Falcon 9 expendable performance and re-use penalty estimated from SES-9 mission

[IainMcClatchie]

The umbilicals seem to release after the rocket is under way.

I've always wondered if there might be some advantage to refilling the tanks from the ground during the first couple of seconds of flight.  The rocket rises about 6 meters in the first two seconds, which doesn't seem like such a stretch for a fuel and oxidizer line.

I figure the acceleration at takeoff is 3.11 m/s^2, so two seconds of added burn at the beginning without draining the tanks would give 6.2 m/s of extra delta-V.  It's hard to imagine that turning into much added payload.

[Robotbeat]

You want total acceleration relative to free fall, so on the order of 25m/s.

[meekGee]

You want total acceleration relative to free fall, so on the order of 25m/s.

No, he's right - geometrical acceleration since he's figuring out how long he can keep replenishing the stage until he runs out of hose.

It's neat, but as he points out, not neat enough.

[wardy89]

The umbilicals seem to release after the rocket is under way.

I've always wondered if there might be some advantage to refilling the tanks from the ground during the first couple of seconds of flight.  The rocket rises about 6 meters in the first two seconds, which doesn't seem like such a stretch for a fuel and oxidizer line.

I figure the acceleration at takeoff is 3.11 m/s^2, so two seconds of added burn at the beginning without draining the tanks would give 6.2 m/s of extra delta-V.  It's hard to imagine that turning into much added payload.

That just sounds highly risky.

As far as i was aware the first stage is fuelled from the bottom of the vehicle not through one of the visible umbilicals that we can see on the TEL which would make replenishing after liftoff not possible.

I am no engineer but in my mind i would want as little fuel as possible in the TEL and around the pad at lift off to limit the amount of potential damage to it and ground support equipment because of fire.

[ChrisC]

Upper stage can already do multiple restarts.

Yes, but perhaps not reliably, hence a batch of improvements.

[Jim]

The umbilicals seem to release after the rocket is under way.

Because they are pulled free.  No need to add a mechanism when the rocket motion is available.

I've always wondered if there might be some advantage to refilling the tanks from the ground during the first couple of seconds of flight.  The rocket rises about 6 meters in the first two seconds, which doesn't seem like such a stretch for a fuel and oxidizer line.

Refilling what tanks?  Those umbilicals are for the second stage, which is not firing and hence not depleting its tanks.
The first stage umbilicals are in the tail service masts and are disconnected at vehicle release.

There is no advantage and only increased risks.  The tanks are sealed and pressurized.  The fill and drain valves are closed.  The umbilical lines are purged empty.  There is no way of quantifying how much fluid to add. 

There are risks in a disconnect not sealing.

[Hobbes-22]

How long can they stretch F9?  It seems an underlying assumption that the vehicle at its max.   If not, and changes to GSE aren't a show stopper,  that opens the door to a bigger S2.

IIRC road transportability limits the length of the stage. Might not be a hard limit, but if a longer stage needs twice as much time to be driven to the launch site than the current design...

[Stan-1967]

How long can they stretch F9?  It seems an underlying assumption that the vehicle at its max.   If not, and changes to GSE aren't a show stopper,  that opens the door to a bigger S2.

IIRC road transportability limits the length of the stage. Might not be a hard limit, but if a longer stage needs twice as much time to be driven to the launch site than the current design...

Yes, S1 is supposedly at its max length that still enables road transport, so that sets a bound on S1 length.  The fineness ratio was also supposedly at it maximum, which bounds the length of the entire stack.  So the fineness ratio is then bounding how big of a S2 can be made. 

Which boundary is more amendable to an engineering path to expand it?

Can you get such a big payload improvement (8.3t) from changing the S1/S2 lengths within the constraints of the existing overall length?   Perhaps if they take the weight savings from new legs, fins, etc., and put that mass into stiffening the S1 core, thereby enabling a longer S2?  Alternately, does a new transport route, or fixing a few low hanging overpasses remove a bottleneck for transportation?

[MP99]

First stage thrust is going up 17% over the SES-9 launch. That's not a small thing.

Given the limitations of S1 and total stack length, maybe a 5m S2? How would stage mass compare with the existing stage for the same prop mass?

If M1Dvac thrust is also increasing, S2 could cope with a larger prop load.

Cheers, Martin

[Space Ghost 1962]

First stage thrust is going up 17% over the SES-9 launch. That's not a small thing.

Given the limitations of S1 and total stack length, maybe a 5m S2? How would stage mass compare with the existing stage for the same prop mass?

If M1Dvac thrust is also increasing, S2 could cope with a larger prop load.

And, you could also incorporate a new fairing, possibly larger ... possibly reusable ...

[LouScheffer]

How long can they stretch F9?  It seems an underlying assumption that the vehicle at its max.   If not, and changes to GSE aren't a show stopper,  that opens the door to a bigger S2.

IIRC road transportability limits the length of the stage. Might not be a hard limit, but if a longer stage needs twice as much time to be driven to the launch site than the current design...

Yes, S1 is supposedly at its max length that still enables road transport, so that sets a bound on S1 length.  The fineness ratio was also supposedly at it maximum, which bounds the length of the entire stack.  So the fineness ratio is then bounding how big of a S2 can be made. 

Which boundary is more amendable to an engineering path to expand it?

If you want a wild suggestion, perhaps SpaceX could go to even larger fineness by active control of bending modes (assuming this is what limits fineness).  Unlike most rockets, they have nitrogen jets and grid fins more or less in the middle of the assembly.  Creative use of these on the way up could ameliorate bending forces, allowing a finer rocket without buckling.  See, for example, Towards Intelligent Structures: Active Control of Buckling.

On the other hand, I don't think stretching the second stage will yield much.  It's already something like 95% fuel, which is a very good fraction indeed.  It's hard to see the faction getting much better with a stretch, and the fraction determines the delta-V.

[Robotbeat]

Moar thrusters to fight bendiness is very Kerbal. But no match for Moar Struts!

[MP99]

On the other hand, I don't think stretching the second stage will yield much.  It's already something like 95% fuel, which is a very good fraction indeed.  It's hard to see the faction getting much better with a stretch, and the fraction determines the delta-V.

A stage which is x% bigger for the same prop fraction can give the same dV to a payload which is x% bigger.

Obviously, this does not take into account that the first stage won't be expanding in line.

Cheers, Martin

[whitelancer64]

Echostar23 appears to be only just in excess of the payload margin for recovery. Does that sound right to everyone?

[AncientU]

They seemed to be exploring the envelope around that 5,600kg mark.  After the AMOS disaster, they appeared to back off getting 5,600 in the landing range.

[TrueBlueWitt]

Moar thrusters to fight bendiness is very Kerbal. But no match for Moar Struts!

And what are their chances of getting either approved for Crewed missions within the next 5 years?

[TrueBlueWitt]

On the other hand, I don't think stretching the second stage will yield much.  It's already something like 95% fuel, which is a very good fraction indeed.  It's hard to see the faction getting much better with a stretch, and the fraction determines the delta-V.

A stage which is x% bigger for the same prop fraction can give the same dV to a payload which is x% bigger.

Obviously, this does not take into account that the first stage won't be expanding in line.

Cheers, Martin

Longer 2nd stage doing more work makes sense now.. 1st/2nd stage ratios were set with much lower thrust Merlins. I think they might change the ratio now if they could. 

Anyway, bigger 2nd stage means lower/slower staging.. which means less fuel needed for 1st stage recovery. Which would help overall performance.