SpaceX: Merlin 1D Updates and Discussion Thread 2

[Chris Bergin]

New thread for the Merlin 1Ds.

Thread 1:
http://forum.nasaspaceflight.com/index.php?topic=26388.0

Resources:
Merlin 1D News Articles:
http://www.nasaspaceflight.com/?s=%22Merlin+1D%22

Remember to stay on topic and not to wander into different subjects (even if they are SpaceX related). We have lots of threads and you can even start splinter threads if the discussion warrants it.

[Okie_Steve]

Something on another thread got me to wondering. What are the fundamental limits to throttling a kero-lox turbopump engine like the Merlin. I believe that pressure fed pintle engines can throttle much deeper. Is the 70% envelope on the Merlin-1D a fairly hard limit or just a design trade off? Anyone have any insight?

[Elmar Moelzer]

Someone mentioned the possibility of flow separation if the M1D is throttled too low. From my limited understanding that among other things depends on the shape and size of the nozzle extension. So maybe it could be possible to throttle a M1d more deeply with a different nozzle design. That might mean less efficiency in other parts of the flight envelope though.
I am sure that someone with more rocket engine design knowledge will immediately jump in and correct me in 10...9...

[cambrianera]

Something on another thread got me to wondering. What are the fundamental limits to throttling a kero-lox turbopump engine like the Merlin. I believe that pressure fed pintle engines can throttle much deeper. Is the 70% envelope on the Merlin-1D a fairly hard limit or just a design trade off? Anyone have any insight?

No insight, but some thinking.
To throttle your engine you reduce propellant flow; you can do this increasing pressure drop on the injector or reducing the RPM of the turbopump.
In the first case you have a net loss of energy, in the second you actually increase the percentage of propellant flowing through the combustion chamber (in GG cycle the propellant through GG and turbine is lost for thrust)

But turbomachinery doesn't like big variations of RPM, normally they are highly optimized for a small range.
Therefore throttling range only from 100% to 70%.
I believe SpaceX went this way.

Usual disclaimer, only speculation.

BTW: expert's opinion highly appreciated.

[baldusi]

Someone mentioned the possibility of flow separation if the M1D is throttled too low. From my limited understanding that among other things depends on the shape and size of the nozzle extension. So maybe it could be possible to throttle a M1d more deeply with a different nozzle design. That might mean less efficiency in other parts of the flight envelope though.

The Merlin 1D has the whole nozzle made of a structure that circulates RP-1 to cool it (it's called regeneratively cooled nozzle). It would look, from the extremely clean look of it and amazing T/W, that they have the inner and oute linings and use a corrugated separator in the middle. Then they would braze it. Like the NK-33. You can add an extension, that will have to be cooled differently (either ablatively, or film-cooled+radiatively, like the Vacuum version of the Merlin 1D). But you can't cut it easily. You'd have to modify or even get new tooling, and the pressure loss on the nozzle cooling circuit will be reduced, which I don't know how would impact the power balance on the engine. Thus, is not easy to reduce the thrust because of flow separation.

[Okie_Steve]

Hmm, thanks for the food-for-thought. It sounds like applying various already-done-elsewhere ideas could address most of these. Route the turbopump output to the nozzle along the lines of the Merlin-Vac, use a double bell nozzle, recycle some of the pump output back to the input etc. I had not considered the reg-cooling but I suspect it's not a show stopper. Based on the immediate discussion it sounds to me like there's nothing really fundamentally limiting just that they applied the KISS principle to the tubopump design because the design trade offs for deeper throttling were not worth it.

[malu5531]

While trying to understand Merlin 1D and in particular "Merlin 1D+"* in depth, I've iterated my calculations a few times and have reached internal coherence and good balance with reality using the following characteristics/specs. 

	Merlin 1D	..	Merlin 1D Vac	..	Merlin 1D+	..	Merlin 1D+ Vac
Chamber Pressure, MPa	9.7		9.7		10.8		10.8
Pressure exhaust, bar	0.43		0.03		0.475		0.034
Throat area, m^2	0.042		0.042		0.042		0.042
Nozzle area, m^2	0.9		7.22		0.9		7.22
Nozzle diameter, m	1.07		3.03		1.07		3.03
Mixture ratio	2.34		2.36		2.36		2.375
Mass flow chamber, kg	229		229		253.5		253.5
Mass flow gg, +%	3.25		3.25		3.25		3.25
Net mass flow, kg	236		236		262		262
SL Thrust, klbf	147 klbf				165 klbf
SL Isp, s	282 s				286 s
Vac Thrust, klbf	167 klbf		181 klbf		185 klbf		200 klbf
Vac Isp, s	320 s		347 s		321 s		347 s

*) Merlin 1D+ is a nick-name for the engine Elon hinted during the SES8 teleconference; a 165 klbf SL "Merlin 1D"

Disclaimer; IANARS & many constants are read from analogue charts, so there are some wiggle room.
Nozzle diameter should probably be a few cm less.

[Prober]

Someone mentioned the possibility of flow separation if the M1D is throttled too low. From my limited understanding that among other things depends on the shape and size of the nozzle extension. So maybe it could be possible to throttle a M1d more deeply with a different nozzle design. That might mean less efficiency in other parts of the flight envelope though.

The Merlin 1D has the whole nozzle made of a structure that circulates RP-1 to cool it (it's called regeneratively cooled nozzle). It would look, from the extremely clean look of it and amazing T/W, that they have the inner and oute linings and use a corrugated separator in the middle. Then they would braze it. Like the NK-33. You can add an extension, that will have to be cooled differently (either ablatively, or film-cooled+radiatively, like the Vacuum version of the Merlin 1D). But you can't cut it easily. You'd have to modify or even get new tooling, and the pressure loss on the nozzle cooling circuit will be reduced, which I don't know how would impact the power balance on the engine. Thus, is not easy to reduce the thrust because of flow separation.

The extension can be cut very easy.  Water jet can cut it without a problem.

[pagheca]

Hi everyone,

this question maybe obvious for many of you - please excuse me if this is the case but I'm just in learning mode.

As well known, the Merlin 1D engine can be throttled down from 100% to 60%.

The vacuum thrust of a rocket engine is proportional to the exhaust velocity and to the mass flow rate of the gas exhaust: dm/dt * V_exh.

In vacuum and if also the gravity drag is negligible, when a Merlin 1D engine is throttled down, is dm/dt only changing, OR also V_exh (aka Isp_vacuum) is affected someway, with a consequent reduction in the engine efficiency in using the available propellant?

If the second hypothesis is right (as I would say by looking at the physical principles the engine is based on), does someone know a reasonable way to write the relation Thrust(%) = f(%,dm/dt, V_exh)?

Thanks for any hint,

[cambrianera]

Someone mentioned the possibility of flow separation if the M1D is throttled too low. From my limited understanding that among other things depends on the shape and size of the nozzle extension. So maybe it could be possible to throttle a M1d more deeply with a different nozzle design. That might mean less efficiency in other parts of the flight envelope though.

The Merlin 1D has the whole nozzle made of a structure that circulates RP-1 to cool it (it's called regeneratively cooled nozzle). It would look, from the extremely clean look of it and amazing T/W, that they have the inner and oute linings and use a corrugated separator in the middle. Then they would braze it. Like the NK-33. You can add an extension, that will have to be cooled differently (either ablatively, or film-cooled+radiatively, like the Vacuum version of the Merlin 1D). But you can't cut it easily. You'd have to modify or even get new tooling, and the pressure loss on the nozzle cooling circuit will be reduced, which I don't know how would impact the power balance on the engine. Thus, is not easy to reduce the thrust because of flow separation.

The extension can be cut very easy.  Water jet can cut it without a problem.

I guess baldusi was suggesting that it's easy to add an extension (increasing the expansion) but not to cut the nozzle (decreasing the expansion).

[Elmar Moelzer]

And I was talking about a different/ new nozzle design, not just cutting or extending the current nozzle.

[cambrianera]

Two things about throttling:
-thanks to aero for this interesting link
http://www.ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100033271_2010034521.pdf
the post
http://forum.nasaspaceflight.com/index.php?topic=33142.msg1130796#msg1130796
-in last GH1 flight, it's likely that throttling of merlin 1D went to about 60%.

[Joffan]

Two things about throttling:
-thanks to aero for this interesting link
http://www.ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100033271_2010034521.pdf
the post
http://forum.nasaspaceflight.com/index.php?topic=33142.msg1130796#msg1130796
-in last GH1 flight, it's likely that throttling of merlin 1D went to about 60%.

.. and that's 60% of whatever they were using as 100% lift-off thrust. If top thrust can be increased by (say) 10%, still leaving some margin, and the throttling back can still get down to the same low level, it would be about 55%.

[aero]

While trying to understand Merlin 1D and in particular "Merlin 1D+"* in depth, I've iterated my calculations a few times and have reached internal coherence and good balance with reality using the following characteristics/specs. 

	Merlin 1D	..	Merlin 1D Vac	..	Merlin 1D+	..	Merlin 1D+ Vac
Chamber Pressure, MPa	9.7		9.7		10.8		10.8
Pressure exhaust, bar	0.43		0.03		0.475		0.034
Throat area, m^2	0.042		0.042		0.042		0.042
Nozzle area, m^2	0.9		7.22		0.9		7.22
Nozzle diameter, m	1.07		3.03		1.07		3.03
Mixture ratio	2.34		2.36		2.36		2.375
Mass flow chamber, kg	229		229		253.5		253.5
Mass flow gg, +%	3.25		3.25		3.25		3.25
Net mass flow, kg	236		236		262		262
SL Thrust, klbf	147 klbf				165 klbf
SL Isp, s	282 s				286 s
Vac Thrust, klbf	167 klbf		181 klbf		185 klbf		200 klbf
Vac Isp, s	320 s		347 s		321 s		347 s

*) Merlin 1D+ is a nick-name for the engine Elon hinted during the SES8 teleconference; a 165 klbf SL "Merlin 1D"

Disclaimer; IANARS & many constants are read from analogue charts, so there are some wiggle room.
Nozzle diameter should probably be a few cm less.

I have a question for you. If SpaceX were to try the D+, what would they do for propellant? Mass flow goes from 236 to 262 so the burns would be shorter on both stages. Sure, the Isp is up a little but in the wash, it really doesn't make much difference to payload capability. With more powerful engines, will they extend the length of the tanks by 10 % (a guess) to keep the lift-off T/W the same or will they just drive the engines harder for little benefit?

And by the way, does anyone know the actual propellant mass for the stages of the existing F 9? Simulating the F 9 is a trick because each time the Isp changes, the lift-off mass changes, or the dry weight of the stages changes to compensate.

I know the mixture ratio so if I knew the length of the tank I could estimate the propellant mass probably more accurately than using the fuel flow rate and burn time since fuel flow rate is a mystery anyway.

[Elmar Moelzer]

I have a question for you. If SpaceX were to try the D+, what would they do for propellant? Mass flow goes from 236 to 262 so the burns would be shorter on both stages. Sure, the Isp is up a little but in the wash, it really doesn't make much difference to payload capability. With more powerful engines, will they extend the length of the tanks by 10 % (a guess) to keep the lift-off T/W the same or will they just drive the engines harder for little benefit?

If they did not need the center engine for breaking, the extra thrust of the 1D+ should allow them to just get rid of it and save the weight. I don't think an 8 engine layout would work... hmmm...

And by the way, does anyone know the actual propellant mass for the stages of the existing F 9? Simulating the F 9 is a trick because each time the Isp changes, the lift-off mass changes, or the dry weight of the stages changes to compensate.

I know the mixture ratio so if I knew the length of the tank I could estimate the propellant mass probably more accurately than using the fuel flow rate and burn time since fuel flow rate is a mystery anyway.

Not sure how accurate this is, but there are some numbers here:
http://www.spacelaunchreport.com/falcon9v1-1.html#components

[aero]

Thanks for the link. It's an update from the old one that I've been referring to.

[Joffan]

If SpaceX were to try the D+, what would they do for propellant? Mass flow goes from 236 to 262 so the burns would be shorter on both stages. Sure, the Isp is up a little but in the wash, it really doesn't make much difference to payload capability.

Yes, it does make a difference. The reason is that with higher thrust you get a quicker ascent and reduced gravity losses. So the answer is that they could increase the maximum payload even without changing the amount of propellant.

[Elmar Moelzer]

Yes, it does make a difference. The reason is that with higher thrust you get a quicker ascent and reduced gravity losses. So the answer is that they could increase the maximum payload even without changing the amount of propellant.

But wouldn't that also increase the g forces on the payload?

[MikeAtkinson]

I have a question for you. If SpaceX were to try the D+, what would they do for propellant? Mass flow goes from 236 to 262 so the burns would be shorter on both stages. Sure, the Isp is up a little but in the wash, it really doesn't make much difference to payload capability. With more powerful engines, will they extend the length of the tanks by 10 % (a guess) to keep the lift-off T/W the same or will they just drive the engines harder for little benefit?

My guess is that they would "spend" the extra thrust in two ways:

Firstly, so that 8 M1D+ engines can give the thrust of 9 current M1D, they avoid having to keep a propellant reserve for the extra gravity losses during engine out. So no stretch of the 1st stage, and run the engines at less than their full thrust.

This would up the payload to LEO to about 16 tonnes and to GTO to about 5.5 tonnes [guesses, not run any simulations].


Secondly, a modest stretch of the 2nd stage [guesses, not run any simulations].

Together these would up the payload to LEO to about 20 tonnes and GTO to about 6.5 tonnes.

[mheney]

Another consideration is that, if you want to reuse the engines, you extend their life by not red-lining them.  When you optimize for cost rather than raw performance, SOP may well be to run the engines at 80% of theoretical max performance rather than pedal-to-the metal ...