Author Topic: Raptor Upper Stage consolidated thread  (Read 124102 times)

Offline Kaputnik

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Re: Raptor Upper Stage consolidated thread
« Reply #140 on: 05/10/2017 04:51 pm »
Is there a GG kerolox engine that outperforms Merlin? I can't name one.
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Offline hkultala

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Re: Raptor Upper Stage consolidated thread
« Reply #141 on: 05/10/2017 05:20 pm »
RS-68 and RS-25 don't count as they are LH2/LOX engines.

If we are talking about all liquid engines, then they do count. LH2/LOX are liquids.

That spacex chose to use low-isp fuel does not make the isp of their engines any better, but it does make impulse DENSITY better, and if we were talking about impulse density, then in that Merlin might beat those LH2/LOX engines.

Quote
The RD-180 has similar ISP (SL,VAC) to Merlin 1D

Wrong. It has over 25, almost 30 seconds better isp.
« Last Edit: 05/10/2017 05:24 pm by hkultala »

Offline hkultala

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Re: Raptor Upper Stage consolidated thread
« Reply #142 on: 05/10/2017 05:26 pm »
Is there a GG kerolox engine that outperforms Merlin? I can't name one.

doesn't matter.

Is there a biplane faster than polikarpov I-15? I can't name one.

Still there are lots of aeroplanes much faster than polikarpov I-15, and polikarpov I-15 could not have been considered a fast aeroplane for the last 75 years.

« Last Edit: 05/10/2017 05:27 pm by hkultala »

Offline Lars-J

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Re: Raptor Upper Stage consolidated thread
« Reply #143 on: 05/10/2017 06:02 pm »
The RD-180 has similar ISP (SL,VAC) to Merlin 1D

Wrong. It has over 25, almost 30 seconds better isp.

If any engine has proved the folly of ISP measuring, it would have to be Merlin.

Offline gospacex

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Re: Raptor Upper Stage consolidated thread
« Reply #144 on: 05/10/2017 06:08 pm »
It's remarkable that Elon thinks engines are their weak point- the Merlin has outstanding T:W, impressive isp, and they churn out more of them than any other engine builder.
It's the structures that have caused their LOVs so far- the engines have had a good track record.

isp of merlin is the worst of all liquid fueled rocket engines used in any orbital-capable rocket in USA today.

RD-170-derivates used by Atlas and Antares, RS-68 used by Delta, RS-25 that will be used by SLS all have considerably better isp.

True.
That's why Sea Launch has such a huge backlog. Their engine is so much better! :P

Offline DanielW

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Re: Raptor Upper Stage consolidated thread
« Reply #145 on: 05/10/2017 06:33 pm »
The RD-180 has similar ISP (SL,VAC) to Merlin 1D

Wrong. It has over 25, almost 30 seconds better isp.

If any engine has proved the folly of ISP measuring, it would have to be Merlin.

True-ish, Merlin and falcon 9 have amply demonstrated that there are big wins to be had in mass fraction. But the returns are logarithmic. Eventually you do need to start paying attention to ISP again.

Offline Kaputnik

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Re: Raptor Upper Stage consolidated thread
« Reply #146 on: 05/10/2017 08:24 pm »
Is there a GG kerolox engine that outperforms Merlin? I can't name one.

doesn't matter.

Is there a biplane faster than polikarpov I-15? I can't name one.

Still there are lots of aeroplanes much faster than polikarpov I-15, and polikarpov I-15 could not have been considered a fast aeroplane for the last 75 years.



The point of this whole diversion was that I was surprised that Elon considers SpaceX to be weak when it comes to engines.
The only type of engine they make is a GG, so that is all we can judge them by. Does it stand judgement against its peers? Hell yeah, it's perhaps the best example of a GG engine that we can find. To me that suggests that they are pretty good at building engines.
If their GG engine was inferior to other engines using the same prop and cycle, then I would agree that they suck at making engines.
Perhaps what Elon is alluding to is that they are weak when it comes to engines running on different cycles or propellants. Hardly surprising given that they have yet to finish developing one.
"I don't care what anything was DESIGNED to do, I care about what it CAN do"- Gene Kranz

Offline JamesH65

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Re: Raptor Upper Stage consolidated thread
« Reply #147 on: 05/10/2017 09:02 pm »
It's remarkable that Elon thinks engines are their weak point- the Merlin has outstanding T:W, impressive isp, and they churn out more of them than any other engine builder.
It's the structures that have caused their LOVs so far- the engines have had a good track record.

isp of merlin is the worst of all liquid fueled rocket engines used in any orbital-capable rocket in USA today.

RD-170-derivates used by Atlas and Antares, RS-68 used by Delta, RS-25 that will be used by SLS all have considerably better isp.

And yet, here it is, powering the F9 to orbit, launching payloads cheaper than anyone else, returning the booster and landing it, unlike anyone else.

It may be the worst liquid fuel rocket - but just look at what it achieves. More than any of the other engines quoted are doing.

The engine is a triumph of pragmatism.


Offline TrueBlueWitt

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Re: Raptor Upper Stage consolidated thread
« Reply #148 on: 05/10/2017 11:03 pm »
It's remarkable that Elon thinks engines are their weak point- the Merlin has outstanding T:W, impressive isp, and they churn out more of them than any other engine builder.
It's the structures that have caused their LOVs so far- the engines have had a good track record.

isp of merlin is the worst of all liquid fueled rocket engines used in any orbital-capable rocket in USA today.

RD-170-derivates used by Atlas and Antares, RS-68 used by Delta, RS-25 that will be used by SLS all have considerably better isp.

And yet, here it is, powering the F9 to orbit, launching payloads cheaper than anyone else, returning the booster and landing it, unlike anyone else.

It may be the worst liquid fuel rocket - but just look at what it achieves. More than any of the other engines quoted are doing.

The engine is a triumph of pragmatism.



Merlin may have the "Lowest" ISP.. But that's just one part of the equation..
It's far from a slouch..
For it's Fuel and engine cycle I believe it has the highest ISP. 
Also the best T/W of any liquid Fueled engine in the US, or being used anywhere!
T/W is almost as big a deal as ISP.. Which is part of the reason they also have super high stage pmfs.. 
Most likely the lowest $/performance of any engine..
It's about optimizing the whole system, not just one piece.
ISP will only help so much if you have terrible T/W and pfm for the stage. 
« Last Edit: 05/10/2017 11:03 pm by TrueBlueWitt »

Offline TomH

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Re: Raptor Upper Stage consolidated thread
« Reply #149 on: 05/10/2017 11:37 pm »
...the lowest $/performance of any engine..

THIS.......matters most.

Online rakaydos

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Re: Raptor Upper Stage consolidated thread
« Reply #150 on: 05/11/2017 03:06 am »
Is there a GG kerolox engine that outperforms Merlin? I can't name one.

doesn't matter.

Is there a biplane faster than polikarpov I-15? I can't name one.

Still there are lots of aeroplanes much faster than polikarpov I-15, and polikarpov I-15 could not have been considered a fast aeroplane for the last 75 years.



The point of this whole diversion was that I was surprised that Elon considers SpaceX to be weak when it comes to engines.
The only type of engine they make is a GG, so that is all we can judge them by. Does it stand judgement against its peers? Hell yeah, it's perhaps the best example of a GG engine that we can find. To me that suggests that they are pretty good at building engines.
If their GG engine was inferior to other engines using the same prop and cycle, then I would agree that they suck at making engines.
Perhaps what Elon is alluding to is that they are weak when it comes to engines running on different cycles or propellants. Hardly surprising given that they have yet to finish developing one.
its not that the engine design sucked. Its that, in 2014, they couldnt build them well enough to get full performance from them. Then in 2015, they improved to the point elon called "full thrust".
Block 5 is another step past that.

Offline hkultala

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Re: Raptor Upper Stage consolidated thread
« Reply #151 on: 05/11/2017 05:21 am »
It's remarkable that Elon thinks engines are their weak point- the Merlin has outstanding T:W, impressive isp, and they churn out more of them than any other engine builder.
It's the structures that have caused their LOVs so far- the engines have had a good track record.

isp of merlin is the worst of all liquid fueled rocket engines used in any orbital-capable rocket in USA today.

RD-170-derivates used by Atlas and Antares, RS-68 used by Delta, RS-25 that will be used by SLS all have considerably better isp.

And yet, here it is, powering the F9 to orbit, launching payloads cheaper than anyone else, returning the booster and landing it, unlike anyone else.

It may be the worst liquid fuel rocket - but just look at what it achieves. More than any of the other engines quoted are doing.

The engine is a triumph of pragmatism.

I have not claimed it as worst liquid fuel rocket.

I've just said it has the worst isp, and so it has ONE clear weakness.

For practically all the OTHER metrics, it's great engine, and falcon 9 is a great rocket,
and overall I consider merlin 1d to be much better engine than for example RS-68. Because impulse density, price/thrust, thrust/weight are all MUCH better with merlin 1d AND the smaller size allows better flexibility for EELV-size vehicle (for upper stage use, landing and engine-out capability)





« Last Edit: 05/11/2017 05:22 am by hkultala »

Online rsdavis9

Re: Raptor Upper Stage consolidated thread
« Reply #152 on: 05/11/2017 10:41 am »
So how is impulse density defined?
Weight of fluids and engine/weight of engine?
Thrust/weight of engine?

EDIT:
Sorry answered my own question:

Quote
Higher density fuels have a higher Impulse Density because Impulse density is basically the propellants Specific Impulse multiplied by it's density.
« Last Edit: 05/11/2017 03:44 pm by rsdavis9 »
With ELV best efficiency was the paradigm. The new paradigm is reusable, good enough, and commonality of design.
Same engines. Design once. Same vehicle. Design once. Reusable. Build once.

Offline DanielW

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Re: Raptor Upper Stage consolidated thread
« Reply #153 on: 05/11/2017 01:59 pm »
So how is impulse density defined?
Weight of fluids and engine/weight of engine?
Thrust/weight of engine?

Force per flowrate? which give Newton Seconds per Kilogram? Not sure that is any different than ISP.

Though I suppose if you want a definition that Merlin wins at maybe Force per volumetric flow rate.
« Last Edit: 05/11/2017 02:02 pm by DanielW »

Offline JamesH65

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Re: Raptor Upper Stage consolidated thread
« Reply #154 on: 05/11/2017 03:36 pm »
So how is impulse density defined?
Weight of fluids and engine/weight of engine?
Thrust/weight of engine?

Force per flowrate? which give Newton Seconds per Kilogram? Not sure that is any different than ISP.

Though I suppose if you want a definition that Merlin wins at maybe Force per volumetric flow rate.

There really doesn't need to be a definition at what the Merlin 'wins' at - just look at the accomplishments of the whole system. Those are wins all the way down.

Offline DanielW

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Re: Raptor Upper Stage consolidated thread
« Reply #155 on: 05/11/2017 04:00 pm »
So how is impulse density defined?
Weight of fluids and engine/weight of engine?
Thrust/weight of engine?

Force per flowrate? which give Newton Seconds per Kilogram? Not sure that is any different than ISP.

Though I suppose if you want a definition that Merlin wins at maybe Force per volumetric flow rate.

There really doesn't need to be a definition at what the Merlin 'wins' at - just look at the accomplishments of the whole system. Those are wins all the way down.

I fully agree, I was just responding to a specific question and disregarding the usefulness of such definitions. Merlin is excellent in that it does many things "good enough" within the design requirements of the falcon9 reusable launch system.

Offline Roy_H

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Re: Raptor Upper Stage consolidated thread
« Reply #156 on: 05/11/2017 06:04 pm »
isp of merlin is the worst of all liquid fueled rocket engines used in any orbital-capable rocket in USA today.

RD-170-derivates used by Atlas and Antares, RS-68 used by Delta, RS-25 that will be used by SLS all have considerably better isp.
I have not seen an ISP figure from SpaceX since the first version of the Merlin 1D. Since then we have seen a 26% increase in thrust. Now I am not a rocket scientist, but as I understand, increasing thrust requires either larger throat or higher ISP. I have no reason to believe the throat or other physical dimensions of the Merlin 1D have changed so I believe the ISP has gone up a lot.
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Online rsdavis9

Re: Raptor Upper Stage consolidated thread
« Reply #157 on: 05/11/2017 07:35 pm »
I have not seen an ISP figure from SpaceX since the first version of the Merlin 1D. Since then we have seen a 26% increase in thrust. Now I am not a rocket scientist, but as I understand, increasing thrust requires either larger throat or higher ISP. I have no reason to believe the throat or other physical dimensions of the Merlin 1D have changed so I believe the ISP has gone up a lot.

probably just higher chamber pressure which translates to higher exhaust velocity which is ISP.
With ELV best efficiency was the paradigm. The new paradigm is reusable, good enough, and commonality of design.
Same engines. Design once. Same vehicle. Design once. Reusable. Build once.

Offline acsawdey

Re: Raptor Upper Stage consolidated thread
« Reply #158 on: 05/11/2017 07:55 pm »
I have not seen an ISP figure from SpaceX since the first version of the Merlin 1D. Since then we have seen a 26% increase in thrust. Now I am not a rocket scientist, but as I understand, increasing thrust requires either larger throat or higher ISP. I have no reason to believe the throat or other physical dimensions of the Merlin 1D have changed so I believe the ISP has gone up a lot.

probably just higher chamber pressure which translates to higher exhaust velocity which is ISP.

Increasing chamber pressure will improve ISP a little but the trouble is with a GG cycle you spend more propellant in the gas generator to provide the increased power to the pumps which does not contribute to thrust. The big gains would be if they were able to improve pump efficiency and GG turbine efficiency. Has anybody tried to back-calculate ISP from those simulations of F9 flights?

Offline Roy_H

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Re: Raptor Upper Stage consolidated thread
« Reply #159 on: 05/11/2017 08:25 pm »
I have not seen an ISP figure from SpaceX since the first version of the Merlin 1D. Since then we have seen a 26% increase in thrust. Now I am not a rocket scientist, but as I understand, increasing thrust requires either larger throat or higher ISP. I have no reason to believe the throat or other physical dimensions of the Merlin 1D have changed so I believe the ISP has gone up a lot.

probably just higher chamber pressure which translates to higher exhaust velocity which is ISP.

Increasing chamber pressure will improve ISP a little but the trouble is with a GG cycle you spend more propellant in the gas generator to provide the increased power to the pumps which does not contribute to thrust. The big gains would be if they were able to improve pump efficiency and GG turbine efficiency. Has anybody tried to back-calculate ISP from those simulations of F9 flights?
Ok, I buy the concept that the GG has to provide more pressure and will consume energy to do so. Does this mean the GG runs hotter? We are still told that there is a 26% increase in thrust, if most of this is not due increased chamber pressure, then is there significant increased thrust from the GG exhaust?
"If we don't achieve re-usability, I will consider SpaceX to be a failure." - Elon Musk
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