Jim pointed out that a layer of paint alone was sufficient to reduce LOX boil off to very low levels.
Quote from: JCRM on 01/25/2016 11:57 pmI thought it referred to hydrogen at it's triple-point of 14.01 KIndeed.
I thought it referred to hydrogen at it's triple-point of 14.01 K
Randy has pointed out US aerospace R&D seems to have gone off the rails somewhere in the 60's when they became obsessed with SCRamjets.
My feeling is the same applies to their pursuit of "Slush" Hydrogen. It's that same sense of "performance Uber Alles" of pursuing the absolute best performance (in this case in terms of density) regardless of the practical problems, when more viable options, which are simpler to implement, exist.
This is a side point. AIUI the stated concern is that people doubt a Skylon could stay on the runway for 2 hrs without venting.
I agree that a system that carries out the chill down LH2 sub cooling will need to vent gas to the atmosphere and that will be burnt off. The question is with all that pre cooled hardware filled with sub cooled propellant resist the heat coming in from the environment long enough to eliminate venting?
LACE, now don't forget LACE as well
Actually in most cases it's invoked for the simple reason of trying to reduce the tankage size of an LH2 system. In rarer cases it's used to allow "recycling" of gaseous LH2 into at least a semi-liquid state by running it back through the 'slush' after its used for some cooling purpose. From what I've read it usually isn't worth doing unless there is a very compelling reason, on the gripping-hand though there usually IS a very compelling reason which is why it gets invoked
"MY" main point was the question of why NOT vents? (Thanks for the calc's JCRM )
Didn't say anything about the boil off. Just that LOX tanks only have paint and no insulation. LOX is boiling off and is replenished up to a few minutes before launch. The open vents are closed but the boil off still causes the vent to open occasionally.
Quote from: RanulfC on 01/27/2016 04:16 pmLACE, now don't forget LACE as well Indeed. LACE is the "performance Uber Alles" approach. SABRE is the "good enough to get the job done" approach.
QuoteActually in most cases it's invoked for the simple reason of trying to reduce the tankage size of an LH2 system. In rarer cases it's used to allow "recycling" of gaseous LH2 into at least a semi-liquid state by running it back through the 'slush' after its used for some cooling purpose. From what I've read it usually isn't worth doing unless there is a very compelling reason, on the gripping-hand though there usually IS a very compelling reason which is why it gets invoked I don't know about the 60's concepts (IIRC some of Bono's SSTO ideas had it in the small print, along with Beryllium alloy structures) but I get the feeling a lot of the time it comes in when someone has mis-estimated the performance (either engine or structural) of their planned design. It's from the same place that caused HOTOL to develop a jet engined takeoff trolley.
Quote"MY" main point was the question of why NOT vents? (Thanks for the calc's JCRM )Well there's the practical and the philosophical.Skylon is moved after propellant load to its start point on the runway. Yes you could run some kind of flexible vent pipe to a burn off stack but imagine how clumsy that would be Option b would be to stick vent system on the vehicle. H2 is very light and would disperse very quickly but I'm not sure that would be enough to convince the CAA it would not need some kind of burn off system. Then there's the philosophical point. IE the COP.Aircraft don't have flare stacks for fuel vapors from their tanks. VTO ELV's do. AFAIK the nearest they get are Nitrogen generators, to purge fuel tanks on large aircraft of explosive vapors, are quite a recent feature (although IIRC SOP for military aircraft)
It would be about 30 tonnes of hydrogen that would need to be evaporated to chill (my estimate of) 180 tonnes of fuel. That's a lot to burn, even if one was happy to just throw it away. On the other hand, it's a lot to pressurise and store - so who knows what actual operational procedure would be.
Quote from: JCRM on 01/28/2016 12:40 amIt would be about 30 tonnes of hydrogen that would need to be evaporated to chill (my estimate of) 180 tonnes of fuel. That's a lot to burn, even if one was happy to just throw it away. On the other hand, it's a lot to pressurise and store - so who knows what actual operational procedure would be.The usual figures for Skylon are it holds about 150 tonnes of LO2 and about 60 tonnes of LH2.John Whitehad's team estimated LV propellant tanks are about 1% of contents weight except for LH2, when (IIRC) it's nearer 12% (Shuttle ET H2 is about 1/8 mass of LH2 it holds).That's about 9 tonnes of Aluminium to cool down. With engines being roughly 24 tonnes of superalloy.
The shuttle tank also contains 630 tonnes of O2, the 26 tonne total weight the total weight of the O2+H2 tanks.So your 12% number is total rubbish.
Quote from: SICA Design on 01/28/2016 02:30 pmQuote from: hkultala on 01/28/2016 01:40 pmThe shuttle tank also contains 630 tonnes of O2, the 26 tonne total weight the total weight of the O2+H2 tanks.So your 12% number is total rubbish.Wikipedia gives the (SLWT) dry ET mass as 26.5T1% of the LO2 mass would be 6.3T12% of the 106T LH2 mass would be 12.7T (1/8th would be 13.3T)This leaves 6.9 - 7.5T for the intertank and other miscellaneousGiven the limited info, 12% seems plausible to me - and certainly doesn't deserve to be called "total rubbish".12% seems to be stated a little too precise given the way it was estimated. No need, however for the LWT:Quote from: http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/et.htmlLiquid Hydrogen Maximum 227,641 pounds[...]The liquid hydrogen tank is 331 inches in diameter, 1,160 inches long, and has a volume of 53,518 cubic feet and a dry weight of 29,000 poundsor 12.74%Now, Skylon's tanks aren't structural, so it may be lighter - but they're less optimally shaped, so it's probably a good estimate.Disappointingly, the SHC of aluminuim is ridiculous at cryogenic temperatures, with just 0.055 J/g to raise its temperature from 16 to 22 K and its thermal conductivity remains high at 27W/m K - so it wont be doing much to slow the temperature rise,
Quote from: hkultala on 01/28/2016 01:40 pmThe shuttle tank also contains 630 tonnes of O2, the 26 tonne total weight the total weight of the O2+H2 tanks.So your 12% number is total rubbish.Wikipedia gives the (SLWT) dry ET mass as 26.5T1% of the LO2 mass would be 6.3T12% of the 106T LH2 mass would be 12.7T (1/8th would be 13.3T)This leaves 6.9 - 7.5T for the intertank and other miscellaneousGiven the limited info, 12% seems plausible to me - and certainly doesn't deserve to be called "total rubbish".
Liquid Hydrogen Maximum 227,641 pounds[...]The liquid hydrogen tank is 331 inches in diameter, 1,160 inches long, and has a volume of 53,518 cubic feet and a dry weight of 29,000 pounds
A ?new? promotional puff:http://www.msn.com/en-us/video/t/the-new-engine-that-could-revolutionise-aviation/vp-BBoFjko"destroy all the other naysayers that say this can;t be done"
Quote from: SICA Design on 01/29/2016 08:38 amThis piece implies that BAe's interest is more in LAPCAT/Scimitar - media bias, perhaps?Scimitar is a better missile engine? Four hours to Australia is of more interest to the general public than space access? (weird right? but then the general public watch Big Brother and the X Factor)
This piece implies that BAe's interest is more in LAPCAT/Scimitar - media bias, perhaps?
Quote from: JCRM on 01/29/2016 08:02 amA ?new? promotional puff:http://www.msn.com/en-us/video/t/the-new-engine-that-could-revolutionise-aviation/vp-BBoFjko"destroy all the other naysayers that say this can;t be done"This piece implies that BAe's interest is more in LAPCAT/Scimitar - media bias, perhaps?
Quote from: SICA Design on 01/29/2016 08:38 amQuote from: JCRM on 01/29/2016 08:02 amA ?new? promotional puff:http://www.msn.com/en-us/video/t/the-new-engine-that-could-revolutionise-aviation/vp-BBoFjko"destroy all the other naysayers that say this can;t be done"This piece implies that BAe's interest is more in LAPCAT/Scimitar - media bias, perhaps?The LAPCAT stuff was a fairly small part of the report I thought. LAPCAT remains a very different engine from SABRE. The joker in the pack remains coping with the prolonged heating on a fuselage during cruise.
Make 'er out of Titanium, then...
Quote from: john smith 19 on 01/29/2016 08:54 pmQuote from: SICA Design on 01/29/2016 08:38 amQuote from: JCRM on 01/29/2016 08:02 amA ?new? promotional puff:http://www.msn.com/en-us/video/t/the-new-engine-that-could-revolutionise-aviation/vp-BBoFjko"destroy all the other naysayers that say this can;t be done"This piece implies that BAe's interest is more in LAPCAT/Scimitar - media bias, perhaps?The LAPCAT stuff was a fairly small part of the report I thought. LAPCAT remains a very different engine from SABRE. The joker in the pack remains coping with the prolonged heating on a fuselage during cruise.I agree; more than 75% of the piece was dedicated to SABRE and reusable space vehicles. However my point was based on the last minute where the video and talk appeared all about LAPCAT, with the quote at 3:51 -"The project [LAPCAT visuals playing] received a huge boost at the end of 2015: a $100M investment from BAe Systems and the British government" followed shortly after at 4:06 -Varvill: "When that day happens and that AEROPLANE [my emphasis] rolls out on the tarmac will be a pretty emotional moment..."Hope I'm wrong...