Quote from: Greg Hullender on 08/14/2022 08:35 pmQuote from: OTV Booster on 08/14/2022 07:30 pmThe tiles mentioned were not really intended or explored for EDL. I wonder if anybody's been looking at different fillers. If the heat shield tiles had a Y2O3 glaze they might be able to thin them for lower mass.Edit to add: Y2O3 is hydrophobic. That's a big thing for the tiles. Not a total solution but a step in the right direction.I think the trouble is that the heat-shield tiles need to be emissive in the range from about 1 to 8 microns. That rules out coating them with anything like Y2O3. Or anything else that reflects most solar energy, for that matter.Maybe I'm misunderstanding. See pg3, bottom chart showing a piece of the Y2O3 transmission spectrum. I'd cut n paste but it's not cooperating. [size=78%]https://iopscience.iop.org/article/10.1088/1757-899X/1240/1/012001/pdf[/size]The chart shows low transmission from about 2 to 10 microns. I've assumed this to mean it readily adsorbs these wavelengths which should also mean it readily emits at these wavelengths. Wrong?I've tried looking up an emission spectra and hit paywalls and yttria mixed with other things. Also, why 2-10 microns? Isn't this a bit past the far UV? I thought the ideal emission range was in the near IR.
Quote from: OTV Booster on 08/14/2022 07:30 pmThe tiles mentioned were not really intended or explored for EDL. I wonder if anybody's been looking at different fillers. If the heat shield tiles had a Y2O3 glaze they might be able to thin them for lower mass.Edit to add: Y2O3 is hydrophobic. That's a big thing for the tiles. Not a total solution but a step in the right direction.I think the trouble is that the heat-shield tiles need to be emissive in the range from about 1 to 8 microns. That rules out coating them with anything like Y2O3. Or anything else that reflects most solar energy, for that matter.
The tiles mentioned were not really intended or explored for EDL. I wonder if anybody's been looking at different fillers. If the heat shield tiles had a Y2O3 glaze they might be able to thin them for lower mass.Edit to add: Y2O3 is hydrophobic. That's a big thing for the tiles. Not a total solution but a step in the right direction.
Quote from: OTV Booster on 08/14/2022 09:36 pmQuote from: Greg Hullender on 08/14/2022 08:35 pmQuote from: OTV Booster on 08/14/2022 07:30 pmThe tiles mentioned were not really intended or explored for EDL. I wonder if anybody's been looking at different fillers. If the heat shield tiles had a Y2O3 glaze they might be able to thin them for lower mass.Edit to add: Y2O3 is hydrophobic. That's a big thing for the tiles. Not a total solution but a step in the right direction.I think the trouble is that the heat-shield tiles need to be emissive in the range from about 1 to 8 microns. That rules out coating them with anything like Y2O3. Or anything else that reflects most solar energy, for that matter.Maybe I'm misunderstanding. See pg3, bottom chart showing a piece of the Y2O3 transmission spectrum. I'd cut n paste but it's not cooperating. [size=78%]https://iopscience.iop.org/article/10.1088/1757-899X/1240/1/012001/pdf[/size]The chart shows low transmission from about 2 to 10 microns. I've assumed this to mean it readily adsorbs these wavelengths which should also mean it readily emits at these wavelengths. Wrong?I've tried looking up an emission spectra and hit paywalls and yttria mixed with other things. Also, why 2-10 microns? Isn't this a bit past the far UV? I thought the ideal emission range was in the near IR. Ah. In this case, low transmission means it reflects those wavelengths, which means it is almost completely non-emissive in that range. Otherwise it wouldn't look white in the visible.The problem is that the heat tiles get up to 1650 Kelvin, which means their emission curve (see the chart I posted two pages back) has a lot of overlap with the solar one. That chart shows that over 90% of the IR spectrum at 1650 is between 1 and 8 microns. That has way too much overlap with the solar spectrum. Solar white, in particular, is super reflective from 1/4 micron to 8 microns.That means you can't make a single kind of tile that serves both purposes.
An idea I've put out is for a constant rpm constant load two stroke. Liquid propellant used for cooling and combustion. Inexpensive, stone simple to maintain. When run off of liquid propellant the compression stroke has, in effect, already been done. Should be crazy efficient at small scale.For those with minimal experience with piston ICE's, with constant rpm and load the complexity of 'breathing' and mixing are simplified and efficiency can approach the theoretical Carnot limits. Get rid of the compression stroke, add in the perfectly characterized cooling requirements, and it's better yet. The major trade to look at is tapping unvaporized propellant vs increased efficiency. I noodled feeding it with boiloff but the cooling gets tricky and the compression stroke shows up again. Maybe gas fed can work, maybe not. Being a simple easily maintained system, it's good for Mars use too.
Where there would be a difference is when the plasma starts to build up and radiative input becomes dominant. The solar white will reject more of this than the black, dropping the peak temperature and shifting the black body emissions away from the visible and deeper into the IR. How much? I have not a clue.
Quote from: OTV Booster on 08/14/2022 04:40 pmAn idea I've put out is for a constant rpm constant load two stroke. Liquid propellant used for cooling and combustion. Inexpensive, stone simple to maintain. When run off of liquid propellant the compression stroke has, in effect, already been done. Should be crazy efficient at small scale.For those with minimal experience with piston ICE's, with constant rpm and load the complexity of 'breathing' and mixing are simplified and efficiency can approach the theoretical Carnot limits. Get rid of the compression stroke, add in the perfectly characterized cooling requirements, and it's better yet. The major trade to look at is tapping unvaporized propellant vs increased efficiency. I noodled feeding it with boiloff but the cooling gets tricky and the compression stroke shows up again. Maybe gas fed can work, maybe not. Being a simple easily maintained system, it's good for Mars use too. some related infohttps://www.ulalaunch.com/docs/default-source/extended-duration/development-status-of-an-integrated-propulsion-and-power-system-for-long-duration-cryogenic-spaceflight-2012.pdf
Quote from: Jim on 08/15/2022 04:52 pmhttps://www.ulalaunch.com/docs/default-source/extended-duration/development-status-of-an-integrated-propulsion-and-power-system-for-long-duration-cryogenic-spaceflight-2012.pdfVery interesting. I like the idea of eliminating hydrazine, helium, most of the batteries, etc. That document seems to be from about 2011, so I wondered whatever happened to the idea.
https://www.ulalaunch.com/docs/default-source/extended-duration/development-status-of-an-integrated-propulsion-and-power-system-for-long-duration-cryogenic-spaceflight-2012.pdf
Quote from: OTV Booster on 08/15/2022 04:12 pmWhere there would be a difference is when the plasma starts to build up and radiative input becomes dominant. The solar white will reject more of this than the black, dropping the peak temperature and shifting the black body emissions away from the visible and deeper into the IR. How much? I have not a clue.I really liked that idea too, but what I could find wasn't encouraging. "Convection is the primary means of heat transfer to a vehicle entering Earth’s atmosphere at speeds under about 15,000 m/s." (Advanced Aerospace Medicine On-line--Tutorial, Section III Space Operations, Chapter 4 Basic Concepts of Manned Spacecraft Design, 4.1.7 Returning from Space, p 322, Federal Aviation Administration, Retrieved 15 Aug 2022).15 kps is well above Earth escape velocity (about 11 kps), and very-low-earth-orbit velocity is under 8 kps. So the heat transfer will mostly be convective, not radiative, and, hence, I don't think Solar White is going to help.
Quote from: Greg Hullender on 08/15/2022 05:47 pmQuote from: Jim on 08/15/2022 04:52 pmhttps://www.ulalaunch.com/docs/default-source/extended-duration/development-status-of-an-integrated-propulsion-and-power-system-for-long-duration-cryogenic-spaceflight-2012.pdfVery interesting. I like the idea of eliminating hydrazine, helium, most of the batteries, etc. That document seems to be from about 2011, so I wondered whatever happened to the idea. ULA's parents (and Shelby) put a stop to it. Would jeopardize SLS.
If you put something like this on Starship, I'm wondering where the exhaust would go. Some of the documentation for ACES makes it sound like it used the main engine bell. Does that make sense?
Quote from: Jim on 08/15/2022 04:52 pmQuote from: OTV Booster on 08/14/2022 04:40 pmAn idea I've put out is for a constant rpm constant load two stroke. Liquid propellant used for cooling and combustion. Inexpensive, stone simple to maintain. When run off of liquid propellant the compression stroke has, in effect, already been done. Should be crazy efficient at small scale.For those with minimal experience with piston ICE's, with constant rpm and load the complexity of 'breathing' and mixing are simplified and efficiency can approach the theoretical Carnot limits. Get rid of the compression stroke, add in the perfectly characterized cooling requirements, and it's better yet. The major trade to look at is tapping unvaporized propellant vs increased efficiency. I noodled feeding it with boiloff but the cooling gets tricky and the compression stroke shows up again. Maybe gas fed can work, maybe not. Being a simple easily maintained system, it's good for Mars use too. some related infohttps://www.ulalaunch.com/docs/default-source/extended-duration/development-status-of-an-integrated-propulsion-and-power-system-for-long-duration-cryogenic-spaceflight-2012.pdfVery interesting. I like the idea of eliminating hydrazine, helium, most of the batteries, etc. That document seems to be from about 2011, so I wondered whatever happened to the idea. Looking further, I also found "ACES Stage Concept: Higher Performance, New Capabilities, at a Lower Recurring Cost," Section IV: Integrated Vehicle Fluids (IVF), Jonathan Barr, ULA, 2015, which was a little easier read, albeit less detailed.ULA abandoned ACES in 2020 in favor of the Centaur V, but I can't find any indication whether Centaur V uses the internal-combustion-engine approach of IVF or not. It certainly offers long-duration missions with multiple relights though.If you put something like this on Starship, I'm wondering where the exhaust would go. Some of the documentation for ACES makes it sound like it used the main engine bell. Does that make sense?
The tiles mentioned were not really intended or explored for EDL. I wonder if anybody's been looking at different fillers. If the heat shield tiles had a Y2O3 glaze they might be able to thin them for lower mass.
One obvious solution here is...don't use a solar white paint with a cutoff at 8 microns. Use something with a different cutoff.Based on your earlier graphs, the ideal cutoff wavelength for a "designer" selective surface material would be somewhere around 1-1.5 microns.
No no, by "optimum" I mean that a cutoff at 1-1.5 microns will result in the coldest equilibrium temperature. Colder than 8 microns.The optimum spectral strategy AIUI is to be 100% reflective in parts of the electromagnetic spectrum where incoming flux exceeds black body emission, and 100% emissive otherwise.If the vertical axis is spectral power (ie the area under the curve is power), the ideal wavelength cutoff is simply where the curves cross.
Quote from: Twark_Main on 08/20/2022 08:14 amNo no, by "optimum" I mean that a cutoff at 1-1.5 microns will result in the coldest equilibrium temperature. Colder than 8 microns.The optimum spectral strategy AIUI is to be 100% reflective in parts of the electromagnetic spectrum where incoming flux exceeds black body emission, and 100% emissive otherwise.If the vertical axis is spectral power (ie the area under the curve is power), the ideal wavelength cutoff is simply where the curves cross.Ah. No, the curves I showed above are normalized so the area under each one is 1. If you look at the unnormalized curves, the radiance from the sun is overwhelming and the curves don't cross at all.
Then derate the insolation to account for orbital night (~1/2), the projected geometry of a cylinder (1/pi), and vehicle pointing (for the tank wall this is sin(θ), where θ is the angle between the nose pointing direction and the Sun).Plus add the emitted heat and reflected light from the Earth, averaged over one orbit. The emitted heat is simply a ~250 K blackbody emitting 235 watts per square meter, derated 50% for view factor. The reflected light can be approximated as Lambertian solar reflector (black body temperature = 6000 K, times the Sun view factor above) with a reflectivity of 31% (Earth's mean albedo), plus 50% derate for view factor, and 1/pi for Earth's projected geometry.Then see where the curves cross.
Quote from: Twark_Main on 08/22/2022 10:28 amThen derate the insolation to account for orbital night (~1/2), the projected geometry of a cylinder (1/pi), and vehicle pointing (for the tank wall this is sin(θ), where θ is the angle between the nose pointing direction and the Sun).Plus add the emitted heat and reflected light from the Earth, averaged over one orbit. The emitted heat is simply a ~250 K blackbody emitting 235 watts per square meter, derated 50% for view factor. The reflected light can be approximated as Lambertian solar reflector (black body temperature = 6000 K, times the Sun view factor above) with a reflectivity of 31% (Earth's mean albedo), plus 50% derate for view factor, and 1/pi for Earth's projected geometry.Then see where the curves cross. The view factor for Earth-to-Starship in LEO is roughly 45%, irrespective of orientation. I was kinda surprised by this.
Doing (roughly) the above math, I don't know where this 8,000 nanometer spectral cutoff comes from. By my curves, to minimize tank heating the surface should ideally be maximally reflective through the entire infrared band.