totm nov 2023 SpaceX Discussion Thread

[Streetwind]

3 minutes ago, magnemoe said:

I wonder if it wather resistance rather than reentry who is the main issue, Space shuttle had no issues with the landing legs, some capsule designs even had hatches in heat shield. 

The shuttle didn't use an ablative material, though. If the shielding physically melts away, how do you guarantee that it doesn't open a dangerous gap midway through - or fuse the gap shut?

Of course, that's not saying that water intrusion would not be an issue...

[kerbiloid]

16 minutes ago, Streetwind said:

If the shielding physically melts away, how do you guarantee that it doesn't open a dangerous gap midway through - or fuse the gap shut?

The mentioned TKS VA had a kind of ablative material (the bottom hatch penetrated through it) and this didn't stop it from being happily reused and be "floatable".

Legs just have no purpose without landing engines.

Upd.: Legs just shift the center of mass up, increasing chances of an overturn. They make sense when engines kill the horizontal speed and the capsule lands verically. Then they absorb the hit energy.

Edited July 20, 2017 by kerbiloid

[Streetwind]

6 minutes ago, kerbiloid said:

The mentioned TKS VA had a kind of ablative material (the bottom hatch penetrated through it) and this didn't stop it from being happily reused and be "floatable".

That's nice. I can't offer you more than a statement given by SpaceX, though; I'm not part of the company and I'm not privy to the details.

[CatastrophicFailure]

21 minutes ago, Streetwind said:

The shuttle didn't use an ablative material, though. If the shielding physically melts away, how do you guarantee that it doesn't open a dangerous gap midway through - or fuse the gap shut?

Of course, that's not saying that water intrusion would not be an issue...

IIRC, reentry welding the heat shield hatch shut wasn't just expected on Gemini B, it was required for the system to work right in the first place. So definitely a valid concern. 

[DDE]

6 hours ago, tater said:

They're not ditching retropropulsion, the dracos are still there, and the capsule could still land... they are ditching the landing gear. 

But if they are now opting for a parachute landing (what I'm taking from Musk's quote) why even have the engines?

[kerbiloid]

Just btw.
They say "hypergolic rockets are unsafe and toxic", but they find absolutely normal to land with a tank of the same hydrazine+NTO inside a crewed commercial civil capsule...
At least PTK NP is going to use powder rockets.

Edited July 20, 2017 by kerbiloid

[CatastrophicFailure]

Mostly a rehash of the above but somewhat relevant:

https://spaceflightnow.com/2017/07/19/propulsive-landings-nixed-from-spacexs-dragon-spaceship/

[rudi1291]

28 minutes ago, DDE said:

But if they are now opting for a parachute landing (what I'm taking from Musk's quote) why even have the engines?

On orbit maneuvering and LES. Probably also for revisiting the concept later. The problem are not the engines, or landing, they are the landing legs...

[DDE]

16 minutes ago, rudi1291 said:

On orbit maneuvering and LES. Probably also for revisiting the concept later. The problem are not the engines, or landing, they are the landing legs...

I was unaware that they've completely ditched the use of the trunk for fuel carriage.

[rudi1291]

http://www.ulalaunch.com/ula-signs-contract-with-sierra-nevada-crs2.aspx

Quote

Sierra Nevada Corporation (SNC) announced that it selected United Launch Alliance’s (ULA’s) commercially developed Atlas V rocket to launch the first two missions of its Dream Chaser cargo system in support of NASA’s Cargo Resupply Services 2 (CRS2) contract.

There are currently 2 planned missions, one in 2020, another in 2021. The Atlas V configuration chosen is the Atlas V 552 (5 SRBs, 5.4m fairing, 2-engine upper stage), which until today never flew in that configuration.

Edited July 20, 2017 by rudi1291

[tater]

2 hours ago, DDE said:

But if they are now opting for a parachute landing (what I'm taking from Musk's quote) why even have the engines?

SpaceX commercial crew has always been a parachute landing at sea. Nothing changed.

The dracos were always the LES system. Landing was secondary for them. Right now they will still offer a backup should the parachute fail, D2 can land, it just needs a soft pad since there are no feet. They judged making the feet safe harder than it was worth it since they have to demonstrate very low %s for LOC incidents and any break in the TPS is a possible failure mode.

[NSEP]

Oh really? I forgot about Dream Chaser completely! I thought the project was dead. Chase for your dreams i geuss.

[kerbiloid]

53 minutes ago, tater said:

Right now they will still offer a backup should the parachute fail

With empty tanks?

53 minutes ago, tater said:

The dracos were always the LES system. Landing was secondary for them.

Looks like the primary was landing, though.

4x2 engines, making T/W = 6 together, capable to be throttled down to 70% iirc.
Exactly for landing if even two engines are gone, and two opposite are disabled.

While T/W = 6 look very weak compared to 12-20 g on other spacecrafts.

Also, iirc, the hype was mostly about a soft landing taxi for civil passengers.

Also, iirc, there was some disappointment on the forum when NASA required only parachute landing for their flights, instead of the planned, on engines.

54 minutes ago, tater said:

D2 can land, it just needs a soft pad since there are no feet.

And D2 is vertical-shaped. Looks like it was designed exactly to be vertically landed. 
Without engine it will fall aside even more often than Soyuz.

Edited July 20, 2017 by kerbiloid

[NSEP]

I never expected the crewed D2 to have powered landing anyways, at least on their first flights. You know, the same way the first Falcon 9 did not try to land in its first flight.

Edited July 20, 2017 by NSEP

[DDE]

@tater, I agree with @kerbiloid that this system is overdesigned for LES work alone. Basically, there's no point of having it around at all if you're not using it for propulsive landing.

If you can recover a fairing, why not recover a fairing with some expended solid motors strapped to them?

Edited July 20, 2017 by DDE

[KSK]

Pinched from the comments over at Ars Technica

 

Elon's comments on the first FH flight are also candid.

Quote:

First of all, I should say that Falcon Heavy requires the simultaneous ignition of 27 orbital class engines. There's a lot that could go wrong there. And I encourage people to come down to the Cape and see the first Falcon Heavy mission. It's guaranteed to be exciting. But it's one of those things that's really difficult to test on the ground. We can fire the engines on the ground and try to simulate the dynamics of having 27 orbital booster engines and the airflow as it goes transonic. It's going to see heavy transonic buffeting. It's behavior at Max Q, there's a lot of risks associated with Falcon Heavy. Real good chance that that first vehicle doesn't make it to orbit. So I want to make sure to set expectations accordingly. I hope it makes it far enough away from the pad that it's not going to cause damage. I would consider that a win, honestly. And yeah. Major pucker factor is the only way to describe it. I think Falcon Heavy is going to be a great vehicle. There's just a lot that's impossible to test on the ground. And we'll do our best.

And it ended up being way harder to do Falcon Heavy than we thought. Because at first it sounds really easy to just stick to first stages on as strap-on side boosters. But then everything changes. The loads change, the air dynamics totally change. You triple the vibration and acoustics. So you break the qualification levels and so much of the hardware. The amount of load you’re putting through that center core is crazy because you have two super powerful boosters also shoving that center core. So we had to redesign the whole center-core airframe on the Falcon 9 because it’s going to take so much load. And then you’ve got the separation systems... and, yeah, it just ended up being way way more difficult than we originally thought. We were pretty naive about that.

But the next thing is that we're going to fully optimize it. It has about 2.5 times the payload capacity of the Falcon 9. We’re well over 100,000 lb to LEO payload capability. And then it has enough thrust performance to put us in a loop with Dragon 2 around the moon. And Dragon itself, the heat shield is designed with a huge amount of margin. So it has enough margin to handle a lunar reentry. But no question, whoever is on the first flight, brave.

 

Emphasis added. I'm sure the comment about naivete will have caused some 'we told you so' eye-rolling at other companies but I think the lack of sugarcoating from Musk is refreshing. Likewise, there's something endearing about a CEO who will cheerfully admit to 'major pucker factor' like that. Or maybe that's just the six year old in me.  

On a more serious note, that last paragraph is interesting. Looks like FH is going to get some love, rather than being quickly passed over for the Small Falcon Booster / Raptor 9 / whatever they decide to call the recently announced vehicle.

Edit: I think I may keep a copy of that second paragraph around somewhere for use in the more off-the-wall armchair rocket design discussions that seem to happen round this way.

Edited July 20, 2017 by KSK

[Elthy]

9 minutes ago, KSK said:

I think I may keep a copy of that second paragraph around somewhere for use in the more off-the-wall armchair rocket design discussions that seem to happen round this way.

Yeah, KSP makes rockets seem way, way to easy.

So are they only building a special version of the Falcon 9 for the center core of the Falcon Heavy or will those neccessary changes also make their way into the "normal" Falcon 9? Both versions are viable, you either save mass or keep the manufacturing costs down by having less differences between the rockets...

[tater]

They won't be changing D2 this late in the game, that's why. It will fly crew, and a major redesign would delay them. It has margin to ISS anyway, so D2 propulsion will be a backup to chutes for splashdown.

[kerbiloid]

After they had spent most of fuel to reach ISS and deorbit?

Probably, its fuel tank will be not a backup, but a mine made of flammable and toxic chemicals.

Maybe, if they are going to refuel from ISS with the fuel delivered by a cargo ship.

Edited July 20, 2017 by kerbiloid

[tater]

Apollo used hypergolics up top. So did shuttle. It's not a big deal.

The D2 design gelled (it's not just Dragon) and since they were not going to do propulsive landing as part of commercial crew, they decided to not waste time and effort on making the gear work.

I see this as a plus if they get working on the next project in earnest sooner. They have said that they want block 5 and crew flying for NASA first, then FH as needed, then people can transition to whatever they are calling BFR these days in real numbers.

[Kryten]

2 minutes ago, tater said:

Apollo used hypergolics up top. So did shuttle. It's not a big deal.

The Apollo system put the ASTP crew in hospital, and could've easily done worse.

[tater]

3 minutes ago, Kryten said:

The Apollo system put the ASTP crew in hospital, and could've easily done worse.

Hypergolic's are nasty, we all know that. That incident didn't stop them from using it on Shuttle. If NASA was concerned, it would not be there. Ditto Orion. And CST-100, I presume, they're using hypergolic thrusters as well, right?

[TheEpicSquared]

46 minutes ago, kerbiloid said:

After they had spent most of fuel to reach ISS and deorbit?

Probably, its fuel tank will be not a backup, but a mine made of flammable and toxic chemicals.

Maybe, if they are going to refuel from ISS with the fuel delivered by a cargo ship.

Eight SuperDracos are overpowered for orbital maneuvers. I think they'll use the Draco thrusters for orbital maneuvering.  

[tater]

Ninjaed. Dragon already uses the thrusters for maneuvering and deorbit. The superdracos would not be used at all on D2 I think. I would imagine that they might experiment with them at some point for cargo returns, particularly with capsules nearing end of life. They could soft-land in the water, for example.

Edited July 20, 2017 by tater

[kerbiloid]

21 minutes ago, TheEpicSquared said:

Eight SuperDracos are overpowered for orbital maneuvers.

Forget dracos, They use same fuel tank, that's what I mean.

We all could see their launch abort test.
6 g acceleration, 6 s duration.
We can presume, that all its engines have more or less same ISP, that means: Dragon v2 has enough fuel for ~360 m/s delta-V.

Deorbiting takes 100...150 m/s.
Reaching ISS, say, at least ~100 m/s.

So, other spaceships have 300..400 m/s delta-V onboard.

After a Dragon spent that (at least) 100+150 = 250 m/s for ISS and deorbiting, it has a rest of fuel ~100 m/s.
Its free-fall speed, say, 150 m/s (as usual), and it must have even more because it doesn't burst it at once, but needs, say, 10 seconds, to land after engines got ignited.
So, gravitational loss ~ 9.81 * 10 = 100 m/s more.

So, to land on engines it needs at least 250 m/s while it has at most 100 m/s.

Which engines to use here doesnt matter.

Edited July 20, 2017 by kerbiloid