Jump to content

SpaceX Discussion Thread


Recommended Posts

12 minutes ago, Wjolcz said:

So since it's rideshare (which means mainly cubesats, right?) The payload is probably so light they don't need the barge for landings and can do RTLS at Vandenberg instead.

They could do that with SAOCOM 1B also, though (that's what they did with SAOCOM 1A), and yet they still changed it to the Cape. So it's not just about the droneship.

Seriously speculating here, but... maintaining facilities is expensive. Cutting an entire launch site would be a fairly effective cost-saving measure, and maybe there just aren't enough SSO/Polar missions to justify having their own launchpad. Just "Cape Kennedy" and Boca Chica might just be their new plan.

Edited by ThatGuyWithALongUsername
Link to post
Share on other sites
12 hours ago, sevenperforce said:

Ever since Elon's interview with Everyday Astronaut, I've been kicking around the utility of employing some kind of altitude-compensated Raptor...not for Starship, per se, but for something smaller. Elon claims Starship will cost on the order of $2-3M per launch, which I think is rather extraordinarily low, but even at $20M (twice his highest estimate) it's still game-changing. Yet presumably there will be many instances where you simply do not need 100 tonne or fifty people delivered to, say, the ISS.

I decided to play with a Raptor-based SSTO using GNOM-style air-augmentation and altitude compensation, with the dedicated goal of developing a vehicle with the same capacity as Falcon 9+Dragon 2. With a fixed vacuum-sized nozzle integral with an inlet duct, the Raptor would have air-augmented altitude compensation as well as full expansion in vacuum, without the pain and suffering of an aerospike. I went with a single-engine design.

Yes, there is a reason why airplanes range from single-seaters that can fly a few hundred miles to those that can carry 500-600 halfway around the world. No matter how efficient your big vehicle is on a per-payload-mile basis, it can still be absolutely cheaper to use the smallest, least-capable vehicle that will do the required mission.

A single engine design is a problem for SpaceX, though, because it likely has too much TWR for landing. They might need to (gasp!) design it to land aerodynamically.

Edited by mikegarrison
Link to post
Share on other sites
16 minutes ago, mikegarrison said:

A single engine design is a problem for SpaceX, though, because it likely has too much TWR for landing. They might need to (gasp!) design it to land aerodynamically.

Even for larger vehicles, the question is worth asking: What is the mass of wheels vs reserved propellants for landing? If it flew the same profile, then instead of plummeting straight down, it glided (steeply) as a lifting body... Nothing chnages except they trade landing props and a burn for a runway.

Like this General Dynamics concept for the upper stage, anyway:

NASM-A19760781000_PS01.jpg?itok=73DqjivA

(this is obviously a dedicated LEO vehicle)

Edited by tater
Link to post
Share on other sites
5 hours ago, ThatGuyWithALongUsername said:

A Shortfall Of Gravitas is cryptically still on the table. Out of curiosity, what kind of ASDS might Starship/SuperHeavy need?

 

 

If it’s an ASDS designed specifically with Super Heavy in mind, then that’s a big droneship :o

Link to post
Share on other sites
4 minutes ago, RealKerbal3x said:

If it’s an ASDS designed specifically with Super Heavy in mind, then that’s a big droneship :o

Or it could be a Falcon sized droneship, so they can have 2 on the east coast if they want to do a center core expended, dual side core droneship falcon heavy Launch (might come in handy for Artemis if that happens) and one on the west coast.

I don't see why they would want a starship droneship except for maybe some testing. I would guess that Point to point requires more of an oil rig than a droneship.

It would increase payload by a bit, but what needs more than 100 tons in one launch but doesn't need refueling (refueling requires rapid turnaround and drone ship landings do not offer that)?

Link to post
Share on other sites

We know Musk had a secondary motive for deciding to use propulsive landing. He wanted to develop it for use in places without a runway (like Mars). But so far the only reusable space vehicles that have actually returned from orbit have all either used parachutes or gliding landings. I assume SpaceX would not rule out glide landings where they made sense.

Link to post
Share on other sites
7 hours ago, tater said:
7 hours ago, mikegarrison said:

A single engine design is a problem for SpaceX, though, because it likely has too much TWR for landing. They might need to (gasp!) design it to land aerodynamically.

Even for larger vehicles, the question is worth asking: What is the mass of wheels vs reserved propellants for landing? If it flew the same profile, then instead of plummeting straight down, it glided (steeply) as a lifting body... Nothing chnages except they trade landing props and a burn for a runway.

It's not just the wheels; you do need a certain minimum L/D ratio to make horizontal landing possible, setting aside the mass of the landing gear. Starship would likely need double the "wing" area to make it work, plus more control surfaces.

My concept would use hot-gas thrusters for auxiliary attitude control on entry and descent as well as for the terminal landing burn. 

 

Link to post
Share on other sites

Besides, wheels and wings are kind of useless when you want to land on airless or near-airless bodies. Developing a special horizontal landing variant would probably cost more to develop and test than what they already have.

That being said, I have to say I do love the look of this:

images?q=tbn:ANd9GcTlvBY6LwVj1hOKOWRZbgt

Edited by Wjolcz
Link to post
Share on other sites
9 hours ago, mikegarrison said:

Yes, there is a reason why airplanes range from single-seaters that can fly a few hundred miles to those that can carry 500-600 halfway around the world. No matter how efficient your big vehicle is on a per-payload-mile basis, it can still be absolutely cheaper to use the smallest, least-capable vehicle that will do the required mission.

A single engine design is a problem for SpaceX, though, because it likely has too much TWR for landing. They might need to (gasp!) design it to land aerodynamically.

And that's one reason SpaceX wont touch that design with a 10' pole. Leave runway landings to Virgin or Boeing.

Link to post
Share on other sites
5 hours ago, sevenperforce said:

you do need a certain minimum L/D ratio to make horizontal landing possible

L/D controls your glide ratio, so if it's bad enough your sink rate becomes unusably high. But when you flare for landing you move outside of your sustainable glide ratio and turn now-unwanted horizontal speed energy into lift to counter your sink rate. I suppose there is still some minimum value to L/D that allows this, but it's not very high. The lifting bodies demonstrated this. The HL-10 had a max L/D of 3 and landed at a descent angle of about -18 degrees (compared to the typical -3 degrees for an airplane). But it only crashed one time, and that was probably due to pilot error more than anything else (the pilot was worried about hitting a helicopter and failed to deploy the landing gear in time).

Edited by mikegarrison
Link to post
Share on other sites
12 minutes ago, mikegarrison said:

...when you flare for landing you move outside of your sustainable glide ratio and turn now-unwanted horizontal speed energy into lift to counter your sink rate. I suppose there is still some minimum value to L/D that allows this, but it's not very high. The lifting bodies demonstrated this. The HL-10 had a max L/D of 3 and landed at a descent angle of about -18 degrees (compared to the typical -3 degrees for an airplane).

I would hazard that the Starship has a considerably lower L/D ratio than the HL-10. Whether it would be enough to execute a landing flare is anyone's guess.

Link to post
Share on other sites
1 hour ago, sevenperforce said:

I would hazard that the Starship has a considerably lower L/D ratio than the HL-10. Whether it would be enough to execute a landing flare is anyone's guess.

Probably not, but it wasn't designed for it. I'm not even sure they did any trade studies considering it, because Musk wanted vertical landing right from the start in order to fit his Mars paradigm. We know they did consider parachutes early on for the Falcon, but that's different. (Gemini considered a hybrid idea, using a parawing, but eventually went with the already-developed parachute system.)

Edited by mikegarrison
oops, vertical landing not horizontal landing
Link to post
Share on other sites

Probably they could use only drogue chute + rocket landing from tens of meters, not fully parachute landing.
When even short Soyuz overturnes "every second" landing due to the horizontal speed inevitably caused by wind. Better chute = better sail.

Edited by kerbiloid
Link to post
Share on other sites
6 hours ago, sevenperforce said:

It's not just the wheels; you do need a certain minimum L/D ratio to make horizontal landing possible, setting aside the mass of the landing gear. Starship would likely need double the "wing" area to make it work, plus more control surfaces.

My concept would use hot-gas thrusters for auxiliary attitude control on entry and descent as well as for the terminal landing burn. 

 

This, also feel that other stuff also get much more complicated fast. 
Has been loads of talk of an flyback booster or first stage, pretty much all used wings and none got developed. 
Yes foldable wings works for cruise missiles but get harder and harder as size go up. 
Add that the extra fuel is also an safety margin. Say an falcon 9 have an engine out some seconds after launch, this does not affect the mission but you loose the first stage as it has to use the landing fuel to do an longer burn because lower TWR.
Starship has the abort to orbit and refuel down the line. 
So even if an runway landing is more economical its more expensive to develop and less flexible.  

Link to post
Share on other sites
2 minutes ago, magnemoe said:

This, also feel that other stuff also get much more complicated fast. 
Has been loads of talk of an flyback booster or first stage, pretty much all used wings and none got developed. 
Yes foldable wings works for cruise missiles but get harder and harder as size go up. 
Add that the extra fuel is also an safety margin. Say an falcon 9 have an engine out some seconds after launch, this does not affect the mission but you loose the first stage as it has to use the landing fuel to do an longer burn because lower TWR.
Starship has the abort to orbit and refuel down the line. 
So even if an runway landing is more economical its more expensive to develop and less flexible.  

Hence not using a runway landing.

Link to post
Share on other sites
19 hours ago, ThatGuyWithALongUsername said:

They could do that with SAOCOM 1B also, though (that's what they did with SAOCOM 1A), and yet they still changed it to the Cape. So it's not just about the droneship.

Seriously speculating here, but... maintaining facilities is expensive. Cutting an entire launch site would be a fairly effective cost-saving measure, and maybe there just aren't enough SSO/Polar missions to justify having their own launchpad. Just "Cape Kennedy" and Boca Chica might just be their new plan.

Boca Chica has an pretty limited window to launch. Not many degree south to enter Mexican water. 
Mush should buy an Caribbean island for flexibility, he can probably get an white cat as an bonus :)
 

Link to post
Share on other sites

The American flag being backwards really bugs me. (guess the wall direction theoretically matters, but I'm used to the Union on the left)

Edited by tater
Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...