totm nov 2023 SpaceX Discussion Thread

[mikegarrison]

Please stop this wave of puns.

[JoeSchmuckatelli]

1 hour ago, zolotiyeruki said:

What I'm imagining is airflow running from tail-to-nose and trying to get under the scales.

Which should help it decelerate... and presuming they are not ripped off during descent, wouldn't that be a good thing?

 

[sevenperforce]

1 hour ago, zolotiyeruki said:

What I'm imagining is airflow running from tail-to-nose and trying to get under the scales.

The angles of entry would make the scaled tiles problematic. The direction of airflow goes in one way on ascent, another way during peak hearing, another way on terminal descent, and yet another way during the landing burn. The shear forces from the airflow are nonlinear with respect to air density and air speed. I can’t imagine there would be a scale tile orientation that would work for all of those. 

[JoeSchmuckatelli]

Makes sense

 

[derega16]

I'm thinking about scale tile, what if the cross section of the tile is wedge or slope shape? It should have less heat spot

[JoeSchmuckatelli]

22 minutes ago, derega16 said:

I'm thinking about scale tile, what if the cross section of the tile is wedge or slope shape? It should have less heat spot

I think @sevenperforce answers this best: the problem isn't about making a good streamlined shape for one direction of movement.  The reentry vehicle is going to approach the air from a whole variety of differing angles at different times in its flight.   Scales are excellent if you want to go nose- forward... but SS only flies that way during launch.

Like you, I first thought that a scale design makes more sense - but when you look at the answers to my question above - you see that smooth aerodynamics on the way up isn't really an issue, and the added cost and complexity for such minor nose forward gain isn't worth the hassle.  Back to what sevenperforce said - it's going to belly flop the reentry - not dive nose in.  Then its going to fall, again belly horizontal for a ways, before kick-flipping for a tailward descent.  Scales won't help with most of that.

[JoeSchmuckatelli]

How much can SS bring back with it and still land?

(presuming they figure out how to take it up and back down successfully).

 

We often talk about the payload to LEO or beyond... but that's usually a limit based on lift capability.  I've not seen here much discussion about descent capacity.

i.e. lets say EM and SX succeed beyond all rational hope and within 10 years we have a hundred SS's flying regularly and costs are waaaaay down.  Intrepid AsMining GmbH Ltd Partners and Associates Incorporated LLC decides to buy their own SS and configures it for asteroid mining and retrieval.  They find a likely candidate asteroid, shred it and centrifuge out the good stuff... how much goodstuff could they reasonably bring back in a single load?

[magnemoe]

4 hours ago, JoeSchmuckatelli said:

How much can SS bring back with it and still land?

(presuming they figure out how to take it up and back down successfully).

We often talk about the payload to LEO or beyond... but that's usually a limit based on lift capability.  I've not seen here much discussion about descent capacity.

i.e. lets say EM and SX succeed beyond all rational hope and within 10 years we have a hundred SS's flying regularly and costs are waaaaay down.  Intrepid AsMining GmbH Ltd Partners and Associates Incorporated LLC decides to buy their own SS and configures it for asteroid mining and retrieval.  They find a likely candidate asteroid, shred it and centrifuge out the good stuff... how much goodstuff could they reasonably bring back in a single load?

Don't think its known, my guess is 100 ton or the same as it can take into orbit, it makes sense if you want to take something to space for testing. 
Its three constrains I know about, first is heating, an heavier ship need to get rid of more kinetic energy as heat. Second is aerodynamic or how nose heavy it can be, last is fuel needed to land and here you are limited by the header tanks, you could add an  set of rearward header tanks for more landing fuel. 

[Deddly]

Asporationally, the idea is to be able to send 100 people at a time to Mars, right? One would hope they would be capable of also bringing 100 home again - so it should be able to land with whatever 100 people + luggage + consumables + life support weighs. 

[kerbiloid]

7 hours ago, JoeSchmuckatelli said:

How much can SS bring back with it and still land?

Still not proven.

[Deddly]

10 minutes ago, kerbiloid said:

Still not proven.

This is of course true. I think it's clear that @JoeSchmuckatelliwas talking about the projected numbers, don't you think? 

[sh1pman]

2 hours ago, kerbiloid said:

Still not proven.

At this point, the only untested part of landing is atmospheric entry. Which was done hundreds of times by a similarly shaped and heat-shielded thing - Shuttle orbiter. Also, USSR did it the first time with Buran. Everything after atmospheric entry is already tested.

[kerbiloid]

12 minutes ago, sh1pman said:

Which was done hundreds of times by a similarly shaped and heat-shielded thing - Shuttle orbiter.

Nothing common in shape with the Shuttle orbiter, as it was described not once before.

[sh1pman]

16 minutes ago, kerbiloid said:

Nothing common in shape with the Shuttle orbiter, as it was described not once before.

I said similar, not same. Flat vs cylindrical bottom, static wings vs moving flaps. Big deal? Other than that, doesn’t look radically different. Heat tiles are very similar. Stainless steel body is much harder to melt than aluminium. Attitude control with flaps is already tested (just not at hypersonic speeds).

Also, simulations are now much better than what Shuttle designers had.

Edited September 29, 2021 by sh1pman

[Beccab]

Please not this discussion again

[sevenperforce]

15 minutes ago, kerbiloid said:

Nothing common in shape with the Shuttle orbiter, as it was described not once before.

The Buran and STS orbiters had a triangular cross section with flat control surfaces. Triangular-cross-section capsules work. The Buran and STS orbiters worked.

Starship has a circular cross section with flat control surfaces.  Circular-cross-section capsules work. Starship will work.

 

[Lisias]

5 hours ago, magnemoe said:

Don't think its known, my guess is 100 ton or the same as it can take into orbit, it makes sense if you want to take something to space for testing. 

Nope. The descent cargo capacity is usually way less than the ascent!

5 hours ago, magnemoe said:

Its three constrains I know about, first is heating, an heavier ship need to get rid of more kinetic energy as heat. Second is aerodynamic or how nose heavy it can be, last is fuel needed to land and here you are limited by the header tanks, you could add an  set of rearward header tanks for more landing fuel. 

You forgot a 4th constrain. Time.

When you ascend, the vehicle has about 10 minutes from launch to orbit, only about 2 or 3 inside atmosphere on the initial stages of flight when the speed is the lower.

When you descent, you need to dissipate almost the same amount of energy in about 2 minutes (taking Mercury as benchmark - the Space Shuttle had wings, SS does not!) - inside the atmosphere. The amount of heat and stress the structure needs to withhold are considerably higher than on ascent.

So this affects the payload. If (I'm guessing) the descent structural stress is twice the ascend, then the payload to descend will be the half - unless you oversize the vehicle to allow this - but by then, you are wasting fuel on the ascend due the extra structural mass.

(and I'm ignoring the particularities of the vehicle - since it will enter atmosphere belly down, it need to have the CoM cautiously positioned, otherwise the fins will not be able to keep it on the needed attitude and the thing will burn up - so, without the fuel on the tail, there's a well defined limit of how much mass you can carry down on the nose without having to add some ballast somehow on the fuel tanks).

[Beccab]

16 minutes ago, Lisias said:

taking Mercury as benchmark - the Space Shuttle had wings, SS does not!

That's not entirely true, the fins on starship are a very significant portion of the drag it can create. Each is currently more than 5 meters wide, and while the aft flaps are going to be smaller in S22+, nothing keeps spacex from making them a bit bigger again

[Lisias]

1 hour ago, Beccab said:

That's not entirely true, the fins on starship are a very significant portion of the drag it can create. Each is currently more than 5 meters wide, and while the aft flaps are going to be smaller in S22+, nothing keeps spacex from making them a bit bigger again

Drag,  not lift.

The Space Shuttle managed to get way less heat and stress (proportionally) than a hypothetical Mercury style capsule of the same size because it could control how fast it would sink in the atmosphere by using lift and then staying a bit longer 'up there' where the air friction is sensible less.

Time is the key. You don't want to dissipate all that speed at once (ask the few Cosmonauts that had to do a ballistic reentry). But the heavier you are, more drag you will need to prevent that. But there's a limit on the size of the fins you can attach to SS. 

It's not only the weight of the fins, but also that hydraulics to move them.

[RCgothic]

Starship is much less dense than shuttle. It doesn't need as much lift. Nor did shuttle need as much lift as it had. The enormous wings were for cross-range capability, not re-entry performance.

[Deddly]

1 hour ago, Lisias said:

It's not only the weight of the fins, but also that hydraulics to move them.

Electric motors, I believe they use. But yes, they can only do so much. 

[Lisias]

2 hours ago, RCgothic said:

Starship is much less dense than shuttle. It doesn't need as much lift. Nor did shuttle need as much lift as it had. The enormous wings were for cross-range capability, not re-entry performance.

But have no doubt that these huge wings helped a lot on increasing the descent cargo capability of it! 

[RCgothic]

Maybe. That's much more important for a vehicle that glides to a landing.

[sevenperforce]

6 hours ago, Lisias said:

The Space Shuttle managed to get way less heat and stress (proportionally) than a hypothetical Mercury style capsule of the same size because it could control how fast it would sink in the atmosphere by using lift and then staying a bit longer 'up there' where the air friction is sensible less.

Time is the key. You don't want to dissipate all that speed at once (ask the few Cosmonauts that had to do a ballistic reentry). But the heavier you are, more drag you will need to prevent that. But there's a limit on the size of the fins you can attach to SS. 

The only pure ballistic re-entry vehicles (i.e., no lift) that humans have ridden are the Soyuz capsules. And even in a Soyuz, ballistic entry is only a contingency.

The Apollo capsule had an L/D ratio of 0.38 at hypersonic speeds, increasing to about 0.68 just below Mach 2. This was somewhat better than the Mercury capsule.

In contrast, the Shuttle had a higher L/D ratio during hypersonic flight, but not dramatically higher. The STS orbiter could get an L/D ratio of about 1.5 during the peak-heating, hypersonic phase, increasing to 4.5 at subsonic speeds.  This was actually lower that what the orbiter could have attained; wind tunnel testing showed that the orbiter could have achieved an L/D ratio of up to 1.9 in hypersonic flight at the proper AoA (in this case, ~17.5°). However, the orbiter's heat shield couldn't handle entering at that angle; it had to hold an AoA of ~40° during peak heating to avoid burning off the nose and windows. 

But Starship is not the Shuttle orbiter. It has fewer design constraints since it doesn't have to be able to land horizontally, which means its heat shield can be designed for whatever AoA will provide peak L/D. A flat plate can achieve an L/D ratio of 2.2 at 17.5° or even higher at more shallow AoA (up to 3.5 at 10°).  And an ogive cylinder can achieve an L/D ratio of up to 0.85 at hypersonic speeds.

Thus, between its flaps and its body lift, it's very possible that Starship can get a hypersonic L/D ratio just as high or higher than the Shuttle orbiter. And since Starship will be much fluffier than the Shuttle, it should have even less loading. Starship's entry profile will be designed to have the highest possible AoA during the initial entry before the air gets thick so that you can slow down as much as possible, then nose farther forward to increase lift to prevent extreme gees, then tilt the nose back again for the terminal descent.

[Lisias]

21 minutes ago, sevenperforce said:

Thus, between its flaps and its body lift, it's very possible that Starship can get a hypersonic L/D ratio just as high or higher than the Shuttle orbiter. And since Starship will be much fluffier than the Shuttle, it should have even less loading. Starship's entry profile will be designed to have the highest possible AoA during the initial entry before the air gets thick so that you can slow down as much as possible, then nose farther forward to increase lift to prevent extreme gees, then tilt the nose back again for the terminal descent.

But such L/D on subsonic speeds will not hold, and the thing would fall as a rock.

Not to mention that the damned thing still needs to land!

The Shuttle could fly into an airstrip and land on her wheels on it, again thanks to her wings that would be  working to the very end.

The SS needs to do a flip and land standing on her feet - what means that the total weight at landing must not exceed the available thrust (and maneuverability) from two engines - otherwise you would need yet more fuel on landing, what implied on less payload on descent (and on ascent too!).

Edited September 29, 2021 by Lisias
Tyops! surprised?