Author Topic: SpaceX: Merlin 1D thread  (Read 520704 times)

Offline cambrianera

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Re: SpaceX: Merlin 1D thread
« Reply #680 on: 04/24/2013 04:54 pm »
A doubt I had (and can't answer).
M1D is throttleable and SpaceX plans to leverage throttleability at the end of the first stage burn, instead of turning off two engines.
When this thing surfaced some consideration were done if this is negative in terms of ISP (throttling normally is bad for ISP).

But M1D has an underexpanded nozzle at altitude; isn't possible that throttling makes the nozzle less underexpanded (gaining some ISP).
Furthermore throttle can be done sending less propellant to the TPA preburner, increasing the percentage of propellant passing in the CC.

Any thought?
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Offline ugordan

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Re: SpaceX: Merlin 1D thread
« Reply #681 on: 04/24/2013 05:02 pm »
But M1D has an underexpanded nozzle at altitude; isn't possible that throttling makes the nozzle less underexpanded (gaining some ISP).

That just means your Isp loss due to throttling won't be as bad.

When you kill 2 engines after hitting a G limit (4.5G or whatever it is on F9), your accel immediately drops to 7/9 of that G limit. With throttling, you could keep the accel right at the G limit so the Isp hit of throttling might actually be offset by slightly lower gravity losses - higher overall acceleration and quicker propellant depletion.

The Isp hit due to throttling is likely to be a non-issue.
« Last Edit: 04/24/2013 05:03 pm by ugordan »

Offline ClaytonBirchenough

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Re: SpaceX: Merlin 1D thread
« Reply #682 on: 05/05/2013 04:55 pm »
Is the only difference between a Merlin 1D and a Merlin 1D Vac the nozzle extension on the latter?
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Offline deltaV

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Re: SpaceX: Merlin 1D thread
« Reply #683 on: 05/05/2013 05:55 pm »
Is the only difference between a Merlin 1D and a Merlin 1D Vac the nozzle extension on the latter?
No there are other differences. For example the M1D gas generator exhaust has a separate nozzle and the M1D Vac gas generator exhausts into the main nozzle F-1 style.

Offline ClaytonBirchenough

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Re: SpaceX: Merlin 1D thread
« Reply #684 on: 05/05/2013 06:12 pm »
Is the only difference between a Merlin 1D and a Merlin 1D Vac the nozzle extension on the latter?
No there are other differences. For example the M1D gas generator exhaust has a separate nozzle and the M1D Vac gas generator exhausts into the main nozzle F-1 style.

Thanks. It was probably relatively easy to convert Merlin 1D into Merlin 1D VAC though, right?
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Offline Prober

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Re: SpaceX: Merlin 1D thread
« Reply #685 on: 05/05/2013 07:19 pm »
Is the only difference between a Merlin 1D and a Merlin 1D Vac the nozzle extension on the latter?
No there are other differences. For example the M1D gas generator exhaust has a separate nozzle and the M1D Vac gas generator exhausts into the main nozzle F-1 style.

maybe, we have only seen "tease" photos up to now.
 
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Offline sdsds

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Re: SpaceX: Merlin 1D thread
« Reply #686 on: 05/05/2013 11:07 pm »
It was probably relatively easy to convert Merlin 1D into Merlin 1D VAC though, right?

My phrasing of the question would be, "How much more did it cost to create both variants, over what it would have cost to create only the sea-level variant?"

My guess is that they actually saved money, compared with the other likely option of continuing Merlin 1C Vac production!
« Last Edit: 05/05/2013 11:08 pm by sdsds »
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Offline beancounter

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Re: SpaceX: Merlin 1D thread
« Reply #687 on: 05/06/2013 05:06 am »
Just throwing this one out there to see if anyone has any ideas.

Apologies if it's been discussed previously however it seems that SpaceX require some form of high energy upper stage to increase the throw mass of their payloads.  Would adding another one or two M1Dv engines do the same as a single say Raptor?  I imagine you'd need to stretch the tanks a bit but that shouldn't be insurmountable.
Am I missing something obvious to rocket scientists that makes this a 'stupid' question?
Thanks.
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Offline QuantumG

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Re: SpaceX: Merlin 1D thread
« Reply #688 on: 05/06/2013 05:17 am »
It's not a stupid question.. but the answer isn't obvious. There is some advantage to adding more thrust to an upper stage, due to the Oberth effect, but it is minor compared to improving ISP.. so, it's only really a sensible tradeoff between comparable engines. Improving ISP of an upper stage gives you an exponential improvement, which can be "spent" in making the stage more bulky in order to hold the more bulky propellants, for a net win.
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Offline beancounter

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Re: SpaceX: Merlin 1D thread
« Reply #689 on: 05/06/2013 05:45 am »
It's not a stupid question.. but the answer isn't obvious. There is some advantage to adding more thrust to an upper stage, due to the Oberth effect, but it is minor compared to improving ISP.. so, it's only really a sensible tradeoff between comparable engines. Improving ISP of an upper stage gives you an exponential improvement, which can be "spent" in making the stage more bulky in order to hold the more bulky propellants, for a net win.

Ok thanks.  Had a quick look at this effect.  Interesting and clearly not intuitive.  I need to think about this a bit.  Raptor is, I believe, a far higher isp engine than the Merlin and this is what provides the upper stage with the improved throw ability, not simply more engines?
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Offline baldusi

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Re: SpaceX: Merlin 1D thread
« Reply #690 on: 05/06/2013 01:16 pm »
Also, adding thrust means worsening your fmp (if you don't enlarge the whole stage). A worse fmp is just as critical as isp. And adding a second engine usually means a smaller nozzle.
For purely orbital maneuvers, isp trumps thrust almost every time. As stated before, you have the Oberth Effect, and the more T/W the more your maneuver approaches an instant impulse, which might be more critical on certain application. But for uppers stages, the general rule is that thrust is not critical unless you are sub orbital. Thus, highly capable second stages, like the Centaur or DIVUS, could take the fmp hit of a second RL10, and improve their LEO performance, because of the gravity losses, but for higher energy orbits, the performance is worse with two engines, since the suborbital time is much less.
In general, the Falcon 9 US has a lot of T/W, but that's the consequence of using a 9 to 1 engine relationship. And with the Merlin 1C, the Vac version had less thrust than the first stage version. Since the 1D is the same engine (turbopump, chamber, etc.) with a better nozzle expansion, the Vac version will be more powerful than the first stage version. This might worsen the high energy performance of the Falcon 9, but might be needed for the Falcon Heavy.

Offline beancounter

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Re: SpaceX: Merlin 1D thread
« Reply #691 on: 05/07/2013 01:00 am »
Also, adding thrust means worsening your fmp (if you don't enlarge the whole stage). A worse fmp is just as critical as isp. And adding a second engine usually means a smaller nozzle.
For purely orbital maneuvers, isp trumps thrust almost every time. As stated before, you have the Oberth Effect, and the more T/W the more your maneuver approaches an instant impulse, which might be more critical on certain application. But for uppers stages, the general rule is that thrust is not critical unless you are sub orbital. Thus, highly capable second stages, like the Centaur or DIVUS, could take the fmp hit of a second RL10, and improve their LEO performance, because of the gravity losses, but for higher energy orbits, the performance is worse with two engines, since the suborbital time is much less.
In general, the Falcon 9 US has a lot of T/W, but that's the consequence of using a 9 to 1 engine relationship. And with the Merlin 1C, the Vac version had less thrust than the first stage version. Since the 1D is the same engine (turbopump, chamber, etc.) with a better nozzle expansion, the Vac version will be more powerful than the first stage version. This might worsen the high energy performance of the Falcon 9, but might be needed for the Falcon Heavy.

Sorry, you lost me in the first sentence:  'fmp'?  Also the 2nd stage should be able to handle the width of 2 engines without nozzle diameter reduction - shouldn't it?
Thanks.
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Offline CuddlyRocket

Re: SpaceX: Merlin 1D thread
« Reply #692 on: 05/07/2013 02:44 am »
A second engine doubles the total thrust, which means you'll accelerate quicker, but it also doubles the rate you use the propellant, which means you can't fire the engines for as long. These effectively cancel out, but you're also carrying the additional weight of the extra engine, so performance is poorer.

Because you have more thrust, you could extend the stage and carry more propellant, but this reduces your acceleration. Also, the first stage now has more mass to carry and this reduces that stage's performance.

There's plenty of room on a Falcon 9 for additional engines, as the diameter of the two stages are the same. The fact that there is only one second stage engine, despite SpaceX's fondness for redundancy, tells you something.

Offline beancounter

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Re: SpaceX: Merlin 1D thread
« Reply #693 on: 05/07/2013 06:41 am »
A second engine doubles the total thrust, which means you'll accelerate quicker, but it also doubles the rate you use the propellant, which means you can't fire the engines for as long. These effectively cancel out, but you're also carrying the additional weight of the extra engine, so performance is poorer.

Because you have more thrust, you could extend the stage and carry more propellant, but this reduces your acceleration. Also, the first stage now has more mass to carry and this reduces that stage's performance.

There's plenty of room on a Falcon 9 for additional engines, as the diameter of the two stages are the same. The fact that there is only one second stage engine, despite SpaceX's fondness for redundancy, tells you something.
Ok thanks for that.  More to think about.
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Offline simonbp

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Re: SpaceX: Merlin 1D thread
« Reply #694 on: 05/07/2013 06:51 am »
But, it's not a Kestrel. You don't need much thrust, but you still want the thrust/weight to be greater than about 0.25 to cancel out the gravity and drag losses.

If you ever see an RL-10, it's a pretty small little engine, especially without any nozzle extensions. Centaur and DCSS get away with using a single one for GTO flights both because of the low thrust requirements and and the low mass of the hydrogen fuel.

Offline baldusi

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Re: SpaceX: Merlin 1D thread
« Reply #695 on: 05/07/2013 05:13 pm »
Also, adding thrust means worsening your fmp (if you don't enlarge the whole stage). A worse fmp is just as critical as isp. And adding a second engine usually means a smaller nozzle.
For purely orbital maneuvers, isp trumps thrust almost every time. As stated before, you have the Oberth Effect, and the more T/W the more your maneuver approaches an instant impulse, which might be more critical on certain application. But for uppers stages, the general rule is that thrust is not critical unless you are sub orbital. Thus, highly capable second stages, like the Centaur or DIVUS, could take the fmp hit of a second RL10, and improve their LEO performance, because of the gravity losses, but for higher energy orbits, the performance is worse with two engines, since the suborbital time is much less.
In general, the Falcon 9 US has a lot of T/W, but that's the consequence of using a 9 to 1 engine relationship. And with the Merlin 1C, the Vac version had less thrust than the first stage version. Since the 1D is the same engine (turbopump, chamber, etc.) with a better nozzle expansion, the Vac version will be more powerful than the first stage version. This might worsen the high energy performance of the Falcon 9, but might be needed for the Falcon Heavy.

Sorry, you lost me in the first sentence:  'fmp'?  Also the 2nd stage should be able to handle the width of 2 engines without nozzle diameter reduction - shouldn't it?
Thanks.
Fuel Mass Percentage, i.e. you need the extra dry weight of the second engine, TVC and associated plumbing, APU, etc. Thus, the percentage of fuel for the total mass of the stage worsen significantly. And in a second stage, if gravity losses are not critical, an extra kilogram of upper stage weight means one less kilogram of payload.

Offline Okie_Steve

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Re: SpaceX: Merlin 1D thread
« Reply #696 on: 06/28/2013 08:48 pm »
There was some discussion on another thread about the difference between running engines dry and shutting down with some small abount of propellant in the lines and pumps, which got me to wondering.

What is the likely failure mode if you run a kero-lox turbopump engine like the Merlin-1D dry? I've always assumed it would be the turbos going overspeed and self destructing, but I don't really know. Maybe it's something else? And, is RUD pretty much guaranteed, likely or just possible in this scenerio?

Offline Jcc

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Re: SpaceX: Merlin 1D thread
« Reply #697 on: 06/28/2013 11:24 pm »
There was some discussion on another thread about the difference between running engines dry and shutting down with some small abount of propellant in the lines and pumps, which got me to wondering.

What is the likely failure mode if you run a kero-lox turbopump engine like the Merlin-1D dry? I've always assumed it would be the turbos going overspeed and self destructing, but I don't really know. Maybe it's something else? And, is RUD pretty much guaranteed, likely or just possible in this scenerio?

For one, RP-1 is used to cool the nozzle, though I don't know if there would be enough time for it to overheat. For two, it also is used for hydraulic fluid to gimble the engine. Again, maybe not critical if it is about to conk out anyway. If the tank loses pressure maybe it can deform.

As far as the turbo pump, that won't be driven without fuel, the question is would there be a reservoir of fuel for the turbo pump when the stream it is pumping runs dry.

Also with a multi-engine system, you can have unpredictable forces if some engines cut when others continue for a while.
« Last Edit: 06/28/2013 11:50 pm by Jcc »

Offline simonbp

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Re: SpaceX: Merlin 1D thread
« Reply #698 on: 06/30/2013 08:30 pm »
Yeah, I doubt you could even run them actually dry; the control system is too smart for that. Rather, you'd run them until the control system determines that the stage has hit a predetermined low fuel safety point and does controlled shutdown of the engines.

Offline Okie_Steve

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Re: SpaceX: Merlin 1D thread
« Reply #699 on: 06/30/2013 10:33 pm »
That was the point of the original question about running dry. What *BAD* thing will happen and how probable is it? I had not considered the multi-engine case and can see the problem with differential shutdown there. I was thinking more of single engine upper stage or maybe center stage on a returning first stage if the F9R works out. There is apparently not much propellent left at auto shutdown. What sort of small gain might there be to running dry to maybe prevent loss of vehicle/mission at the risk of possible loss of vehicle/mission if it comes apart. Clearly it's not done, probably for a very good reason, just wondering what/why. Specifically for kero-lox engines like the Merlin-1d. Hydrogen-oxygen or methane-oxygen might very well be different.

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