totm nov 2023 SpaceX Discussion Thread

[tater]

Would love to see an image with both F9s vertical.

[Guest]

47 minutes ago, RCgothic said:

Yes, but not thanks to Centaur or DCSS being better than FUS. Centaur and DCSS light higher and faster than F9US so they don't have to do as much work. That's thanks to DIVH and AtlasV.

F9US has at least 4.4km/s more DV than DCSS and Centaur for any mass of payload between 0t and 16t. F9US does more work than the second stage of any other operational rocket.

That's only because it's about 2.5 times the weight of a Centaur. This is also the reason it looks so good in comparison. It's got more dV because it's bigger. It's not humongous in volume because it's using kerolox, which is much denser than hydrolox. 

A better comparison would be the "big Centaur" from Vulcan, because that one is looking to be in the 50T class, like the FUS.

[sevenperforce]

Just now, Dragon01 said:

That's only because it's about 2.5 times the weight of a Centaur. This is also the reason it looks so good in comparison. It's got more dV because it's bigger. It's not humongous in volume because it's using kerolox, which is much denser than hydrolox. 

Nah, it's not about gross weight; it's about prop fraction. Centaur has a prop fraction of 9.3:1, not much better than the tanks in KSP. The propellant fraction for F9US is more than double that. It's not even close.

Just now, Dragon01 said:

A better comparison would be the "big Centaur" from Vulcan, because that one is looking to be in the 50T class, like the FUS.

It will have slightly better mass fractions but not dramatically so. Insulation is heavy and hydrogen is fluffy. These are not true balloon tanks; tank mass is closer to being linear to prop mass than it is to following the square-cube law.

 

[Guest]

3 minutes ago, sevenperforce said:

Nah, it's not about gross weight; it's about prop fraction.

No it isn't. A kerolox stage of any kind will have a better prop fraction than a hydrolox one. Even Soyuz has a 2nd stage with a prop fraction of more than 10. That's not difficult to achieve. Additionally, with higher gross weight of these stages, the payload becomes, proportionally, a smaller fraction of the whole vehicle. They pay for it with specific impulse, but overall, an optimized kerolox upper stage will be heavier and have more dV than a hydrolox stage, given they were both designed for a vehicle in the same class.

All of this comes out of 1st stage performance, however. To loft the FUS as high up as the Centaur, you'd need the 1st stage the size of a Saturn I one, and then you're pretty much going up a size class (sort of what FH does). A comparison between something like Atlas and F9 is tricky, because their flight profiles are very different. This, BTW, is why reusability is far less trouble with F9 than with Atlas. More work, proportionally, has to be done by the 2nd stage in an all-kerolox setup, so the 1st stage simply doesn't go as high as with a hydrolox upper stage. 

[tater]

 

[wumpus]

51 minutes ago, Dragon01 said:

No it isn't. A kerolox stage of any kind will have a better prop fraction than a hydrolox one. Even Soyuz has a 2nd stage with a prop fraction of more than 10. That's not difficult to achieve. Additionally, with higher gross weight of these stages, the payload becomes, proportionally, a smaller fraction of the whole vehicle. They pay for it with specific impulse, but overall, an optimized kerolox upper stage will be heavier and have more dV than a hydrolox stage, given they were both designed for a vehicle in the same class.

All of this comes out of 1st stage performance, however. To loft the FUS as high up as the Centaur, you'd need the 1st stage the size of a Saturn I one, and then you're pretty much going up a size class (sort of what FH does). A comparison between something like Atlas and F9 is tricky, because their flight profiles are very different. This, BTW, is why reusability is far less trouble with F9 than with Atlas. More work, proportionally, has to be done by the 2nd stage in an all-kerolox setup, so the 1st stage simply doesn't go as high as with a hydrolox upper stage. 

Don't forget the whole thrust issue for hydrolox first stages.

On the other hand, Delta-IV appears to have a TWR similar to Falcon 9 (~1.3) and a much longer first stage burn time 245s vs 180s.  So you can build a higher performance rocket than falcon 9 with hydrolox (and presumably make a profit with DoD/NASA pricing).

[sevenperforce]

1 hour ago, Dragon01 said:

No it isn't. A kerolox stage of any kind will have a better prop fraction than a hydrolox one. Even Soyuz has a 2nd stage with a prop fraction of more than 10. That's not difficult to achieve. Additionally, with higher gross weight of these stages, the payload becomes, proportionally, a smaller fraction of the whole vehicle. They pay for it with specific impulse, but overall, an optimized kerolox upper stage will be heavier and have more dV than a hydrolox stage, given they were both designed for a vehicle in the same class.

I mean, the rocket equation is the rocket equation. dV is linearly proportional to specific impulse and proportional to the natural log of mass fraction. Since the specific impulse range of most bipropellant rocket engines is within a factor of 1.35, which is dramatically less than e, the advantage will always go to mass fraction.

Obviously there are issues like staging velocity and TWR which are larger forcing functions on total dV.

6 minutes ago, wumpus said:

Don't forget the whole thrust issue for hydrolox first stages.

On the other hand, Delta-IV appears to have a TWR similar to Falcon 9 (~1.3) and a much longer first stage burn time 245s vs 180s.  So you can build a higher performance rocket than falcon 9 with hydrolox (and presumably make a profit with DoD/NASA pricing).

The longer first-stage burn time actually hurts you with respect to TWR, because the Falcon 9's TWR grows much faster than the Delta IV's. And FH is simply a much bigger rocket than DIVH. It is twice as heavy.

 

[CatastrophicFailure]

3 hours ago, tater said:

Would love to see an image with both F9s vertical.

Boom. 

if you squint reeeeeeal hard...

[Geonovast]

1 minute ago, CatastrophicFailure said:

Boom. 

if you squint reeeeeeal hard...

Julia and I have different definitions of the word "see"

[Guest]

1 hour ago, wumpus said:

Don't forget the whole thrust issue for hydrolox first stages.

On the other hand, Delta-IV appears to have a TWR similar to Falcon 9 (~1.3) and a much longer first stage burn time 245s vs 180s.  So you can build a higher performance rocket than falcon 9 with hydrolox (and presumably make a profit with DoD/NASA pricing).

A hydrolox first stage is a suboptimal decision, TBH. The whole reason why hydrolox upper stages are great is that they're light, which means the first stage has to lift less, thus allowing it to be smaller and lighter, both because of less propellant and less structural strength to support a large upper stage. Mass savings in the upper stage typically yield large overall dV gains, because any mass there stick with you longer. This isn't so in case of first stages, where in most cases, adding propellant and thrust is more straightforward, and you throw it away relatively quickly. Indeed, since square-cube law means the larger the rocket, the better mass fraction it will have, making a bigger first stage usually doesn't hurt.

1 hour ago, sevenperforce said:

I mean, the rocket equation is the rocket equation. dV is linearly proportional to specific impulse and proportional to the natural log of mass fraction. Since the specific impulse range of most bipropellant rocket engines is within a factor of 1.35, which is dramatically less than e, the advantage will always go to mass fraction.

Rocket equation is not the whole story, though. In this case, we're discussing upper stages, and my point is, FUS seems a lot better because in Falcon 9, a proportionally larger fraction of dV is allocated to the upper stage than in other rockets. Effectively, a hydrolox upper stage allows you to get more out of the lower stage than a kerolox one. An upper stage needs to be considered within context of the lower one.

[CatastrophicFailure]

1 hour ago, Geonovast said:

Julia and I have different definitions of the word "see"

 

Better? 

[Geonovast]

3 minutes ago, CatastrophicFailure said:

Better? 

No. 

I dunno about @tater, but I was looking for something more along the lines of this:

[tater]

Just now, Geonovast said:

I dunno about @tater, but I was looking for something more along the lines of this:

Yep.

39A and SLC-40 are farther apart than that, but an aerial shot with a long lens would do nicely.

[StrandedonEarth]

48 minutes ago, tater said:

Yep.

39A and SLC-40 are farther apart than that, but an aerial shot with a long lens would do nicely.

Near sunrise/sunset for longer shadows may make things more apparent 

[CatastrophicFailure]

2 hours ago, tater said:

an aerial shot with a long lens would do nicely.

Um... how ‘bout this?

Spoiler

 

[StrandedonEarth]

Looks a little crinkly. Hopefully under pressure it'll plump up like a sausage skin.

[tater]

9 minutes ago, StrandedonEarth said:

Looks a little crinkly. Hopefully under pressure it'll plump up like a sausage skin.

SN6 is more smooth. They seem to be getting nicer each iteration.

Edited June 25, 2020 by tater

[AngrybobH]

2 hours ago, StrandedonEarth said:

Looks a little crinkly. Hopefully under pressure it'll plump up like a sausage skin.

looks like the worlds largest beer keg.

[StrandedonEarth]

36 minutes ago, AngrybobH said:

looks like the worlds largest beer keg.

With a very frosty yet fiery brew inside

[Silavite]

1 hour ago, AngrybobH said:

looks like the worlds largest beer keg.

 

32 minutes ago, StrandedonEarth said:

With a very frosty yet fiery brew inside

All it needs is a fuel shift from methane/LOX to ethanol/LOX.

[kerbiloid]

38 minutes ago, Silavite said:

All it needs is a fuel shift from methane/LOX to ethanol/LOX.

And produce it from grain right in the rocket, like they are going to do with methane on Mars.

[tater]

16 hours

 

[magnemoe]

16 hours ago, RCgothic said:

Yes, but not thanks to Centaur or DCSS being better than FUS. Centaur and DCSS light higher and faster than F9US so they don't have to do as much work. That's thanks to DIVH and AtlasV.

F9US has at least 4.4km/s more DV than DCSS and Centaur for any mass of payload between 0t and 16t. F9US does more work than the second stage of any other operational rocket.

 

SpaceX don't have any reason to do hydrolox.

Would their first stage be improved by hydrolox? No. It couldn't get off the pad without boosters.

Would the second stage be improved? No. The extra insulation would reduce the mass fraction more than the ISP gains.

Most rockets in falcon 9 weight class is 2.5 stages who let you drop the upper stage later, using hydrolox also give you more dV with the expense of an larger stage, insulation and often having to handle two types of fuel. Saturn 9 did and New Glen plan to do this. 
Problem with 2.5 stages is reuse, it can be done as in falcon heavy but its not very efficient. 
 

[RCgothic]

18 hours ago, Dragon01 said:

That's only because it's about 2.5 times the weight of a Centaur. This is also the reason it looks so good in comparison. It's got more dV because it's bigger. It's not humongous in volume because it's using kerolox, which is much denser than hydrolox. 

A better comparison would be the "big Centaur" from Vulcan, because that one is looking to be in the 50T class, like the FUS.

Edit: retracted due to maths error. *Embarrassed*

Edited June 26, 2020 by RCgothic

[mikegarrison]

There are some completely different design philosophies at work here.

Falcon 9 is optimized for a relatively short first stage burn, because that's necessary to allow the first stage to land. So it needs a high thrust, big delta-v upper stage, which is why the upper stage is designed the way it is. It's all a fallout of the need to optimize the first stage for landing.

Rockets that don't try to land their first stage tend to get more of the total launch energy out of the first stage, which makes a light, lower-thrust, hydrogen-burning upper stage a really attractive option.

Falcon's upper stage wouldn't make a lot of sense on an Atlas or Delta, but Centaur wouldn't make a lot of sense on a Falcon 9. Neither one is independent of the overall design philosophies of their respective launch systems.