Author Topic: ITS Development Updates and Discussion Thread  (Read 254813 times)

Offline GORDAP

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Re: ITS Development Updates and Discussion Thread
« Reply #1020 on: 08/01/2017 02:37 PM »
Do Block 4 and 5 leave the merlin unchanged?


If the iteration period is shortened (with higher flight rates), can a smaller vehicle be fielded and matured faster?


I get all the stuff about isp, coking, density, but I'm wondering if programmatically there is an opportunity cost of taking bigger slower steps. And, also added risks. 


What about lost revenue derived from more capable in-space hardware (like constellations, space stations, ..) that would be enabled by an earlier, but less dramatic, reduction in cost/lb to orbit that intermediate steps might offer.

Yes, there might be a rationale for taking a 'halfway' step to increased performance while not impacting operations and infrastructure very much, but the only one that makes sense is the opposite of what you've proposed.  That is, a new Raptor based ITS-like upper stage atop a Falcon Heavy.   This at least puts the ISP/Density fuel tradeoff in the right stages.  But I no longer think this is anywhere in their plans.  I expect an 8+ meter 'ITSy' full bore.  A Merlin based upper stage atop an ITSy booster is a non-starter, for all the reasons above.

Offline spacenut

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Re: ITS Development Updates and Discussion Thread
« Reply #1021 on: 08/01/2017 03:09 PM »
Why 8m if the booster is going to be 9m?  9m would be the core barrel for ITSy.  Then add return shielding, legs, etc to the core.  Same width barrels, same tooling. 

Offline speedevil

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Re: ITS Development Updates and Discussion Thread
« Reply #1022 on: 08/01/2017 03:28 PM »
The optimum no. of engines on a booster is 7-9. More than that increases the risk of a catastrophic engine failure destroying the LV.

If you can mitigate failures such that the worst possible failure is contained to one engine, and design your craft appropriately, it is in principle possible to run your engines at a thrust where they risk failure in use, but complete the mission entirely nominally.

More engines make this easier.
This does depend on well understood engines, but can open up some interesting trades in the maintenance/reliability design space.

Offline GORDAP

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Re: ITS Development Updates and Discussion Thread
« Reply #1023 on: 08/01/2017 03:32 PM »
I say "8+" for both booster and upper stage (I think they'll be the same)  as I don't think Elon committed to exactly 9, just said the present factory could accommodate up to 9.  It would be unsurprising to me if the ITSy design settled on, say, 8.4 or 8.7 meters for other logistical reasons we don't currently have insight into. 

Online wannamoonbase

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Re: ITS Development Updates and Discussion Thread
« Reply #1024 on: 08/01/2017 03:39 PM »
...If the iteration period is shortened (with higher flight rates), can a smaller vehicle be fielded and matured faster?


I don't think SpaceX will need to iterate as much as they have in the past.  They have way more experience and existing technologies (i.e. Vertical landing and reuse tech on the Block 5).

A new Raptor powered vehicle will have a new air-frame, which is an understood science and the Raptor engine which they have time to develop and test extensively.  Since they have all their Merlin experience and are starting with sub-cooled propellants the first Raptor will be much closer to the final version than Merlin.

It would be fun to see them work on US recovery with the F9/FH family.  But resource wise it's likely more efficient to move onto the next vehicles.

Get the Block 5's flying and ramp up second stage production.  Fly the manifest, book revenue and gain re-use experience. 
SpaceX, just a few things planned for 2018: FH, Starlink Prototypes, Block 5, Dragon 2, Increased launch rate.

Offline gin455res

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Re: ITS Development Updates and Discussion Thread
« Reply #1025 on: 08/01/2017 03:42 PM »
Do Block 4 and 5 leave the merlin unchanged?


If the iteration period is shortened (with higher flight rates), can a smaller vehicle be fielded and matured faster?


I get all the stuff about isp, coking, density, but I'm wondering if programmatically there is an opportunity cost of taking bigger slower steps. And, also added risks. 


What about lost revenue derived from more capable in-space hardware (like constellations, space stations, ..) that would be enabled by an earlier, but less dramatic, reduction in cost/lb to orbit that intermediate steps might offer.

Yes, there might be a rationale for taking a 'halfway' step to increased performance while not impacting operations and infrastructure very much, but the only one that makes sense is the opposite of what you've proposed.  That is, a new Raptor based ITS-like upper stage atop a Falcon Heavy.   This at least puts the ISP/Density fuel tradeoff in the right stages.  But I no longer think this is anywhere in their plans.  I expect an 8+ meter 'ITSy' full bore.  A Merlin based upper stage atop an ITSy booster is a non-starter, for all the reasons above.



Falcon Heavy hasn't flown yet and is much more complicated.

How would this upper-stage land with only one vacuum optimised engine?


If system performance is a function of payload AND no. of reuses; might it be better to make the booster more reusable with Raptors?  It is 70% of the cost of an F9. A single stick Raptor-7 or Raptor-9 booster that requires reduced maintenance and can be reused 100 times could be a money saver.



And if constellations are the primary payload, does their lower orbit counter-act some of the advantage that is gained from methane's improved vacuum isp?


Sea-Level -> Vacuum ISP (from Wikipedia)
Raptor: 334 -> 361
Merlin:  282 -> 311
18%-> 16% difference


Vacuum
Raptor: 382
Merlin: 348
10% difference - improved mass fraction for merlin

Offline rakaydos

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Re: ITS Development Updates and Discussion Thread
« Reply #1026 on: 08/01/2017 04:15 PM »
Do Block 4 and 5 leave the merlin unchanged?


If the iteration period is shortened (with higher flight rates), can a smaller vehicle be fielded and matured faster?


I get all the stuff about isp, coking, density, but I'm wondering if programmatically there is an opportunity cost of taking bigger slower steps. And, also added risks. 


What about lost revenue derived from more capable in-space hardware (like constellations, space stations, ..) that would be enabled by an earlier, but less dramatic, reduction in cost/lb to orbit that intermediate steps might offer.

Yes, there might be a rationale for taking a 'halfway' step to increased performance while not impacting operations and infrastructure very much, but the only one that makes sense is the opposite of what you've proposed.  That is, a new Raptor based ITS-like upper stage atop a Falcon Heavy.   This at least puts the ISP/Density fuel tradeoff in the right stages.  But I no longer think this is anywhere in their plans.  I expect an 8+ meter 'ITSy' full bore.  A Merlin based upper stage atop an ITSy booster is a non-starter, for all the reasons above.



Falcon Heavy hasn't flown yet and is much more complicated.

How would this upper-stage land with only one vacuum optimised engine?
Falcon heavy is complicated because it's trying to be a "Lego Rocket"- Saving costs by reusing an already functional component in a different context. It's complicated because they cant solve problems with falcon heavy by making changes to the heavy boosters, because those heavy boosters must remain Falcon 9 cores. So there's a lot of issues in the 3 core problem that they cannot fix "correctly", and have to cobble together a solution to a problem with the booster by modifying the core.

As for the upper stage, you need to cut into your payload capacity, and glue a modified Dragon 2 to it's nose. Heat shield, landing legs, propulsive landing, hypergolic fuel for GTO orbit stability, independant long-duration electronics, all of which move the CG foreward so it's stable nose first, AND all of which is an optional, though common, module than can be left off an any mission that needs disposable performance.

Offline spacenut

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Re: ITS Development Updates and Discussion Thread
« Reply #1027 on: 08/01/2017 04:37 PM »
The problems with FH gives a reusable metholox upper stage more reason to develop as a way forward to ITSy. 

landing a second stage could be just the addition of the Super Dracos, enough hypergolic fuel, and landing legs.  The increased capability of a the sub-scale Raptor might keep payloads the same, and with an expendable upper stage based Raptor and expendable first stage, may make FH unnecessary.  Someone figured about 28 tons to LEO with this.  GEO would be reachable with a 8+ ton satellite in expendable mode.   Again no need for FH. 

They are going to have to put liquid methane facility in place before the ITS or ITSY can fly anyway.  This could be used to load fuel for a methane upper stage designed like a mini-ITSy.  At least to test components before the full blown ITS or the 9m ITSy is built.   

Offline envy887

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Re: ITS Development Updates and Discussion Thread
« Reply #1028 on: 08/01/2017 05:26 PM »
Do Block 4 and 5 leave the merlin unchanged?


If the iteration period is shortened (with higher flight rates), can a smaller vehicle be fielded and matured faster?


I get all the stuff about isp, coking, density, but I'm wondering if programmatically there is an opportunity cost of taking bigger slower steps. And, also added risks. 


What about lost revenue derived from more capable in-space hardware (like constellations, space stations, ..) that would be enabled by an earlier, but less dramatic, reduction in cost/lb to orbit that intermediate steps might offer.

Yes, there might be a rationale for taking a 'halfway' step to increased performance while not impacting operations and infrastructure very much, but the only one that makes sense is the opposite of what you've proposed.  That is, a new Raptor based ITS-like upper stage atop a Falcon Heavy.   This at least puts the ISP/Density fuel tradeoff in the right stages.  But I no longer think this is anywhere in their plans.  I expect an 8+ meter 'ITSy' full bore.  A Merlin based upper stage atop an ITSy booster is a non-starter, for all the reasons above.



Falcon Heavy hasn't flown yet and is much more complicated.

How would this upper-stage land with only one vacuum optimised engine?


If system performance is a function of payload AND no. of reuses; might it be better to make the booster more reusable with Raptors?  It is 70% of the cost of an F9. A single stick Raptor-7 or Raptor-9 booster that requires reduced maintenance and can be reused 100 times could be a money saver.



And if constellations are the primary payload, does their lower orbit counter-act some of the advantage that is gained from methane's improved vacuum isp?


Sea-Level -> Vacuum ISP (from Wikipedia)
Raptor: 334 -> 361
Merlin:  282 -> 311
18%-> 16% difference


Vacuum
Raptor: 382
Merlin: 348
10% difference - improved mass fraction for merlin

Most of that difference in ISP is due to chamber pressure and larger expansion ratio, not fuel type. Higher chamber pressure engines are more efficient at sea level because of the greater difference between nozzle exit pressure and atmospheric pressure.

In vacuum, the chamber pressure becomes more or less irrelevant and only expansion ratio and fuel type matter.

A single-engine upper stage will most likely land like the fairings (either midair catch or into a floating airbag). But ITS upper stages will land propulsively; the 9m version can land on 1 Raptor, as it is nearly a 1/3 mass and thrust scale model of the 12m version. Or it can land on multiple smaller (~1,000 kN) Raptors.

Offline Lar

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Re: ITS Development Updates and Discussion Thread
« Reply #1029 on: 08/01/2017 06:56 PM »
Raptor upper stage has its own thread and is off-topic for this one. Please consider that thanks.
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Offline TomH

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Re: ITS Development Updates and Discussion Thread
« Reply #1030 on: 08/01/2017 08:22 PM »
Raptor upper stage has its own thread and is off-topic for this one. Please consider that thanks.

Not only is it off topic, we know it is not going to happen. Thus it is totally moot and needs to become mute as well.

Offline TomH

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Re: ITS Development Updates and Discussion Thread
« Reply #1031 on: 08/01/2017 08:37 PM »
Do Block 4 and 5 leave the merlin unchanged?

This is a different question from the one above where you stated:

Has the merlin reached its final version yet? No.

This is a matter of semantics. Just because Block 5 has not flown does not mean the Merlin has not reached its final iteration. It has. All the engineering is done. The rocket just has not flown yet.

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