totm nov 2023 SpaceX Discussion Thread

[Grand Ship Builder]

Are they even considering using the SuperDracos as retrorockets to minimize Gee force?

Like the Soyuz?

Or not, because it's landing on water.

[Nibb31]

No they are not.

The SDs are the LES and that is all they are being certified and tested for.

They will not be certified or tested for anything else, because it would cost a lot of extra money for no real benefit.

Edited September 14, 2017 by Nibb31

[PakledHostage]

19 hours ago, TheEpicSquared said:

Very kerbal. 

I like the Monty Python references.

Edited September 15, 2017 by PakledHostage

[Fearless Son]

On 9/14/2017 at 2:10 AM, TheEpicSquared said:

Very kerbal. 

I was just on my way to see if someone posted that.  Watching it give me flashbacks!  

[CastleKSide]

Anyone know to what extent the FH core is modified from the F9 FS? Like is it a 90% similar or like 30%?

[cubinator]

7 minutes ago, CastleKSide said:

Anyone know to what extent the FH core is modified from the F9 FS? Like is it a 90% similar or like 30%?

Falcon Heavy needs KJR.

[tater]

 

[Nibb31]

8 hours ago, CastleKSide said:

Anyone know to what extent the FH core is modified from the F9 FS? Like is it a 90% similar or like 30%?

SpaceX doesn't give actual figures, but based on kremlinology, it seems to be closer to 60 or 70%. The octaweb is the main difference, and the structure is reinforced.

[tater]

 

 

 

[tater]

Drogues deployed nominally. 

Mains out.

Splashdown soon. Welcome back, Dragon.

 

 

Edited September 17, 2017 by tater

[TheDestroyer111]

Why does the Dragon V2 trunk have fins? IMO they should attach them to the capsule rather than trunk, and jettison for reentry: that would give higher TWR during abort (relative to carrying trunk on abort) and/or smaller abort engines would be usable. Trunk is not recovered anyway, so no recovery bonus for fins.

Edited September 18, 2017 by TheDestroyer111

[Cuky]

fins are probably to help keep the vehicle stable during launch and especially during abort using LES. Bonus is that trunk looks less boring with them

[tater]

The fins are for LES stability leaving the stack.

[sevenperforce]

1 hour ago, TheDestroyer111 said:

Why does the Dragon V2 trunk have fins? IMO they should attach them to the capsule rather than trunk, and jettison for reentry: that would give higher TWR during abort (relative to carrying trunk on abort) and/or smaller abort engines would be usable. Trunk is not recovered anyway, so no recovery bonus for fins.

The fins are to provide passive aerodynamic stability during launch abort. For passive aerostabilization, you want the center of pressure as far back from the center of mass as you can. Like all capsules, the Dragon V2 has its center of mass very close to the heat shield to ensure that the heat shield faces into the plasma during re-entry. It would not be possible to place fins on the capsule substantially behind the center of mass.

The trunk effectively holds the fins out at a distance, ensuring that the center of pressure is far back enough for passive stabilization.

11 minutes ago, Cuky said:

fins are probably to help keep the vehicle stable during launch and especially during abort using LES. Bonus is that trunk looks less boring with them

Fins do nothing to stabilize the vehicle during launch; they actually make the stack slightly less stable. They are only useful for abort.

[kerbiloid]

According to the official site,

Quote

Dragon’s trunk supports the Dragon spacecraft during ascent to space and contains a cargo carrier designed to hold unpressurized cargo bound for the International Space Station.

Does it mean that LES has to eject also several tonnes (?) of cargo inside the trunk?

[CatastrophicFailure]

33 minutes ago, kerbiloid said:

According to the official site,

Does it mean that LES has to eject also several tonnes (?) of cargo inside the trunk?

I don't think the trunk can hold that much. Probably even less for a manned flight, for safety. The point of the D2 is crew delivery, after all, cargo is secondary. 

[TheDestroyer111]

17 hours ago, sevenperforce said:

It would not be possible to place fins on the capsule substantially behind the center of mass.

You mean that this:

Spoiler

will be unstable after adding a heat shield?

[TheEpicSquared]

6 hours ago, TheDestroyer111 said:

You mean that this:

  Reveal hidden contents

will be unstable after adding a heat shield?

I'm pretty sure that ksp ≠ real life lol

Edited September 19, 2017 by TheEpicSquared

[sevenperforce]

8 hours ago, kerbiloid said:

According to the official site,

Does it mean that LES has to eject also several tonnes (?) of cargo inside the trunk?

Those SuperDracos are more powerful than the Kestrel engine on the second stage of Falcon 1. They have no trouble with aborts.

There may be a mechanism or design to jettison cargo on abort, but I doubt it.

45 minutes ago, TheDestroyer111 said:

17 hours ago, sevenperforce said:

 It would not be possible to place fins on the capsule substantially behind the center of mass.

You mean that this:

  Reveal hidden contents

will be unstable after adding a heat shield?

Note that I said on the capsule. What you've got are fins floating underneath the capsule, held in place by KSP magic juice.

But, by all means, feel free to test it out for yourself. Slap a bunch of separatrons on a Mk1 capsule, add fins, turn off the reaction wheels, and watch it tumble.

[TheDestroyer111]

3 minutes ago, sevenperforce said:

Note that I said on the capsule. What you've got are fins floating underneath the capsule, held in place by KSP magic juice.

But, by all means, feel free to test it out for yourself. Slap a bunch of separatrons on a Mk1 capsule, add fins, turn off the reaction wheels, and watch it tumble.

Actually they are slightly clipped into the capsule rather than floating, and unless the abort mechanism has a manual override for the controls combined with a pilot who never even, the lift created by the fins will be very small so having such a small connection is not an issue.

In KSP (without FAR tho) attaching Sepratrons and Basic Fins to a Mk1 or Mk1-2 (2.5m) command pod's side wall (fins should really be attached a little lower) gives a craft that automatically faces forward when trying to destabilize it with reaction wheels in any part of the flight.

IRL you have a computer do the abort sequence, which won't give you any meaningful sideslip during the abort burn, and once the engines burn out, stability won't matter.

[sevenperforce]

1 hour ago, TheDestroyer111 said:

Actually they are slightly clipped into the capsule rather than floating, and unless the abort mechanism has a manual override for the controls combined with a pilot who never even, the lift created by the fins will be very small so having such a small connection is not an issue.

In KSP (without FAR tho) attaching Sepratrons and Basic Fins to a Mk1 or Mk1-2 (2.5m) command pod's side wall (fins should really be attached a little lower) gives a craft that automatically faces forward when trying to destabilize it with reaction wheels in any part of the flight.

IRL you have a computer do the abort sequence, which won't give you any meaningful sideslip during the abort burn, and once the engines burn out, stability won't matter.

Here, I'll show you. I don't have FAR, but even in stock KSP the aero model is able to accurately model basic CoM/CoP interactions.

No LES:

Spoiler

Basic capsule, no LES. I've set up the abort to ignite a Flea (to simulate a booster RUD) and decouple. Note disabled reaction wheels.

Lifting off without SAS.

Abort!

Starting to roll immediately.

Tumbling wildly.

LES, engines only:

Spoiler

Let's throw some LES engines on here to boost us clear of the fireball!

Approaching Max-Q.

Abort!

So far so good...

Uh oh.

Nooooooo!

Poor Assy is smeared all over the walls of the capsule at this point.

LES, fins on capsule:

Spoiler

Let's add some fins. Fins make things stable, right?

Note where the CoM is. You should already be able to tell that this is a big problem.

Lifting off.

Aborting.

Made it a little farther...

But, nope. Gees are even worse in this example.

Smeared once again.

Obviously, you don't want to flip during an abort because you could end up pushing yourself back into the exploding booster. But flipping under acceleration is just as deadly. At 14 gees, your harness will cut you into ribbons...and even if it doesn't, your organs will scramble.

LES, smaller fins on capsule base:

Spoiler

Solution: move the fins down?

Smaller fins, lower down. Better, right?

Uppppp!

Uh oh.

And, dead once again.

LES, fins on trunk:

Spoiler

Finally, here's how to properly do it. The tank is drained; I only used it as a structural element.

As you can see, the fins are COMPLETELY behind the CoM.

Lifting off.

Abort trigger.

Rapidly pushing out of the fireball.

Still climbing, perfectly straight.

Well clear of any debris or shrapnel.

As you can see, LES is passively stable ONLY when the fins are placed significantly behind the capsule's center of mass. Real-life capsules do not have high-torque reaction wheels like KSP and they cannot rely on computers to use gimbaling or differential throttling in an abort situation.

[TheDestroyer111]

1 hour ago, sevenperforce said:

Here, I'll show you. I don't have FAR, but even in stock KSP the aero model is able to accurately model basic CoM/CoP interactions.

No LES:

  Reveal hidden contents

Basic capsule, no LES. I've set up the abort to ignite a Flea (to simulate a booster RUD) and decouple. Note disabled reaction wheels.

Lifting off without SAS.

Abort!

Starting to roll immediately.

Tumbling wildly.

LES, engines only:

  Reveal hidden contents

Let's throw some LES engines on here to boost us clear of the fireball!

Approaching Max-Q.

Abort!

So far so good...

Uh oh.

Nooooooo!

Poor Assy is smeared all over the walls of the capsule at this point.

LES, fins on capsule:

  Reveal hidden contents

Let's add some fins. Fins make things stable, right?

Note where the CoM is. You should already be able to tell that this is a big problem.

Lifting off.

Aborting.

Made it a little farther...

But, nope. Gees are even worse in this example.

Smeared once again.

Obviously, you don't want to flip during an abort because you could end up pushing yourself back into the exploding booster. But flipping under acceleration is just as deadly. At 14 gees, your harness will cut you into ribbons...and even if it doesn't, your organs will scramble.

LES, smaller fins on capsule base:

  Reveal hidden contents

Solution: move the fins down?

Smaller fins, lower down. Better, right?

Uppppp!

Uh oh.

And, dead once again.

LES, fins on trunk:

  Reveal hidden contents

Finally, here's how to properly do it. The tank is drained; I only used it as a structural element.

As you can see, the fins are COMPLETELY behind the CoM.

Lifting off.

Abort trigger.

Rapidly pushing out of the fireball.

Still climbing, perfectly straight.

Well clear of any debris or shrapnel.

As you can see, LES is passively stable ONLY when the fins are placed significantly behind the capsule's center of mass. Real-life capsules do not have high-torque reaction wheels like KSP and they cannot rely on computers to use gimbaling or differential throttling in an abort situation.

Mounting fins way ahead of where they should be is the best place to start if you want to tumble. What reason is there for you to avoid mounting the fins properly on the back of the vehicle? That's not just KSP magic, as long as the fins touch the capsule and don't float in the air, you can build it IRL.

BTW look at the pad abort test. The dragon stands on the ground facing straight up, but the trunk goes sideways thanks to weird magic™? Actually, no, not magic, differential throttle is at work here. And what makes you think this is not done by a computer but by a human in the capsule or at mission control? And in a real in-flight abort, you also have to go sideways to avoid the launch vehicle so this is not just for pad aborts.

[sevenperforce]

26 minutes ago, TheDestroyer111 said:

What reason is there for you to avoid mounting the fins properly on the back of the vehicle? That's not just KSP magic, as long as the fins touch the capsule and don't float in the air, you can build it IRL.

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.

26 minutes ago, TheDestroyer111 said:

BTW look at the pad abort test. The dragon stands on the ground facing straight up, but the trunk goes sideways thanks to weird magic™? Actually, no, not magic, differential throttle is at work here. And what makes you think this is not done by a computer but by a human in the capsule or at mission control? And in a real in-flight abort, you also have to go sideways to avoid the launch vehicle so this is not just for pad aborts.

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.

[tater]

@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.

 

[sevenperforce]

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.

All because...you want a higher TWR for your abort?