timandy1

Merlin=/=Merlin Vacuum?

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Ok, so I don't know anywhere else to post this, but the Wikipedia page and every other source claims that the "Merlin Vacuum (1D)" engine for the falcon 9 is just a normal Merlin 1D engine with an extended bell, but in every image I've seen the Merlin Vacuum looks HUGE compared to the 1st stage engines? What?

Edited by timandy1

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Size of the engine bell is not indicative to its power. That bell needs to be so large to direct the expanding gas in a vacuum straight down the axis of the rocket. Any exhaust molecule that shoots out at an angle ( over expansion ) is wasted energy.... In a nutshell.

Edited by Motokid600

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The inner workings are similar, but the bell is huge. A rocket engine works optimally when the exhaust pressure is similar to the external pressure (afaik). In a vacuum, the perfect engine would have a bell that's extremely long, since the external pressure is almost zero. So, a bigger bell is better for isp in a vacuum.

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4 minutes ago, Bill Phil said:

The inner workings are similar, but the bell is huge. A rocket engine works optimally when the exhaust pressure is similar to the external pressure (afaik). In a vacuum, the perfect engine would have a bell that's extremely long, since the external pressure is almost zero. So, a bigger bell is better for isp in a vacuum.

I think you guys mean to say longer, as in longer along the bells axis of symmetry. 

Edited by PB666

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34 minutes ago, PB666 said:

I think you guys mean to say longer, as in longer along the bells axis of symmetry. 

Well that's the intent, but it's still bigger along that axis.

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You can expect the MVac to be a customized derivative of the regular 1D. There will be other modifications besides the bell.

 

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The extended nozzle improves efficiency in vacuum. This is why even though the F9 second stage is so small, it can still burn up to 6 minutes. The same concept can be seen on RL10B-2, where they added an extended nozzle for vacuum operations. I think RL10B-2 is the world's most efficient liquid fueled rocket engine, according to spacelaunchreport.com 

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9 minutes ago, Delta_8930 said:

The extended nozzle improves efficiency in vacuum. This is why even though the F9 second stage is so small, it can still burn up to 6 minutes. The same concept can be seen on RL10B-2, where they added an extended nozzle for vacuum operations. I think RL10B-2 is the world's most efficient liquid fueled rocket engine, according to spacelaunchreport.com 

I think it burn so long because it can throttle down to 39% full thrust.

I don't know about the RL-10B-2 being so efficient

Edited by Bill Phil

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Also, the turbopump exhaust goes into the nozzle, while on merlin 1d it ends next to the nozzle. Check a youtube video of a falcon launch, and wait until the second stage kicks in. ;)

Edited by Kartoffelkuchen

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1 hour ago, Delta_8930 said:

RL10B-2's Isp is 462 seconds in vacuum. 

yep, one of the most efficient second stage engines out there.

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2 hours ago, Kartoffelkuchen said:

Also, the turbopump exhaust goes into the nozzle, while on merlin 1d it ends next to the nozzle. Check a youtube video of a falcon launch, and wait until the second stage kicks in. ;)

Reminds me of the original F-1; they piped the exhaust through the engine nozzle to get extra performance

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2 hours ago, Delta_8930 said:

RL10B-2's Isp is 462 seconds in vacuum. 

I know its Isp. The Vinci goes to 465 according to Wikipedia.

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A lithium fluoride hydrogen engine was test fired in the 60s... With vac isp in the 500s.

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21 hours ago, Delta_8930 said:

Reminds me of the original F-1; they piped the exhaust through the engine nozzle to get extra performance

It doesn't necessarily yield extra performance - in fact, the exhaust port usually has a nozzle so that it will produce as much thrust as the exhaust pressure will allow.  The main reason to have the exhaust manifold injecting into the engine bell is to cool the nozzle extension (and thus not have to pump fuel through it).

Also a reminder to all: when comparing absolute Isp, it's important to account for the fuel mixture used.  Hydrolox typically has about 1.3x the Isp of kerolox, all other things being equal, but then you have to deal with much large tank volume and boiloff.

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51 minutes ago, Kryten said:

That was a desktop demonstrator, and used gaseous hydrogen.

It was still test fired. 

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