Second Stage Stretch

[spacenut]

I know a Raptor 5.2 m second stage is out at this time.  Would the F9 or FH get a slight increase in payload if they could stretch the second stage some?  It would be the same diameter so trucking it wouldn't be a problem.  The only think I see as a problem is either the strongback or the umbilicals.  Would a stretch actually improve the payload?  Would the additional payload increase be worth any modifications they would need to make to their ground support?  Would the stretch make the airframe of the rocket too long an cause too much stress?

[John Alan]

I've been saying stretch S2 and shrink S1 same amount for a while now... 

Get some more fluids above the more efficient Mvac... borrowed it from S1 length ... 

Move the movable middle cradle and S2 umbilical down a bit... And Bob's your Uncle.. 

[Lars-J]

They already stretched it for v1.2 by ~2m. They are unlikely to do it again, as they want to have a common upper stage for F9 and FH. This upper stage size appears to be a good compromise for both.

Keep in mind that F9 stages pretty low, so unless M1D-Vac has some serious thrust enhancements, the gravity losses would be an issue. Thrust to weight ratio needs to be taken into account.

[envy887]

The F9 is currently at it's maximum overall length, limited by bending loads because it's so tall and skinny. So to lengthen the second stage, the first stage needs to get shorter - which they aren't likely to do.

And a larger upper stage actually gets less payload unless thrust also increases - it has more gravity losses and more dry mass sent to orbit.

[whitelancer64]

The last part of the question is the most important. The Falcon 9 is already exceptionally tall and thin as it is, with a fineness ratio approaching 1:20 (I've read that the "ideal" for orbital rockets is around 1:14, and Werner Von Braun is anecdotally reported to have said 1:10 is good). Making the rocket taller means it will be more vulnerable to winds during launch causing the rocket to bend - something you do not want your rocket to do.

[Prettz]

I know a Raptor 5.2 m second stage is out at this time.

A second stage that's a different diameter is out for all of time. It will never happen while F9 exists.

[DAZ]

The last part of the question is the most important. The Falcon 9 is already exceptionally tall and thin as it is, with a fineness ratio approaching 1:20 (I've read that the "ideal" for orbital rockets is around 1:14, and Werner Von Braun is anecdotally reported to have said 1:10 is good). Making the rocket taller means it will be more vulnerable to winds during launch causing the rocket to bend - something you do not want your rocket to do.

I would have to agree, with the up-thread comment, that it would seem to be extremely unlikely that SpaceX would make any changes to the length of any of their stages at this time. It would seem that SpaceX wants for the falcon 9 and falcon 9 heavy to be there workhorse rocket and put the majority of their resources into their new rocket.

But this issue of the fitness ratio does bring up an interesting question, at least to me, of can the falcon 9 heavy version actually have a longer 2nd stage?

As I understand it, the issue of the fitness ratio is primarily due to the limited predictability of the stresses of the rocket as it travels through the atmosphere. Once above the atmosphere, the fitness ratio is not anywhere near as important. It would seem to me that the falcon 9 heavy with its 2 strapped on side falcon 9 rockets would inherently change this fitness ratio. The 2 side boosters should help stiffen the lower part of the rocket. These side boosters are only jettisoned once the entire stack is above all of the significant atmosphere. This would seem to mean, as I understand it, that even though the remaining rocket might have a fitness ratio > 1:20 that this really shouldn't matter.

[Jim]

. The 2 side boosters should help stiffen the lower part of the rocket.

They don't

[su27k]

I tested some numbers on http://silverbirdastronautics.com/LVperform.html, it seems a further stretch may be able to improve GTO performance somewhat, so I'm not sure this idea can be easily dismissed on TWR/performance ground.

Note a 2m stretch would change fineness ratio from 19.1 to 19.7, doesn't seem to be a super big change to me, obviously it will have impacts but are we sure the impact will be a showstopper?

[Lars-J]

The F9 is currently at it's maximum overall length, limited by bending loads because it's so tall and skinny. So to lengthen the second stage, the first stage needs to get shorter - which they aren't likely to do.

This keeps being repeated without an actual source. Can you source this? Because this sounds like someones armchair opinion that was quoted enough to become assumed fact.

(I just want clarification - I do not believe that the upper stage will be stretched further, but for other reasons)

[bstrong]

I can't recall an authoritative source that a further stretch is impossible either (especially with the stronger FH center core). But we do have at least one good source that it won't be done (context is speculation that the cradles on the TEL can be easily moved):

At a glance the mounting system can give that impression I guess, but it's just design change.  I can tell you after installing that mess I never ever want to move it.  Ever.

[hkultala]

Where are the second stage umbilicals connected? top or bottom of the stage? would stretching the stage mean changing their position?

[virnin]

Where are the second stage umbilicals connected? top or bottom of the stage? would stretching the stage mean changing their position?

Bottom of the stage.  The payload umbilicals would have to be moved up to the new top pf S2.

[envy887]

The F9 is currently at it's maximum overall length, limited by bending loads because it's so tall and skinny. So to lengthen the second stage, the first stage needs to get shorter - which they aren't likely to do.

This keeps being repeated without an actual source. Can you source this? Because this sounds like someones armchair opinion that was quoted enough to become assumed fact.

(I just want clarification - I do not believe that the upper stage will be stretched further, but for other reasons)

The closest thing I have to a original source in this article

Because of the different densification possible for LOX and RP1, an adjustment of tank sizes on the rocket is necessary to keep the Oxidizer to Fuel ratio required by the Merlin 1D engines.

This is accomplished by shortening the LOX tank on the first stage and stretching the RP-1 tank while retaining the original first stage length. Stretching the first stage beyond the length of the v1.1 first stage is not possible due to bending forces occurring in flight. Widening the diameter of the stages is also no option because of the requirement of road transport, putting a limit on the maximum diameter. The second stage of Falcon 9 FT accommodates the required change in RP-1 volume by stretching the stage.

[emphasis mine]

However, the same page lists the v1.2 S1 as 1.4m longer than the v1.1 S1, for what it's worth.

The fineness of the F9 has often been cited as a area where it's pushing the boundaries of what is possible, according to experts on this site. It's not proof, but certainly supports the stance that fineness would be a substantial issue preventing further lengthening.

....  I am a firm believer in Vicomte de Saint-Exupéry's admonishment:

Perfection is achieved not when there is nothing more to be added, but when there is nothing more to be taken away.

I'd put SpaceX's foray into sub-cooled LOX in the same category as the high fineness ratio issue.  Some decisions increase operational cost and aren't worth the effort.  Launch vehicles are especially unforgiving due to their very nature.  Of course, you may fairly charge me with succumbing to Clarke's first law, too.  I have reached Medicare age so my experience makes me suspect. 

[Zed_Noir]

Instead of stretching the 2nd stage or introducing the Raptor Vac.

Can a new composite 2nd stage of 5.2 meter diameter with a Merlin Vac and the same length as the current stage make sense for the Falcon Heavy in the future?

[macpacheco]

If you want to just dream about a technical possibility then dream away.

But the truly relevant question isn't if SX can technically do it, is if it will do it. The answer I believe is a resounding no, SX isn't going to mess with F9/FH beyond Block V improvements, except to fix issues/risks that might be found and refine booster reuse.
Why ?
F9 block IV already can handle the vast majority of missions with ASDS recovery. With Block V it will be able to handle even more missions with recovery.
The very few missions that F9 can't do with recovery, FH picks up the slack.
Improving upper stage performance isn't going to shift a substantial number of missions from FH to F9.
Improving upper stage performance isn't going to enable FH to perform missions that would realistically be needed prior to ITS / mini ITS be flying.
The fundamental advantage of Raptor for all stages is its design to handle hundreds of full mission firings. Its better ISP and thrust/weight makes it compelling for both stages.
As soon as SX completes its qualification of Raptor, I expect they will announce a F9 on steroids, using Raptor propulsion, perhaps 5.2m fuselage and a fully reusable platform.
Possibly the 2nd stage will be a mini ITS platform that avoids ejecting fairings and having to recover them separately.

Remember SpaceX isn't about the most technically efficient solution but rather bang for the buck, as long as the 2nd stage is expended, the primary goal is to make its production process cheaper rather than making it bigger/harder to make. Even a stretch could be done at all safely, the cost side vs no substantial benefits already settle this issue.

Once FH is flying with Block V improvements and Crew Dragon is done, pretty much every SX rocket design people will focus 99% on ITS/mini ITS/upscale F9.

If your annoyed by my post is because you forgot to put "speculation" on the thread title.

[su27k]

If you want to just dream about a technical possibility then dream away.

But the truly relevant question isn't if SX can technically do it, is if it will do it. The answer I believe is a resounding no, SX isn't going to mess with F9/FH beyond Block V improvements, except to fix issues/risks that might be found and refine booster reuse.

I think the speculation here is that they'll do a tank stretch for block 5. As for why, well one of the reason is to hit their intended performance target. We've seen what a block 3 expendable can do, it is quite a bit distance away from the advertised expendable performance on their website. At least I haven't seen anyone can reproduce the 8.3t to GTO number using the current dimension and known upgrades. Add in the fact that they're building a new structure test stand which seems to fit 2nd stage makes this speculation a bit more substantial than a simple dream...

[Prettz]

Instead of stretching the 2nd stage or introducing the Raptor Vac.

Can a new composite 2nd stage of 5.2 meter diameter with a Merlin Vac and the same length as the current stage make sense for the Falcon Heavy in the future?

No because it raises the costs of launching a Falcon, rather than lowering them.

[Zed_Noir]

Instead of stretching the 2nd stage or introducing the Raptor Vac.

Can a new composite 2nd stage of 5.2 meter diameter with a Merlin Vac and the same length as the current stage make sense for the Falcon Heavy in the future?

No because it raises the costs of launching a Falcon, rather than lowering them.

My previous post was about upgrading the Falcon Heavy upper stage. Nothing was said about the Falcon 9.

Why would a more capable Falcon Heavy upper stage increase the cost of the price per kilogram going up?

[Jim]

My previous post was about upgrading the Falcon Heavy upper stage. Nothing was said about the Falcon 9.

Why would a more capable Falcon Heavy upper stage increase the cost of the price per kilogram going up?

Because it requires a new TEL, new production line, new GSE, new transportation method