totm nov 2023 SpaceX Discussion Thread

[tater]

I think the metal idea might fit. Trying to think what was counterintuitive about that.

 

[Piscator]

It could very well be, that the counter-intuitive part might still refer to something else than the change in material. Having the "countour staying aproximately the same" still allows for the position of the fins to be switched arround, which seemed like the most plausible idea so far. Having a fin on the "underside" of the vessel is pretty counter-intuitive.

[tater]

 

14 hours ago, Piscator said:

It could very well be, that the counter-intuitive part might still refer to something else than the change in material. Having the "countour staying aproximately the same" still allows for the position of the fins to be switched arround, which seemed like the most plausible idea so far. Having a fin on the "underside" of the vessel is pretty counter-intuitive.

Yeah, That's my take as well. Y with the | down, and no major hinge point. Maybe smaller control surfaces at the tips without the huge force required to move them. The hinge seems like a terrible point of failure, since any failure of the fin hinge to completely deploy makes landing impossible.

[sh1pman]

Are there any good estimates of how much fuel F9 S1 has to reserve for ASDS and RTLS landings? I guess it's somewhere around 10% and 20%, respectively, but I may be wrong.

Edited November 26, 2018 by sh1pman

[tater]

5 minutes ago, sh1pman said:

Are there any good estimates of how much fuel F9 S1 has to reserve for ASDS and RTLS landings? I'm guess it's somewhere around 10% and 20%, respectively, but I may be wrong.

Elon said at some point:

Quote

We effectively lose, in terms of performance... It really depends on what we want to do with the stage if we were to do an ocean landing or a return to launch site landing. If we do an ocean landing, the performance hit is actually quite small at maybe in the order of 15%. If we do a return to launch site landing, it's probably double that, it's more like a 30% hit (i.e., 30% of payload lost).

 

[Spaceception]

18 hours ago, RedKraken said:

I can't see elon getting excited about that.

1

Ignoring that this is actually their idea, or whether it's counterintuitive, why not?

Just off the top of my head, not sure if this is completely right;

 

The main drawback with the FH is faring size. It can't do big and bulky payloads.
SpaceX wants to replace the Falcon lineup with the BFR/Starship.
A ship slightly more capable, but with a much wider and taller faring than the FH would allow bigger payloads, and more flexibility.
It'd also be fully reusuable, and refuelable in space, unlike FH/F9. And could still carry an appreciable amount of cargo (And people) to Mars.
And possibly, it'd allow them to build it faster (As mentioned earlier in this thread), saving the carbon fiber BFR/Starship down the line when they're more experienced/making more money.

This would be a somewhat better version of the New Glenn, now that I think about it.

Edited November 26, 2018 by Spaceception
drawback makes more sense

[tater]

That's the cost in payload to LEO not exact propellant mass.

[sh1pman]

2 minutes ago, tater said:

Elon said at some point:

 

Yeah, I'm actually more interested in the prop reserve fraction, because I'm making a rocket calculator that can account for reuse. 

I'll use 20% and 10% for now, these numbers give pretty realistic payload reduction fractions.

Edited November 26, 2018 by sh1pman

[tater]

I've seen simulations (reddit) that show RTLS (for a CRS flight) used 13.6% (56t) was used, and for an ASDS landing (Formosasat), it was 9% (37t).

Both those simulations had reserve props left over upon landing (9 and 4 t, respectively).

[sh1pman]

1 minute ago, tater said:

I've seen simulations (reddit) that show RTLS (for a CRS flight) used 13.6% (56t) was used, and for an ASDS landing (Formosasat), it was 9% (37t).

Both those simulations had reserve props left over upon landing (9 and 4 t, respectively).

thanks!

[RedKraken]

1 hour ago, sh1pman said:

estimates of how much fuel F9 S1 has to reserve for ASDS and RTLS landings? I'm guess it's somewhere around 10% and 20%, respectivel

I've been using 8% and 16% based on the horizontal component of the velocity at meco.

[Xd the great]

3 hours ago, RedKraken said:

I've been using 8% and 16% based on the horizontal component of the velocity at meco.

You still need to take in the vertical acceleration, and killing the vertical velocity.

[RedKraken]

3 hours ago, Xd the great said:

You still need to take in the vertical acceleration, and killing the vertical velocity.

More explicitly :

For RTLS I figured deltaV is

     2 x (Vh @ meco) + (20s Brakeburn sheds 1000m/s + 20x9.8 g loss ) + (3s landing burn sheds 170m/s + 3x9.8 g loss)

Roughly 2 x (1300m/s) + (1200 m/s) + 200m/s.

Or 4000m/s.

Most of that occurs at boostback 311s isp. The brakeburn at whatever its burn average isp is : guess 300 s. Landing burn isp  at 282 s.

Use a weight average isp of 305s for the lot.

So 23t final mass (plus margin if required), 4000m/s deltaV, 305s isp gives a start mass of 87.7t.

Fuel fraction is (87.7 - 23 ) / 411 = 0.157

Reserve is 15.7%

If you need a margin, bump up the reserve. 17% reserve lets you land with 2t of propellant left. 19% reserve lands with 5t left.

 

No boostback for ASDS, just a much bigger (~2x) brakeburn.

Edited November 27, 2018 by RedKraken
deltav

[kerbiloid]

2 hours ago, RedKraken said:

2 x (Vh @ meco) + (20s Brakeburn sheds 1000m/s

Does it require another Vh to return back, when it is already at 60 km (iirc) altitude and has some Vv (probably ~1 km/s, just a guess) ?

And does it brake Vv= - 1000 m/s only by thrust, not by drag? The speed looks aircrafty.

Edited November 27, 2018 by kerbiloid

[RedKraken]

1 hour ago, kerbiloid said:

Does it require another Vh to return back, when it is already at 60 km (iirc) altitude and has some Vv (probably ~1 km/s, just a guess) ?

Meco at +1200m/s, + 1200m/s for a bunch of RTLS (at about +100km downrange, +60km alt).

At +1200m/s in the horizontal, i need -1200m/s(WRONG! way too much) to get back to the launch site....

....if my return time was (100000/1200) 83s, but it isnt...oh boy i've stuffed up.

 

Actual return flight time is loft(1200/9.8)122s  up to 130km + descent time estimate (from 130km) is about 160s going 1600m/s if no drag/ no vert burns)

So about 280s....check landing time for booster should be around (150+280) T+430s or 7m10s.... CRS-13 booster landed at 7m40s..cool.

Say 300s for simplicity.

Estimate we use a 30s boostback burn from 100km downrange to 110km downrange.

So at meco I burn -1200m/s to zero my horizontal velocity, then only another (110km/270s) -400m/s to hit the pad...

All up 1600m/s burned in the horizontal instead of 2400. Ouch.

 

So my new RTLS deltaV =1.35Vhmeco(boostback) +1200(brakeburn) +200(landing)  m/s

assuming meco at (+1200m/s, +1200m/s) or 1700m/s prograde

 

That brings RTLS reserve down from 16% to 11%. Nice! 

Or 43t for the F9 booster.

The boostback burn is not instantaneous.

Estimate (say 66t->40t) 26t of propellant reserve burned at 311s.

26t on three engines at 300kg/s each is about (26/0.9)29s of horizontal burntime. Close to my estimate above. Good.

For (0.75x29)22s of this we are still heading downrange at an average of (1200/2)600m/s. 22s at 600m/s is 13200m.

So 13km further downrange from burn start 100km (~113km) Also close to estimate.

After a 29s burn we are heading home at 400m/s from 113km downrange with another 90s before we start descending and 300s before we land.

****I need to recheck my math and test this against some telemetry.

[kerbiloid]

27 minutes ago, RedKraken said:

1200(brakeburn)

I mean, does it really need a full brakeburn?

Its free fall terminal velocity is probably ~200 m/s.
So, the brakeburn would probably decrease the speed from -1200 to, say, 1.5..2 mach (a fighter speed), i.e. 1200 - 600 + 200 = 800 m/s/)

[RedKraken]

38 minutes ago, kerbiloid said:

I mean, does it really need a full brakeburn?

Its free fall terminal velocity is probably ~200 m/s.
So, the brakeburn would probably decrease the speed from -1200 to, say, 1.5..2 mach (a fighter speed), i.e. 1200 - 600 + 200 = 800 m/s/)

I'll see if i can find the video showing the booster velocity during the brakeburn....it was pretty amazing to me.

The velocity got chopped from 1600m/s  down to 700 m/s in  about 20s. 

Occurs about T+6min, or about 60s before the landing.

They call it the "re-entry burn" in the webcasts.

[RedKraken]

Okay : SOACOM 1A from oct 7 VAFB 

4520kph -> 2122kph in 26s, altitude 55km ->31km 

This is (1255m/s - 583m/s) + (9.8* 26s) = 672 + 255 = 927 m/s ..... closer to your 800 m/s

https://www.youtube.com/watch?v=zw4X8p5zVZE&t=6m

 

[RedKraken]

Zuma was really low.

4500->2800kph in 17s.

(1250 - 778) + (9.8*17s) = 640 m/s

https://www.youtube.com/watch?v=NQVNSwTz3qg&t=20m

 

[tater]

 

[CatastrophicFailure]

Current twitter chatter is saying SSO-A likely delayed till Dec. 1 due to weather, still waiting on official word. This is one delay I wouldn’t complain about. (Weekend launch! )

[CatastrophicFailure]

Aaaaaand Illuminati delay confirmed: 

 

[Ultimate Steve]

On the bright side, if that date sticks, I'll be able to watch it!

[tater]

 

[Spaceception]

1 hour ago, CatastrophicFailure said:

Current twitter chatter is saying SSO-A likely delayed till Dec. 1 due to weather, still waiting on official word. This is one delay I wouldn’t complain about. (Weekend launch! )

 

43 minutes ago, Ultimate Steve said:

On the bright side, if that date sticks, I'll be able to watch it!

This is one thread I'm always catching up on, and I think this is the first time I've ever seen happy reactions to a delay

But what better way to start the month than with a Falcon 9 launch?

Edited November 27, 2018 by Spaceception