New methane SC engine.

[douglas100]

Let's remember, that once they develop a RLV, they could use it to transport itself to the launch pad. Once they are there, you can go as big as you can see a business case for.

Transport itself to the launch pad? From where? Hawthorne? Are you seriously suggesting they launch their reusable first stage (when and if it is developed) from Los Angeles to fly over land to Cape Cananveral?

[strangequark]

Transport itself to the launch pad? From where? Hawthorne? Are you seriously suggesting they launch their reusable first stage (when and if it is developed) from Los Angeles to fly over land to Cape Cananveral?

It'd be a hell of a sight! They'd probably never allow it though, what with the impact to the Delta Smelt.

[baldusi]

Let's remember, that once they develop a RLV, they could use it to transport itself to the launch pad. Once they are there, you can go as big as you can see a business case for.

Transport itself to the launch pad? From where? Hawthorne? Are you seriously suggesting they launch their reusable first stage (when and if it is developed) from Los Angeles to fly over land to Cape Cananveral?

I seriously doubt Hawthorne can even manufacture anything over 5m. I was actually thinking of barge to VAFB. But, if you can go to a Barge, it would actually be cheaper. A transatlantic 40 container can be as little as 4k USD. So a barge should be around 20k, if shared, or 200k if leased. There would be one way to lower the costs of oversized parts, and that would be so ship lots of parts in one single truck caravan. Lot's of permits, fess, and studies, but only twice a year. Of course you'd need to ship something like five to ten cores at a time. I still think that it is 3.6m by truck or anything bigger by barge.
And I still think that they will try a RLV Falcon 9 before thinking of anything bigger. Except, for the unlikely case they actually win the SLS booster competition.

[douglas100]

OK, back to reality. I don't want to be involved in taking this thread any further OT so I'll say no more.

[ArbitraryConstant]

Here's a question I kinda assumed but never really knew the answer to:

Do engine development costs increase in proportion to the size of the engine?

Intuitively I can see why they might, since you'd need things like bigger tooling to build the engine, bigger facilities and more fuel to do even minimal tests on a single prototype, etc. SpaceX talked about needing a billion to develop Merlin 2, when that's more than they needed to take Falcon 9 all the way to orbit on smaller engines.

If they want this SC engine to work on the first stage my suspicion is that they'll want it to be bigger to allow bigger vehicles in potentially reusable, single stick configurations with manageable numbers of engines, but AIUI there would still be fairly hard limits on how big they can afford to go.

[john smith 19]

Here's a question I kinda assumed but never really knew the answer to:

Do engine development costs increase in proportion to the size of the engine?

Yes. But not as much as you might think and the dependencies might also surprise you.

Firstly all mfg and test plant has capacity limits. In principal *up* to those limits you can make something as big as you like. All suppliers have *preferred* ranges they like to supply their materials in and if you've been materials *below* this size your overall bill might go down.

Above those machinery limits it's no longer a case of ordering bigger raw material (assuming its available) you need a whole new *factory* to work it. Note the ET limits for example.

NASA's book on Combustion Instability (SP194, available as a PDF) states that actually cost is proportional to chamber pressure *cubed*. Note that NASA normally tests engines and components by feeding them from pressurized tanks.

Historically increasing the size of parts has not been that big a factor in increasing engine prices. However bigger chamber -> (potentially) more combustion instability -> more testing. Likewise more complex design -> more parts -> more paperwork

On this basis it's not too surprising the SSME cost quite a lot (High pressure, very complex design). The Russian multi chamber designs allow they to create a range (4 chamber RD 170 2chamber RD180 etc) at the expense of lower T/W as the number of chambers goes down and the turbine capacity becomes excessive.

The AIAA papers on the RS68 indicate one of their concerns was to reduce the number of test/analyze/fix cycles in the development work. They budgeted for about 6 but (IIRC) came up with around 26-30, which is what it has been historically.

Note this (hypothetical) new engine has issue of complexity(new fuel) x complexity(new cycle) x complexity (new size) relative to Spacex's existing knowledge base.

Test facilities might be the biggest factor in this. If the test facilities in Texas are built to handle the loads Spacex might do a surprising amount of development. If not then some fairly major construction work has to happen before they can start the process.

[MikeAtkinson]

Another major factor in cost is the experience in the team doing the development. SpaceX are fortunate in having an experienced team with several engine developments behind them. It is also likely that team members have experience of SC engines.

My guess is that with an experienced team development cost will increase a more than linearly with size but less than the square of size.

With an inexperienced team development cost will increase with the square of size (and perhaps more), because mistakes are so costly at large sizes.

[simonbp]

It is also likely that team members have experience of SC engines.

Yes, especially large, cryogenic, reusable SC engines with a long flight history.

SpaceX has developed three engines almost from scratch (Merlin, Kestrel, and Draco); they know how run an engineering project. The new SC engine is just not a priority at the moment, and won't be until both a manned Dragon and Falcon Heavy have flown.

[RedLineTrain]

I imagine that having assembled a top-notch engine development team, Musk would not want it to go idle.  After the Merlin 1D and the Super Dracos, what engine developments do they have left?

[go4mars]

I imagine that having assembled a top-notch engine development team, Musk would not want it to go idle.  After the Merlin 1D and the Super Dracos, what engine developments do they have left?

That is what this thread is about. 

Elon mentioned highly efficient staged combustion and "mostly methane".  The airforce asked everyone for a specified engine and SpaceX asked, 'can it be methane?'.  We're speculating on very little info here, wondering if those facts are all related, and what the plan might be, and potential implications.

Questions like these have arisen: 

Is it methane for both stages?  Will there be commonality? 
Does cross-feed potential factor in (like a 4 engine 1st stage for full-cross-feed or a 5 engine for almost full cross-feed plus landing engine)? 

1) Is methane the choice because of future Mars ISRU? 
2) Is methane the choice because of reusability implications like these:

2a) Amenability to re-entry or re-ignition purposes: Perhaps a component of the ullage after separation can be flashed into compressed methane gas and oxygen gas within their respective systems by a light-weight, carefully controlled, exothermic reaction within the tanks to provide structural rigidity for re-entry (vapour pressure/fugacity), and force the ubiquitous presence of prop reagents through the plumbing as a gas for low initial thrust levels until the liquid component is forced back to the plumbing by acceleration of the stage (changing from gas propellants back to "the usual liquid burn") in a sputtering transient? 
2b) low/no coking
2c) relative low cost and abundance of LNG? 
2d) Improved performance? 

3) Is methane the choice because of concerns about cumulative emmissions at very high future flight rates? 


And other themes:

Is Merlin 2 pushed back forever?  Or is this an intermediate step with better near-term commercial potential and reduced risk?   

Is a crygenic raptor stage pushed back forever?  Or is this an intermediate step with better near-term commercial potential and reduced risk?

If a new engine were announced tomorrow, when might it first see commercial use? 

How might the new SpaceX engine stack up against previous development programs from decades ago? 

Will the bells be retractable or extendible for protection during re-entry and efficiency maximization as the flight progresses?

What will the song be on the announcement video?  Something by the band "30 seconds to Mars" perhaps?

[simonbp]

I imagine that having assembled a top-notch engine development team, Musk would not want it to go idle.  After the Merlin 1D and the Super Dracos, what engine developments do they have left?

Cross-feeding the Falcon Heavy. It's a non-trival problem, which is why it's never been done before (unless you count the original Atlas, which had a torturous and expensive development).

[kevin-rf]

Cross-feeding the Falcon Heavy. It's a non-trival problem, which is why it's never been done before (unless you count the original Atlas, which had a torturous and expensive development).

Saturn I, Though the Atlas is really a better example. Of course the then Soviet solution to this problem ( R-7 ) was to use smaller tanks on the strap ons.

[dunderwood]

Saturn 1 never dropped any tanks, so it was a much simpler task.  It was just manifolding tanks together, not trying to merge multiple boosters.

[go4mars]

2a) Amenability to re-entry or re-ignition: ... Perhaps a component of the ullage after separation can be flashed into compressed methane gas and oxygen gas within their respective systems by a light-weight, carefully controlled, exothermic reaction within the tanks to provide structural rigidity for re-entry (vapour pressure/fugacity), and force the ubiquitous presence of prop reagents through the plumbing as a gas for low initial thrust levels until the liquid component is forced back to the plumbing by acceleration of the stage (changing from gas propellants back to "the usual liquid burn") in a sputtering transient? 

 
It seems to me that this would work better with a staged combustion engine vs GG.  Is that right? 

[sdsds]

Is it methane for both stages?

It's easy to speculate that SpaceX would like it to be.  First stage methane because "2b) low/no coking" supports more rapid turn around of a reusable first stage.  Upper stage methane because easy LEO storability supports depot (or at least two-launch) mission architectures.  And yes, very long term, Mars ISRU.

Is a cryogenic raptor stage pushed back forever?  Or is this an intermediate step with better near-term commercial potential and reduced risk?

You mean hydrolox, right?  Speculating further, it looks like SpaceX can bring its vision into reality without incurring the complications of LH2, and could easily start with the upper stage engine because: (a) it is smaller and could be funded internally, and (b) they won't be able to attract a "buyer" (USAF or NASA) for a methane first stage until they have a proven methane engine design in their stable.  Both those buyers are risk averse and would penalize a proposal based on a technology they perceive to have a low readiness level.

If a new engine were announced tomorrow, when might it first see commercial use?

They'd initially suggest it would be available in two years.  And would continue to make that same claim, year after year!