SpaceX Falcon Heavy Discussion (Thread 6)

[Stan-1967]

The above postings were me thinking out loud what sort of stretch was within doable range...
SpaceX says FH (w/ normal S2) will put 63 metric tons to LEO (all expended), price $150m per EM... ok great...
And yes... I do agree that stretching S2 will improve that 63 number...

My questions are more...
...what sort of % stretch would suit all FH operations better?

I think that can only be answered if you know what gap SpaceX is trying to fill.  You indicated as much in your response.  I think it is doubtful they need to increase LEO capacity, as there is no indication of any payloads approaching the current limit.  However GTO, lunar & other high energy orbits could benefit from a stretched S2.

As to reducing engine count on the core, probably a bad idea.  It will limit GLOW & lead to heavy gravity losses at the beginning of the launch.  A heavier S2 will make this worse.  The stretch of S2 is enabled by the high T/W of the three cores lifting the entire stack.

[John Alan]

The above postings were me thinking out loud what sort of stretch was within doable range...
SpaceX says FH (w/ normal S2) will put 63 metric tons to LEO (all expended), price $150m per EM... ok great...
And yes... I do agree that stretching S2 will improve that 63 number...

My questions are more...
...what sort of % stretch would suit all FH operations better?

I think that can only be answered if you know what gap SpaceX is trying to fill.  You indicated as much in your response.  I think it is doubtful they need to increase LEO capacity, as there is no indication of any payloads approaching the current limit.  However GTO, lunar & other high energy orbits could benefit from a stretched S2.

As to reducing engine count on the core, probably a bad idea.  It will limit GLOW & lead to heavy gravity losses at the beginning of the launch.  A heavier S2 will make this worse.  The stretch of S2 is enabled by the high T/W of the three cores lifting the entire stack.

I look at it this way...
Without cross-feed... the only choice is throttle core down to save some prop for between BECO and MECO...
Some have opined, that on the demo flight... SpaceX averaged about 80% thrust setting between Launch and BECO...
There are pics around showing the center core fire trail to be noticeably shorter, even as it clears the tower...

With a F5 core...
GROSS possible lift off thrust is reduced by only 15% (23/27)... not much of a hit when you think about it.
In effect - your setting core throttle at 55% (of 9 engine max) and leaving it be all the way up...
Your running those 5 at likely the max ISP condition of full throttle...
The boosters get throttled for Max-Q and it's said have to also throttle as they deplete on G forces to payload later...
Well... this way there is more then 40% prop in the core stage at BECO... to push against instead of throttling...
Boosters deplete and stage a bit lower and slower...
Between BECO and MECO it burns thru that 40% prop when it's more downrange speed and not so much climbing and it's gravity implications...
Should be what? ~70 seconds of burn time to depletion of core...

Again... was just putting a possible configuration out there for certain missions...
Boosters to ASDS's or RTLS... core is expended... and that core was cheaper by the cost of 4 engines not used... 

[speedevil]

S2 would  never  be under-filled regardless of size.

Is this strictly true?
At some point, doesn't the low T/W of the second stage mean that with a very heavy payload+stack it will impact the atmosphere, whereas with a lighter fuel load, it might make it to LEO?
This long a stage would be fully filled only for light payloads.

Or, similarly if the fully fuelled stack caused load limits to be exceeded on the core.
I have not done the numbers to see if either of these are plausible limits.

[mme]

S2 would  never  be under-filled regardless of size.

Is this strictly true?
At some point, doesn't the low T/W of the second stage mean that with a very heavy payload+stack it will impact the atmosphere, whereas with a lighter fuel load, it might make it to LEO?
This long a stage would be fully filled only for light payloads.

Or, similarly if the fully fuelled stack caused load limits to be exceeded on the core.
I have not done the numbers to see if either of these are plausible limits.

Fuel's cheap and it allows for making up any underperformance of either stage.

[speedevil]

S2 would  never  be under-filled regardless of size.

Is this strictly true?
At some point, doesn't the low T/W of the second stage mean that with a very heavy payload+stack it will impact the atmosphere, whereas with a lighter fuel load, it might make it to LEO?
This long a stage would be fully filled only for light payloads.

Or, similarly if the fully fuelled stack caused load limits to be exceeded on the core.
I have not done the numbers to see if either of these are plausible limits.

Fuel's cheap and it allows for making up any underperformance of either stage.

Fuel being cheap doesn't help much if your stage has reentered due to lack of T/W.

[hkultala]

S2 would  never  be under-filled regardless of size.

Is this strictly true?
At some point, doesn't the low T/W of the second stage mean that with a very heavy payload+stack it will impact the atmosphere, whereas with a lighter fuel load, it might make it to LEO?
This long a stage would be fully filled only for light payloads.

Or, similarly if the fully fuelled stack caused load limits to be exceeded on the core.
I have not done the numbers to see if either of these are plausible limits.

Fuel's cheap and it allows for making up any underperformance of either stage.

Fuel being cheap doesn't help much if your stage has reentered due to lack of T/W.

First stage can easily give it enough vertical velocity that it won't re-enter.

[whitelancer64]

Adding fuel gives you diminishing payload returns - Tyrrany of the rocket equation.

Seems no one here is sure where that point of diminishing returns is for a S2 stretch.

[intrepidpursuit]

Moving this discussion on FH pricing here since it has nothing to do with the demo flight:

That means a centre core costs only $5M to make. $95M for 0.9*26.7 = 24.03 t is $3,953/kg, which is the cheapest option.

No Legs, no Iconel Heat shield.. No Grid fins..
All of which would not only save cost but also reduce mass on core, which is much bigger benefit than on the boosters.  Also, what's the cost of deep ocean drone ship recovery for what would be a very very toasty core? You have to wonder how many times they figured they could re-use the core to start with.. if only a few times.. then maybe the new numbers make sense.

This isn't the right take away.
From Elon's twitter post if a center core expendable is $95M which is slightly more than an expendable F9 then that means an expendable F9 is about the $90M. That makes perfect sense, since an expendable F9 is roughly the same capacity as the posted price for a recoverable FH. Same capacity - same price. That means throwing away a F9 adds about $30M to its price.

So if an expendable F9 is $90M how is it only $5M more to fly reusable side boosters on an expendable core? That doesn't make sense to current pricing for full recovery FH @ $90M. Then why does a fully expenable FH cost $150M.

Keep in mind posted prices to date have only been for recoverable Falcons on their first flight. The price point has stayed relatively static, with more and more being recoverable as performance increases. So its logical that a fully expendable F9 costs more now that it is more capable.

But with Block V enabling rapid reuse it should be expected that all Falcons are reused unless a customer pays more to insist on first flight OR expendable.
This quote from the recent NSF article provides insight as to what the reuse price will be:

...once Falcon Heavy flies in its fully reusable configuration – essentially lowering its price to just $62 million dollars or the price of a regular, brand new Falcon 9.

Ok $62M for a 3 core + fairing reused and recoverable payload. Now take that $30M adder to throw away a Falcon core and you end up pretty well at $95M for center core expendable. Throw the two side cores away: $150M.

The key thing here isn't to focus on today's pricing, consider it a temporary price point while SpaceX wasn't sure if they could recover cores or refly at a low cost. A rough estimate for reusing block V Falcon 9 is probably $42-45M if similar discounts are applied.

Expended Falcon Heavy = $150m = 2 boosters, 1 center core, 1 second stage expended = nothing recovered
Expended Center Core = $95m = 1 center core, 1 second stage expended = 2 boosters recovered
Expended Falcon 9 = $90m* = 1 booster, 1 second stage expended = nothing recovered
Recovered Falcon Heavy = $90m = 1 second stage expended = 2 boosters, 1 center core recovered
Recovered Falcon 9 = $60m = 1 second stage expended = 1 booster recovered

*This is the only unconfirmed number, but anyone who bought a reusable FH for $90m is having to use an expended F9 so that price makes sense, plus Elon said $95m is slightly more than the cost of an expended F9.

F9 expendable and FH center core expended being about the same price makes perfect sense. They both consume the same amount of hardware if we assume the center core is the reason for the $5 million difference. If we assume sold price for a booster or core is $60m and the upper stage plus overhead and profit is $30m, then all the numbers add up.

IMHO it is obvious they are planning to split the difference in cost savings from reusability with their customers. SpaceX' rockets are cheaper expendable than anyone else's, so they have no motivation to bring the costs lower yet and they need to recoup so R&D cost. So offer the reusable rockets at an even lower cost then your already budget expended rockets and they eat most of the market. They are catching "flying pallets of cash" and splitting them with their customers. That seems like a more than fair deal to me.

It also seems clear that despite what detractors are saying, they couldn't possibly be operating at a significant loss. Private investment is private, but if their prices didn't cover the cost of an expendable rocket it is hard to account for where all that money is coming from. IMHO they could throw away F9s for $60m and stay liquid. Once they stabilize the design and fly reusable regularly they will start making massive profits. If that wasn't true then there would be no dreams of BFR and CommX. Private investors wouldn't get involved for multi-billion dollar secret stakes if there wasn't a clear path to a significant return.

The first part is math, the second two parts are my estimations. Elon's numbers line up exactly with what I've assumed publicly so far. I think it is pretty hard to believe they are on the edge of bankruptcy at this point, though some still seem to.

[speedevil]

The first part is math, the second two parts are my estimations. Elon's numbers line up exactly with what I've assumed publicly so far. I think it is pretty hard to believe they are on the edge of bankruptcy at this point, though some still seem to.

A lot seems to be unpublic.
https://www.cnbc.com/2017/11/28/spacex-raises-100-million-in-funding-round.html

It seems unlikely they'd be selling shares if they have large piles of cash around.

It seems clear they've not been spending money on pads (other than for FH/...).
Similarly, not (as of last Nov at least) large amounts of production tooling for BFR.

I do wonder if there is an inconspicuous warehouse somewhere with a few thousand refrigerator sized boxes that one day will turn up on a suddenly packed launch schedule, once PAZ has launched and revealed any last-minute changes needed and B5 is bedded in.

[Lar]

A lot seems to be unpublic.
https://www.cnbc.com/2017/11/28/spacex-raises-100-million-in-funding-round.html

Conventional thinking is that was a liquidity round. Chance for some employees to shuffle their portfolios and to establish pricing. 100M is way too small to be a useful funding round for an organization the size of SpaceX

[hkultala]

Adding fuel gives you diminishing payload returns - Tyrrany of the rocket equation.

Wrong.

Exactly the opposite of "tyranny of the rocket equation".

It's Tyranny of gravity losses for most rockets.

It there was no gravity losses, increasing propellant (and tankage required to carry it) would give superlinear increase in payload.

But FH has the best T/W of any liquid-fueled rocket so gravity losses are less an issue so there is lots of improvement available by increasing the amount propellant.

Seems no one here is sure where that point of diminishing returns is for a S2 stretch.

Everybody who knows anything seems to understand that for FH it's much bigger than the current S2 size.

The reasons for NOT increasing it are different:

* For F9, the optimal upper stage size is smaller, either go "too big" for F9 or have two separate upper sizes(costly and problematic)
* Would need to change the TEL. Have a pause in launches to change it and if F9 does not change it's upper stage makes FH need different TEL (different launch pad) than F9
* Would be slightly more expensive. More aluminium tankage.
* Simply not needed. Why waste money developing something that is not needed

[dglow]

* Would need to change the TEL. Have a pause in launches to change it and if F9 does not change it's upper stage makes FH need different TEL (different launch pad) than F9

What about a stretched second stage would render its modified TEL incapable of supporting both sizes?

[John Alan]

The reasons for NOT increasing it are different:

* For F9, the optimal upper stage size is smaller, either go "too big" for F9 or have two separate upper sizes(costly and problematic)
* Would need to change the TEL. Have a pause in launches to change it and if F9 does not change it's upper stage makes FH need different TEL (different launch pad) than F9
* Would be slightly more expensive. More aluminium tankage.
* Simply not needed. Why waste money developing something that is not needed

There is literally 4 bolts and some wires and a duct and the top of the TEL tower comes off...

Changing that part out between launches for different configurations is not an issue IMHO... 

On edit...
I would even opine a totally different top will be installed when Falcon Crew get's stood up there later this year...

[whitelancer64]

Adding fuel gives you diminishing payload returns - Tyrrany of the rocket equation.

Wrong.

Exactly the opposite of "tyranny of the rocket equation".

It's Tyranny of gravity losses for most rockets.

It there was no gravity losses, increasing propellant (and tankage required to carry it) would give superlinear increase in payload.

But FH has the best T/W of any liquid-fueled rocket so gravity losses are less an issue so there is lots of improvement available by increasing the amount propellant.

Seems no one here is sure where that point of diminishing returns is for a S2 stretch.

Everybody who knows anything seems to understand that for FH it's much bigger than the current S2 size.

The reasons for NOT increasing it are different:

* For F9, the optimal upper stage size is smaller, either go "too big" for F9 or have two separate upper sizes(costly and problematic)
* Would need to change the TEL. Have a pause in launches to change it and if F9 does not change it's upper stage makes FH need different TEL (different launch pad) than F9
* Would be slightly more expensive. More aluminium tankage.
* Simply not needed. Why waste money developing something that is not needed

The "tyranny of the rocket equation" absolutely does apply here. Adding fuel to the 2nd stage does not provide infinite benefits.

[Paul451]

Adding fuel to the 2nd stage does not provide infinite benefits.

Because hkultala was saying it did? Or anyone?

[Lars-J]

Adding fuel to the 2nd stage does not provide infinite benefits.

Because hkultala was saying it did? Or anyone?

No, but it is a common belief around these parts that stretching the FH upper stage is a cure for almost any problem, even cancer. (that was sarcasm)

[woods170]

There is literally 4 bolts and some wires and a duct and the top of the TEL tower comes off...

Changing that part out between launches for different configurations is not an issue IMHO... 

On edit...
I would even opine a totally different top will be installed when Falcon Crew get's stood up there later this year...

Emphasis mine.
Your guess is very close to what is planned for CCP missions.

[mlindner]

The first part is math, the second two parts are my estimations. Elon's numbers line up exactly with what I've assumed publicly so far. I think it is pretty hard to believe they are on the edge of bankruptcy at this point, though some still seem to.

A lot seems to be unpublic.
https://www.cnbc.com/2017/11/28/spacex-raises-100-million-in-funding-round.html

It seems unlikely they'd be selling shares if they have large piles of cash around.

It seems clear they've not been spending money on pads (other than for FH/...).
Similarly, not (as of last Nov at least) large amounts of production tooling for BFR.

I do wonder if there is an inconspicuous warehouse somewhere with a few thousand refrigerator sized boxes that one day will turn up on a suddenly packed launch schedule, once PAZ has launched and revealed any last-minute changes needed and B5 is bedded in.

That's an incorrect way of thinking about it. You don't sell shares when you need cash, you sell shares when people think you don't need any cash. If you sell shares when you need cash then you'll end up selling a lot more shares for the same amount of cash.

[hkultala]

Adding fuel gives you diminishing payload returns - Tyrrany of the rocket equation.

Wrong.

Exactly the opposite of "tyranny of the rocket equation".

It's Tyranny of gravity losses for most rockets.

It there was no gravity losses, increasing propellant (and tankage required to carry it) would give superlinear increase in payload.

But FH has the best T/W of any liquid-fueled rocket so gravity losses are less an issue so there is lots of improvement available by increasing the amount propellant.

Seems no one here is sure where that point of diminishing returns is for a S2 stretch.

Everybody who knows anything seems to understand that for FH it's much bigger than the current S2 size.

The reasons for NOT increasing it are different:

* For F9, the optimal upper stage size is smaller, either go "too big" for F9 or have two separate upper sizes(costly and problematic)
* Would need to change the TEL. Have a pause in launches to change it and if F9 does not change it's upper stage makes FH need different TEL (different launch pad) than F9
* Would be slightly more expensive. More aluminium tankage.
* Simply not needed. Why waste money developing something that is not needed

The "tyranny of the rocket equation" absolutely does apply here. Adding fuel to the 2nd stage does not provide infinite benefits.

... and the main reason for it is GRAVITY LOSSES, not rocket equation.

And I was not suggesting increasing the fuel load to infinite.

Assuming we did not have any gravity losses:

Example: we have rocket stage with 1.5 tonne of engine and related equipment, 3 tonnes of tank mass and 100 tonnes of propellant.

Assume isp of 340 and the stage has  10.479 km/s of delta-v alone, or can carry a 20-tonne payload to 5.416 km/s.

Keeping the engine same but stretching the tank 1.5 times means we have same 1 tonne of engine, ~4.4 tonnes of tank mass (only sides increase, bulkheads do not) and 150 tonnes of propellant.

Now the stage alone has 10.909 km/s delta-v alone, or can carry a 20-tonne payload to 6.383 km/s, or can carry 30.85-tonne payload to the same 5.416km/s trajectory where the original stage could carry only 20 tonnes.

This is superlinear increase for the stage, 1.54 times more payload for 1.5 times more fuel.


But if we think about the whole rocket:

Though now our staging mass is also 1.5 higher so our 1st stage gives lower impulse to our second stage.

Consider than 20-tonne payload(0.9km/s more with stretched second stage), and not touching the first stage.

Expendable FH would lose about 0.65 km/s staging velocity for that extra staging weight. So ~0.25 km/s more delta-v for that second state stretch for same payload.

But for reusable first stage, the improvement would be much better, because reusable 1st stages have to brake their vertical velocity AND the staging mass will be much greater due to the fuel left for braking and landing.

So, the reusable 1st stage might lose maybe about 0.4 km/s in staging velocity, so total net victory of ~0.5 km/s for the stretched upper stage for the 20-tonne payload.

AND

If we did not have any gravity losses, we could just have ~1.5 times more fuel in our first stage(s), and then have the same staging velocity, but just slightly over 1.5 times higher payload.

But because of gravity(to even liftoff from the pad) and gravity losses, we would also need 1.5 more thrust in our stages.


The problem for stretching tanks is gravity, not rocket equation.

[DeanG1967]

I can't seem to find that the full payload weight of the Tesla and mounting stand (for a better word) that the FH launched.  Basically I am wondering what the total weight.  SpaceX says FH can do 16,800 kg (37,000 lb) to trans-Mars injection.  A Tesla alone obviously doesn't eight 37,000 pounds.