes off with a TWR of 1.3, a 10% increase of power would mean a TWR of 1.4 or a 33% increase in delta-v (at take-off, which would have to be toned down after awhile due to max-Q issues, but would at least get the thing going a lot faster).

In my mind, if you can support the stress, I would go to my down engine velocity (800kmh, 222.22 m/s) speed as rapidly as possible, even if I gain 5 or 10 seconds, that is a savings of fuel I would otherwise spent fighting gravity, it also means I can down my engines more in anticipation of MaxQ, Of course it if was me Given the oversize payloads, I would have a wider shorter 1st and 2nd stage with a capacity for more fuel in both, and allowing for the increased capacity engines.

I think there is probably a risk metric here, since they upgraded the Merlin 1 in rapid succession, I suspect they basically increasing stress until one engine fails, if they increased stress too quickly then the probability of a 2 engine failure increases. The other issue since this is a trial run for recycling, you kind of want the good publicity of a successful relaunch, and you don't want to risk that with a overstressed engine failure. If RP-1 is a fraction of the variable cost, and recycling or de-novo production are the primary costs, then you burn a little extra fuel, so what, you still save alot of money on the recycling. If I am right you probably would see a high tech pipeline and tank engineer out there with a cesium source looking for cracks and stress lines in the engines, asking the basic question how much stress did the last launch induce?

Is it possible for us to get a stat page going for the Merlins, maybe even a Merlin 1D thread so that we can follow the launch stats, there seems to be no certainty of its rated launch capability or its ISP, the wiki's figure appear to be outdated.

 

 

 

[Exoscientist]

It should be noted that the 13 metric ton payload to LEO for the F9 v1.1 that was cited by SpaceX, prior to the FT upgrade, is actually assuming F9 first stage reusability. A fully expendable version of the F9 v1.1 actually has a ca. 16 metric ton payload to LEO.

This was indicated by Gwynne Shotwell in a 2014 interview where she said without reusability the F9 payload is about 30% higher than the cited amount which would put it at about 16 metric tons:

NASA, CNES Warn SpaceX of Challenges in Flying Reusable Falcon 9 Rocket
May 5, 2014 by Amy Svitak in On Space

Quote

SpaceX President Gwynne Shotwell says Falcon 9's reusability is already designed into the rocket's first stage, including the weight of the landing legs that would otherwise detract from the rocket's performance. She also said Falcon 9 retains 30% performance margin over the company's advertised mass-to-orbit capability of 4,850 kg to GTO margin SpaceX is using to conduct operational trials of a reusable Falcon 9 first stage.

http://aviationweek.com/blog/nasa-cnes-warn-spacex-challenges-flying-reusable-falcon-9-rocket

This is confirmed by a NASA launch performance calculator that also gives a 16.6 metric ton payload to LEO for the expendable version of the F9 v1.1:

NASA Launch Services Program's
Launch Vehicle Performance Web Site.
http://elvperf.ksc.nasa.gov/Pages/Query.aspx

On this query page enter 200 km for the altitude and 28.5 degrees for the launch inclination to match the latitude of Cape Canaveral, otherwise the payload will be reduced. 

Since for the Falcon 9 FT, SpaceX said the payload to GTO was increased by 30%, estimates also put the increase in payload to LEO by 30%, which would put it in the 20 to 22 metric ton range.

  Bob Clark 

[Climberfx]

And have you seen the Merlin 2 specs? Will be better than all 9 Merlin 1D together, and will lift the new ones Falcon 9 (with just one Merlin 2 in place of all 9 Merlin 1D), Falcon 9 Heavy, and all the Falcon X, X Heavy, etc...

Edited June 21, 2016 by Climberfx

[Nibb31]

45 minutes ago, Climberfx said:

And have you seen the Merlin 2 specs? Will be better than all 9 Merlin 1D together, and will lift the new ones Falcon 9 (with just one Merlin 2 in place of all 9 Merlin 1D), Falcon 9 Heavy, and all the Falcon X, X Heavy, etc...

Surely you're talking about Raptor, not Merlin 2.

A Raptor-powered Falcon would not be able to throttle down to the level of a single Merlin, which would make landing more difficult. Raptor is supposed to be for the BFR.

[Climberfx]

I doubt that they would substitute land able 1st stage Falcon 9 for a no land able one.

The original name for "Raptor", is Merlin 2. (https://en.wikipedia.org/wiki/Merlin_(rocket_engine_family)#Merlin_2_concept)

Take a look:

 

[Nibb31]

53 minutes ago, Climberfx said:

I doubt that they would substitute land able 1st stage Falcon 9 for a no land able one.

The original name for "Raptor", is Merlin 2. (https://en.wikipedia.org/wiki/Merlin_(rocket_engine_family)#Merlin_2_concept)

Take a look:

 

Yeah, that's like 5 years old at least. The "Falcon XX" concept was abandoned years ago, SpaceX has been using the name Raptor for its engine since 2009, and the "Hydra" pic is fanart.

 

Edited June 21, 2016 by Nibb31