Stoke Space

[RyanRising]

3 hours ago, sevenperforce said:

I think you're making an assumption here about what the "regime of typical LEO spacecraft entries" constitutes. It's not an either/or distinction between radiative and convective heating. Rather, re-entries start with radiative heating as the dominant factor, and then the radiative heating decreases while the convective heating increases.

You're correct that certain vehicles are designed to accept convective heat and then radiate it away. However, the image you provided above (Figure 10) isn't applicable to LEO entries generally. The source you cite says they "apply only to single-pass, nonlifting, parabolic-velocity entries." The real numbers are more complicated.

The actual numbers can be found in this presentation, which explains the relationship between convective heating and radiative heating based on multiple factors. Importantly, one of the factors is the effective radius of the vehicle:

Q_conv ∝ v³(ρ/R)^0.5 but Q_rad ∝ v⁸ρ^1.2R^0.5

As the effective vehicle radius increases, convective heating decreases, but radiation heating increases. For something like the Shuttle, which had a very deep atmospheric entry (to allow a lifting entry) and a very high effective radius, it would make sense that radiative heating was a major component.

It is true that I’m making several assumptions about “what the typical LEO entry is.” Those assumptions are relatively high altitude, >~30km, and speeds of less than 8 km/s. Anything above those speeds wouldn’t be a low earth orbit and anything reaching those speeds at lower altitudes is… well it isn’t a crewed spacecraft like any we’ve seen before. I think these are reasonable assumptions.

The paper you reference does not support your claim that LEO entires are dominated by radiative heating at first and then convective heating as they slow down. It does show that this is the case for higher-velocity entires, and the paper is clearly concerned more with those than LEO ones. These graphs start from 10 km/s and only go up from there.

However, even in the larger 5m radius case the heat fluxes at 10 km/s are roughly equal, and it’s reasonable to extrapolate that it becomes very much lower at LEO entry speeds of 8km/s. (edit: to be clear, this doesn’t mean there is no radiative heating whatsoever in these regimes, only that it is significantly lower i than the convective heating)

The equations you show are proportional, and describe the growth of each. However, this still leaves room for constant scale factors, and the examples they give show that for the size of entry vehicle they consider, those scale factors work out so that convective heating will dominate radiative heating at lower entry speeds near 8 km/s. The presentation says as much: reflective TPS is applicable for high-velocity, interplanetary missions. It doesn’t claim it’s very useful for LEO missions.

The scales that this presentation considers include crewed space capsules that have been fielded. For those, it’s pretty clear that convective heating is higher than radiative at speeds < 8 km/s. However, it’s still possible that the flatter areas of the Space Shuttle experienced much more significant radiative than convective heating. I don’t have data on the type of heat flux experienced in those areas, but maybe I just need to look a little more.

Edited March 16, 2023 by RyanRising

[sevenperforce]

57 minutes ago, RyanRising said:

However, it’s still possible that the flatter areas of the Space Shuttle experienced much more significant radiative than convective heating. I don’t have data on the type of heat flux experienced in those areas, but maybe I just need to look a little more.

It definitely looks like the crossover is lower than I had previously thought.

But (bringing this back to the original thread topic) the black tiles of the Shuttle were nevertheless chosen for their ability to radiate heat out.

We can do the same math as before, but instead just jump directly to the radiative cooling capacity of the Shuttle tiles. The Shuttle tiles reached temperatures of 1533 K and thus would have been rejecting a peak of 313.2 kW/m², per the Stefan-Boltzmann law. If the surface area of the Stoke upper stage heat shield is 16 square meters, then that's 5 MW of heating we need to reject.

Rather higher. But if radiative heating isn't a major issue, then we can perhaps have some of that heat transfer by convection into the hydrogen boundary layer we create.

1 hour ago, AckSed said:

How would water-cooled heat shields (which I've seen mentioned here and there) stack up against H2-cooled? I imagine they have a denser coolant and pretty damn good cooling, but the system is dead weight that doesn't contribute to propulsion.

Water is a very effective coolant in terms of volume, but it doesn't come anywhere close to what liquid hydrogen can do in terms of mass.

[tater]

Stoke says they looked into water, and decided the trades were close enough to go for simplicity, and a single system.

[sevenperforce]

1 hour ago, tater said:

Stoke says they looked into water, and decided the trades were close enough to go for simplicity, and a single system.

Not to mention that liquid hydrogen is self pumping thanks to the expander cycle. 

[tater]

2 hours ago, sevenperforce said:

Not to mention that liquid hydrogen is self pumping thanks to the expander cycle. 

Yeah, I'm kinda loving that aspect.

In the interview, when he points out that the point was not to make an aerospike, but to make a heatshield with holes for engines...

I so want this to work.

[Superluminaut]

On 3/14/2023 at 5:13 PM, sevenperforce said:

What spectral bands was your reflectivity testing in? Infrared light is not visible light. Many things that are opaque to visible light are transparent to infrared. Although the grey-black tiles on the bottom of the Shuttle absorbed the majority of visible light, they were reasonably reflective of infrared radiation.

Usually  250 to 2500 nm

On 3/14/2023 at 5:13 PM, sevenperforce said:

 You're not doing the math.

Imagine your heat shield needs to get rid of 98% of the incoming energy. If you have a white heat shield which reflects 90% of incoming energy but can only radiate away 50% of what it absorbs, it will get rid of 95% of incoming energy, which means it fails. If you have a black heat shield which only reflects 75% of incoming energy but can radiate away 93% of what it absorbs, then it will get rid of 98.25% of the incoming energy, so it survives.

I think this is overly abstracted.

In my measurements --as far as I can remember-- a very reflective black has a reflectivity of about 0.12. Anything at or greater than 0.65 is typically white. White and black coated surfaces will usually have a comparable emittance.

Guesstimating some values to ballpark with (for smooth surfaces) White: 0.90 reflectance, 0.87 emittance, Black: 0.12 reflectance, 0.93  emittance

Let's assume the case that nasa engineered the love out of that black emittance. We can assume that the above given white properties would have been easily attainable. And for this thought experiment, let's use the liberal value of 0.15 reflectivity for black.

0 = (B_em + B_ref - (B_em * B_ref)) - (W_em + W_ref - (W_em * W_ref))

B_em = (0.90 + 0.87 - 0.15 - (0.90 * 0.87))  / 0.85

B_em = 0.985

This estimate is to break even with a simple white.

I have seen emittance measurements of 0.98, but that was on a textured surface.

Even if you engineered something with 0.999 emittance, that's only valuable on paper, it's not very useful as a radiation-heat shield.

Once heat has been absorbed by an envelope it will not stay in that envelope, it will conduct into a cooler interior. Emittance and conduction are simultaneous, a black body will match the temperature of it's environment. A black envelope, in addition to conducting more heat, will emit some portion of it's absorbed energy inwards into the structure. Reflectance of external rays will only occur on the external face and none of the reflected energy will be transmitted into the structure.

Outside of radiation, reflectivity is useless, and emissive materials are the choice. This could also be the case if radiation is a lessor source of heat.

On 3/14/2023 at 5:13 PM, sevenperforce said:

It does, in fact, appear to be the case in LEO. And it is, in fact, the case in LEO.

The evidence seems to support otherwise.

The entire shuttle deals with radiation, but only those surfaces that encounter convective heating are black.

If black was a superior method of shielding from rays, why not make the entire shuttle black.

I found these pictures with google. I haven't had the time to figure out if they are fact or fiction, but they look too esoteric to invent.

On 3/14/2023 at 5:13 PM, sevenperforce said:

 Definitely no eye or skin protection visible in this photo:

  Reveal hidden contents

 

Apollo astronauts were not suited on reentry. I don't have a source on hand, but I believe the reasoning was that wearing a space suit would have been detrimental to crew safety if they needed to make an emergency exit in water, or the capsule was upside down. I've never seen hazardous light as a part of the discussion. The main argument for suits was decompression. I also believe apollo astronauts described the plasma colors they saw, which would suggest they did not have a shielded visor down.

[tater]

 

 

[RyanRising]

6 hours ago, tater said:

 

 

I guess I should have picked up on the double-walled tank during the EDA tour, but that's a neat way to demonstrate its presence.

[sevenperforce]

17 hours ago, tater said:

 

 

It’s so smol!!

[tater]

11 minutes ago, sevenperforce said:

It’s so smol!!

I think it's about the same dia as F9—so it can be transported on roads.

I have to admit that when I first saw F9 in person at Hawthorne, it seemed... small.

 

[tater]

 

[StrandedonEarth]

Voldemort it is then!

[Exoscientist]

14 hours ago, tater said:

 

 Think that’s an April Fools joke. 
 

  Bob Clark

[tater]

Well played, regardless.

[tater]

 

[tater]

 

 

[magnemoe]

On 3/19/2023 at 11:09 PM, tater said:

I think it's about the same dia as F9—so it can be transported on roads.

I have to admit that when I first saw F9 in person at Hawthorne, it seemed... small.

 

More of lack of scale reference, Falcon 9 first stage is not small. Yes its only 3.5 meter in diameter because road transport. 
Now I agree Stoke's second stage looks short, much more so knowing they uses hydrogen on it.  On the other hand its an small payload rocket more in the electron than falcon 9 range. 
 

[tater]

36 minutes ago, magnemoe said:

More of lack of scale reference, Falcon 9 first stage is not small.

I have now seen the F9 booster at SpaceX in Hawthorne, and the FH side core at the KSC visitor center.

As I said in the post you quoted, they seemed small in person—to me. Shuttle, which I knew was big I had only seen in person before (decades ago) on the pad in the distance—until a couple months ago, when I saw Atlantis at KSC a few minutes before I saw the FH side core—seemed huge in person compared to how I had imagined it.

"Seemed" is definitionally my personal perception of it. I am well aware of the dimensions of the vehicle.

Edited April 26, 2023 by tater

[tg626]

Interesting. When i got up close and personal with Endevor and Enterprise, they seemed smaller than I expected. Still huge, but yet smaller than I thought they'd be.

Perspective.

[tater]

35 minutes ago, tg626 said:

Interesting. When i got up close and personal with Endevor and Enterprise, they seemed smaller than I expected. Still huge, but yet smaller than I thought they'd be.

Perspective.

Interesting

The External Tank that you walk under to see Atlantis, OTOH, seemed small to me, and the orbiter seemed huge. lol

 

[tater]

 

[tater]

 

[darthgently]

On 6/9/2023 at 2:45 PM, tater said:

 

This is the way.  Or so I hope.  It looks so very fantastic

[tater]

I'm a total stoke fanboy and they haven't even hopped yet.

[JoeSchmuckatelli]

20 hours ago, tater said:

I'm a total stoke fanboy and they haven't even hopped yet.

They're certainly doing interesting stuff, have a cool name and fun vids