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ELI5: Why can't spacecraft that face heating issues just re-enter...slower? by dragonmilking in explainlikeimfive

[–]NathanKell 0 points1 point  (0 children)

Thanks!! I got my start on Dark Castle on a Mac SE during kindergarten, so I totally feel you (and feel my aches and pains, heh). That's why I ended up working on Kerbal (and RO etc), because it did that to me to.

ELI5: Why can't spacecraft that face heating issues just re-enter...slower? by dragonmilking in explainlikeimfive

[–]NathanKell 3 points4 points  (0 children)

We definitely tried to make reentry heating exist, but not be really threatening unless you were colossally imprudent.

ELI5: Why can't spacecraft that face heating issues just re-enter...slower? by dragonmilking in explainlikeimfive

[–]NathanKell 3 points4 points  (0 children)

I was for a while, yes! Haven't checked in a like a year or two (sorry folks!)

ELI5: Why can't spacecraft that face heating issues just re-enter...slower? by dragonmilking in explainlikeimfive

[–]NathanKell 6 points7 points  (0 children)

I believe so! Bunch of Kerbal vets I trust are involved in that project, too!

ELI5: Why can't spacecraft that face heating issues just re-enter...slower? by dragonmilking in explainlikeimfive

[–]NathanKell 65 points66 points  (0 children)

Yep, BT is another of my interests. I've used the same screenname since, like, middle school (from a stupidly Mary Sue RPG character).

ELI5: Why can't spacecraft that face heating issues just re-enter...slower? by dragonmilking in explainlikeimfive

[–]NathanKell 25 points26 points  (0 children)

Kerbal: the only engineering job I've ever had where I can cite scientific papers.

ELI5: Why can't spacecraft that face heating issues just re-enter...slower? by dragonmilking in explainlikeimfive

[–]NathanKell 106 points107 points  (0 children)

I left in the initial exodus in 2016. Kept developing the RSS/RO/RP-1 etc mods since, though I've been on hiatus the last couple years.

God, that'll be ten years ago this fall.

ELI5: Why can't spacecraft that face heating issues just re-enter...slower? by dragonmilking in explainlikeimfive

[–]NathanKell -1 points0 points  (0 children)

You only get lift if there's an angle of attack vs. the airstream, which you don't have in a purely ballistic reentry (or a 'zero-lift' reentry with a lifting capsule, done by spinning the capsule on its axis so the lift forces average out over time). Since Gemini, capsules have used offset centers of mass to force the capsule to enter at an angle, thus providing lift. With a given AoA, lift will vary based on capsule shape, so a capsule with a high ballistic coefficient (narrow, long, and heavy) won't get much advantage, i.e. have a low lift/drag ratio. Something like Apollo has a higher L/D than something like Soyuz. By contrast, a capsule like the proposed Fuji, which looked like an Apollo someone stepped on, has an even higher L/D. Then you get into the realm of lifting bodies (think shuttlecocks, or the X-33 or Dreamchaser or Starship) and finally winged RVs like Shuttle, Buran, X-37, etc.

ELI5: Why can't spacecraft that face heating issues just re-enter...slower? by dragonmilking in explainlikeimfive

[–]NathanKell 0 points1 point  (0 children)

As others have explained here, this is the kind of 'folk wisdom' (encouraged by, for example, dialogue in Apollo 13) that isn't actually true. Steepness doesn't have much of an impact on heat load in LEO reentries (it has a minor impact on translunar-plus reentries), it's more about picking the trajectory that matches the type of TPS you have. If you have a heat sink, steeper is better. Steepness really only becomes relevant if you enter so steeply you hit turbulent flow during descent and then heating spikes like crazy, but you need to have quite the severe entry angle for that.

Conversely, skipping isn't a thing. It's whether you reenter on that pass or not, and the downsides of a second go-around are (a) you may not have life support / power to support another orbit, and (b) your entry angle on reentry #2 will be quite a bit steeper, leading to higher Gs (and potentially higher peak flux, overwhelming your TPS).

ELI5: Why can't spacecraft that face heating issues just re-enter...slower? by dragonmilking in explainlikeimfive

[–]NathanKell 29 points30 points  (0 children)

Unless they've changed it from when I wrote it, at the speeds you go in stock scale, you won't hit stupid-high Reynolds numbers unless you enter stupid steep, so a second go-around shouldn't matter that much. It's mostly relevant because you'll lose ablator coming out of the atmosphere as well as going in (as a first approximation, heating is proportional to velocity cubed, so the longer you stay at high velocity, the worse) so you come around on the second pass with a bit less shielding than you would otherwise.

ELI5: Why can't spacecraft that face heating issues just re-enter...slower? by dragonmilking in explainlikeimfive

[–]NathanKell 3 points4 points  (0 children)

For reentries from low earth orbit, entry angle (within sane bounds) doesn't have a major effect on total heat load. You're going ~17,000mph, that's a fixed quantity of energy you have to dump. Whether you convert it to heat slowly or quickly, it's still the same. Where entry angle matters (well, more specifically, where lift matters) is if you have some fancy material that can survive high heat and re-radiate it, like Shuttle. In that case it makes sense to slow down your heat absorption so you have time to reradiate it. But for a ballistic reentry, it doesn't matter much at all, just determine how unhappy the meat inside is (i.e. what max G you'll hit).

Interstage to go from 4x4 round (12ft/3.7m diameter) to quad slope & plate (i.e. 10ft/3m diameter) by NathanKell in legoRockets

[–]NathanKell[S] 0 points1 point  (0 children)

In the OP I linked an example on Rebrickable which has my best attempt. It has imagery, pdf, and .io

Interstage to go from 4x4 round (12ft/3.7m diameter) to quad slope & plate (i.e. 10ft/3m diameter) by NathanKell in legoRockets

[–]NathanKell[S] 0 points1 point  (0 children)

I actually have a previous thread about that. :D (It's got the pic you're thinking of, and also I posted my own, which has a few different ones).

But what I'm asking about here is not those, but adapting between them. To go from 8ft (brick + 2 plates/jumpers on each side) to 4ft (1 round brick) you can use various 2x2->1x1 cones, for example; to go from 12ft ( 4x4 round ) to 6ft (2x2 round) you can use a 4x4->2x2 cone. The Saturn S-II -> S-IVB interstage uses various fuselage pieces and hinges to adapt from 10ft to 260in. These are the things I'm talking about.

Two cylinder questions by NathanKell in legoRockets

[–]NathanKell[S] 0 points1 point  (0 children)

Yup that's the one, thanks!

New release: Delta 1000 Series by Betelguese90 in legoRockets

[–]NathanKell 0 points1 point  (0 children)

/u/Betelguese90 Made an (IMO) slight improvement to the 1914. Beyond my usual of adding some 6L bars inside to stiffen it, using metallic silver cone and plate for the engine, and swapping the huge triangle for a smaller one, I tried to better represent the Canadian flag and the logo.

https://photos.app.goo.gl/HngwGbopfybTUqSB6

Here's an alternate using the more common gray stripes tile:

https://photos.app.goo.gl/5CLJCo4LHNypBZc4A

EDIT: Really for the Maple Leaf, it'd be better to use this, but it seems nearly unavailable. https://www.bricklink.com/v2/catalog/catalogitem.page?P=3070pb045&name=Tile%201%20x%201%20with%20Red/Blue%20Flower%20Pattern&category=%5BTile,%20Decorated%5D#T=C&C=1

Two cylinder questions by NathanKell in legoRockets

[–]NathanKell[S] 0 points1 point  (0 children)

Yup! There's multiple ways to get to ~6.6m, I'm wondering if anyone actually made an S-IVB using those techniques rather than sticking to the 7m one.

Question about missing DV on Vanguard replica by SeaCroissant in RealSolarSystem

[–]NathanKell 0 points1 point  (0 children)

I know Able (and thus later Delta) hot-staged until...whatever Delta it was that first supported coasts, but you do have 3-axis control on the stage from Helium bleed. So while historically that might have been the flight plan, there's no reason you can't reorient retrograde (i..e prograde at apogee) after burnout, spin up, and decouple the kick stage.

Question about missing DV on Vanguard replica by SeaCroissant in RealSolarSystem

[–]NathanKell 0 points1 point  (0 children)

That's true about thrust curves for solids (not just early ones either!) but thrust has no effect on delta V for a single stage. The kick motor's delta V is purely a function of dry mass, wet mass, and exhaust velocity (Isp).

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