totm nov 2023 SpaceX Discussion Thread

[TheDestroyer111]

1 hour ago, sevenperforce said:

Um.

As a rule, it is typically considered a good idea to affix all aerodynamic control surfaces firmly to the body of your aircraft. Real life does not have autostrut.

Yes, the computer programming running the abort uses differential thrust to throttle the engines on one side just a touch higher than the other side, causing it to turn slightly and fly free of the booster. What you cannot rely on during abort is for gimbal or differential throttling to fight an inherently unstable aerodynamic system.

Once you put the fins enough backward of CoM in any way, it's not inherently unstable.

I know you don't have autostrut, But it won't be necessary as long as the fins aren't supposed to produce any noticeable amount of lift (that is, as long as you don't turn a capsule into technically a fixed-wing aircraft)

1 hour ago, tater said:

@sevenperforce, clearly the engineers at SpaceX, as monitored by their highly-involved customer, NASA, have not the slightest clue compared to a random KSP player.

 

lol apparently, because there are still satellite builders willing to choose other rockets than spacex or other reusable rocket company

8 minutes ago, sevenperforce said:

I would also point out that placing "floating fins" with the tips attached to the sides of the capsule, intended to break away prior to re-entry, would mean adding four decoupling actuators to the aeroshell of the Dragon 2. The added weight of the structural reinforcements required to keep these fins fixed would decrease payload to orbit. That's also four more chances for a decoupling actuator to fail; if any of them fail, you go nose-first and burn up on re-entry. You would also need to deal with the possibility of the fins colliding with the trunk during an abort.

Now that's quite a thought... Although the fins can be decoupled just when aerodynamics stop affecting abort scenarios, now to calculate if we'll need the added drag (and heat shielding) of going tail-first into the atmosphere in case of abort just after someone realizes the fins don't detach... Someone assemble that frappin' aerodynamics team! 

Edited September 19, 2017 by TheDestroyer111

[sevenperforce]

3 minutes ago, TheDestroyer111 said:

Once you put the fins enough backward of CoM in any way, it's not inherently unstable.

I know you don't have autostrut, But it won't be necessary as long as the fins aren't supposed to produce any noticeable amount of lift (that is, as long as you don't turn a capsule into technically a fixed-wing aircraft)

Gah. What do you think the fins are there for? If there is force due to airflow acting on the fins, then they will be ripped off the capsule if they are only attached at the tip. If there is not enough force acting on the fins to rip them off, they will not provide passive aerodynamic stability.

And it actually IS lift. How do you think a fin provides passive guidance? As long as the vehicle remains on prograde, the fin airfoil's angle of attack is zero. When it starts to pitch or yaw off-prograde, the AoA changes, turning the fin at an angle to the airstream and producing lift opposite the misalignment to push it back toward prograde.

[TheDestroyer111]

45 minutes ago, sevenperforce said:

And it actually IS lift. How do you think a fin provides passive guidance? As long as the vehicle remains on prograde, the fin airfoil's angle of attack is zero. When it starts to pitch or yaw off-prograde, the AoA changes, turning the fin at an angle to the airstream and producing lift opposite the misalignment to push it back toward prograde.

At zero AoA (any stable vehicle constantly tries to get to that) there is no lift, and AoA won't change without reason, maybe except for the first few seconds of the abort when airspeed is... slow... what...

wow i just figured out why spx attached the fins to the trunk, they WILL create a LOT of lift in case of abort at max-q

thnx a looooot for your stubbornness in trying to make me understand the truth behind these fins LOLROFL

 

//EDIT: @CatastrophicFailure whuts wrong w/ski seasons?

Edited September 19, 2017 by TheDestroyer111

[CatastrophicFailure]

THIS JUST IN: MAN ARGUES WITH BRICK WALL, WINS! 

In related news, Hell predicting best ski season in millennia. 

[sevenperforce]

13 minutes ago, TheDestroyer111 said:

At zero AoA (any stable vehicle constantly tries to get to that) there is no lift, and AoA won't change without reason, maybe except for the first few seconds of the abort when airspeed is... slow... what...

wow i just figured out why spx attached the fins to the trunk, they WILL create a LOT of lift in case of abort at max-q

thnx a looooot for your stubbornness in trying to make me understand the truth behind these fins LOLROFL

I love seeing the lightbulb click on!

For a visual:

Spoiler

Even with completely-fixed, bilaterally symmetric fins, changing vehicle attitude causes lift due to AoA, which pulls back toward prograde.

This, incidentally, is why passively-stabilized, vertically-launched model rockets need to have the fins angled through the center of the stack; it's not enough for them to be merely axisymmetric.

10 minutes ago, CatastrophicFailure said:

THIS JUST IN: MAN ARGUES WITH BRICK WALL, WINS! 

In related news, Hell predicting best ski season in millennia. 

I was as surprised as you were!

[tater]

The implication of this post is that all subsequent F9 launches are moving back to SLC-40 after this launch.

[StupidAndy]

...so that means that we're only one launch away from Falcon Heavy is one launch away!

also Wikipedia says that the first launch after Iridium L4 is from CCAFS SLC-40, so yay?

link

[CatastrophicFailure]

14 minutes ago, StupidAndy said:

...so that means that we're only one launch away from Falcon Heavy is one launch away!

also Wikipedia says that the first launch after Iridium L4 is from CCAFS SLC-40, so yay?

link

We can hope, but there may be (probably will be) launches from 40 before FH is ready. 

[StupidAndy]

Just now, CatastrophicFailure said:

We can hope, but there may be (probably will be) launches from 40 before FH is ready. 

oh, forgot something,

one launch FROM 39A till Falcon Heavy! hooray!

[Racescort666]

 

2 minutes ago, StupidAndy said:

oh, forgot something,

one launch FROM 39A till Falcon Heavy! hooray!

I was going to ask how you figured that there was only 1 launch until FH. By my count, there's at least 3.

[StupidAndy]

Just now, Racescort666 said:

I was going to ask how you figured that there was only 1 launch until FH. By my count, there's at least 3.

yeah I miss things sometimes

[Skylon]

So...when falcon heavy flies...will more people want to launch on it? Looking at the schedule on Wikipedia...only three launches (although one is going to the Moon)

[_Augustus_]

5 hours ago, Skylon said:

So...when falcon heavy flies...will more people want to launch on it? Looking at the schedule on Wikipedia...only three launches (although one is going to the Moon)

I think that there will be a lot of DOD launches for big payloads such as Mentor and KH-12 that normally fly on DIVH. And probably more Moon flights with more than 2 passengers (D2 fits 7).....

[Nibb31]

8 hours ago, Skylon said:

So...when falcon heavy flies...will more people want to launch on it? Looking at the schedule on Wikipedia...only three launches (although one is going to the Moon)

The problem is the lack of payloads. There is no real demand for 50t payloads for LEO or 20t GEO birds for two main reasons:

1. There aren't any launchers to for that sort of capacity, so there is no reason to design payloads for them.
2. Large payloads tend to have a large price tag, regardless of launch cost (launch is typically less that 20% of the cost of the payload).

It takes years to design and build a satellite, so it doesn't make sense for an operator to put all their eggs in the basket of a single launch vehicle that has yet to prove it can actually fly, and even Musk's tweets aren't super optimistic about that.

Edited September 21, 2017 by Nibb31

[tater]

http://spacenews.com/spacex-forces-air-force-to-revise-launch-mindset/

Quote

“SpaceX does not launch on schedule,” Monteith said Sept. 20 during a space warfighting panel at the annual Air Force Association Air Space Cyber Conference. “They launch on readiness.”

“They have forced us — and I mean forced us — to get better, infinitely better, at what we do,” he said. “We are adopting commercial business practices and becom[ing] more efficient and more affordable.

“Working with them, we have been able to reduce our main launch footprint by 60 percent and reduce the cost of a single launch by over 50 percent,” he said. “Based on the autonomous flight safety system they developed with us they will help us get to 48 launches a year.”

 

[Grand Ship Builder]

Once I made the trunk made out of service bays directly attached to the capsule and put the rest of the craft down under so the trunk can survive. I had to weld 8 thuds together for the craft to even take off. Meanwhile my experimental re-usable craft which is mostly inflatable heatshield can lift off with only 4 thuds pretty well.

 

Not directly related to SpaceX, but I mention "trunk" and "re-usable", so of course it counts!

[Racescort666]

1 hour ago, Nibb31 said:

The problem is the lack of payloads. There is no real demand for 50t payloads for LEO or 20t GEO birds for two main reasons:

1. There aren't any launchers to for that sort of capacity, so there is no reason to design payloads for them.
2. Large payloads tend to have a large price tag, regardless of launch cost (launch is typically less that 20% of the cost of the payload).

It takes years to design and build a satellite, so it doesn't make sense for an operator to put all their eggs in the basket of a single launch vehicle that has yet to prove it can actually fly, and even Musk's tweets aren't super optimistic about that.

I thought part of the drive for FH was for the 6t+ payloads going to GTO to have a chance at reusability. I was under the impression that the weight limit to GTO for reusable was less than 6t (according to wikipedia it is 5.5t). With the introduction of FH, there is now an increased mass to GTO while recovering the boosters.

[CatastrophicFailure]

9 hours ago, Skylon said:

So...when falcon heavy flies...will more people want to launch on it? Looking at the schedule on Wikipedia...only three launches (although one is going to the Moon)

There have already been several launches slated for FH, but due to the delays wound up on expendable F9s. FH will immediately end that. Every expendable F9 flight over the last couple of years represents a FH customer, so the demand is out there, just because it can lift 20 tonnes to GTO doesn't mean it will on every flight. Even under best case scenario, the FH is only planned to fly a few times a year.

And if they're even remotely successful with the attempt at stage 2 recovery, that opens the possibility of a fully reusable launch vehicle, which changes things yet again. 

[Nibb31]

You're assuming that a reusable FH will be cheaper than an expendable F9. Maybe it will, maybe it won't. Only time will tell.

[Grand Ship Builder]

2 minutes ago, Nibb31 said:

You're assuming that a reusable FH will be cheaper than an expendable F9. Maybe it will, maybe it won't. Only time will tell.

In the long run, yes. On the short term, no.

[Steel]

8 minutes ago, Grand Ship Builder said:

In the long run, yes. On the short term, no.

There's absolutely no guarantee that it will ever be cheaper (and equally no guarantee that it won't be cheaper). There's no way to predict it because nothing like this has ever been done before.

Edited September 21, 2017 by Steel

[CatastrophicFailure]

12 minutes ago, Nibb31 said:

You're assuming that a reusable FH will be cheaper than an expendable F9. Maybe it will, maybe it won't. Only time will tell.

Considering that's what on the primary reasons for the thing, yeah I'm gonna assume that's one of SpaceX's goals for it until demonstrated otherwise. 

But the argument that there just aren't any payloads for it when there have already been payloads for it (that launched on other vehicles or other vendors) seems a bit silly.

[tater]

I see less and less need for FH, honestly. F9 improvements have been so fast and furious that it eventually becomes an issue of having yet more mass capability than is really needed given possible fairing sizes (even with an increase).

The real tell will be Musk's talk in a few days about ITSy. ITSy obviates any need for FH, IMHO. When FH was thought to be easy, then sure, why not. If it turns out to be a problem, then it might not be worth future trouble.

SLS has the benefit of diameter, which for manned spaceflight applications is non-trivial (people live in volumes). NG will be a serious threat to SLS for exactly that reason, even with lower masses to LEO. ITSy kills SLS dead, IMO.

Regardless, I think FH is a dead end, though it might fly for a few years.

Edited September 21, 2017 by tater

[Nibb31]

1 hour ago, CatastrophicFailure said:

Considering that's what on the primary reasons for the thing, yeah I'm gonna assume that's one of SpaceX's goals for it until demonstrated otherwise. 

No, the primary reason for the Falcon line, right now, is to generate revenue to fund ITS. That means that SpaceX launches won't necessarily get much cheaper. SpaceX is already the cheapest shop out there and they don't need more customers. It is quite possible that further cost reductions will go into increasing profit rather than cutting prices.

 

[tater]

Yeah, at this point SpaceX will not leave more money on the table than they have to. A small discount for a "flight-proven" core will be all we see for a while, they have no need to reduce prices beyond that until faced with price competition.