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#280 by woods170 on 30 May, 2022 14:28
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I'm not regular here, and I thank those who answered my previous question from nasaspaceflight article. Now I have another from the latest Starliner article:
"The spacecraft experienced a temporary loss of signal and communication with the ground during the re-entry phase as an envelope of ionized air forms around the spacecraft due to the extreme heat as temperatures can rise to 1,930° C (3,500° F). Approximately four minutes before touchdown, Starliner’s heat shield was jettisoned, followed by the deployment of the drogue parachutes.
These help to slow the spacecraft down, with the drogues making way for the three main parachutes about one minute and 10 seconds later. Just two minutes and 49 seconds before touchdown, rotation handles deployed which aligned the Starliner to land perpendicularly on the land. Shortly after, its base heat shield was separated, followed by the inflation of airbags which cushioned the spacecraft as it landed at the White Sands Space Harbor in New Mexico."
So there are two heatshield ejections? Is the second a frame for the heatshield or is it a second layer of heat shielding? Please explain because it does not make sense to me. One jettison (I would think after parachute deployment best) only to be required.
On Starliner TWO pieces of the heat shield are indeed jettisoned.
A classic "Apollo-like" capsule like Starliner is fully coated in heat shields, or more appropriately: Thermal Protection System (TPS). The main heatshield is the oblate heat shield at the base (or bottom) of the capsule. It is often referred to as the "Base Heat Shield" or "the primary TPS". It is the thing that takes the brunt of the thermal load during reentry because it faces into the direction of travel.
The cone-shaped portion of Starliner also is covered in heat shields (TPS). This is often referred to as "the backshell TPS".
Part of this backshell TPS is the "Forward Heat Shield". It has the shape of a truncated cone and is situated directly beneath the docking system. This Forward Heat Shield covers the parachute assembly.
For the parachutes on Starliner to be deployed, it is necessary to jettison the Forward Heat Shield. So, this part of the backshell is jettisoned, which exposes the parachutes for deployment.
Long after the parachutes have been deployed, but shortly before touchdown, the Base Heat Shield is jettisoned. On Starliner this is done to expose six airbags that help cushion the landing on land.
So: two jettison events:
1. Jettison of the Forward Heat Shield
2. Jettison of the Base Heat Shield -
#281 by mn on 30 May, 2022 15:51
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Suppose somehow the base heat shield fails to separate, is that a LOC event? or just a harder but survivable landing? (I understand that the the heat shield covering parachutes failing to separate is most likely a LOC event)
About the trunk or service module, suppose that fails to separate, it is obviously not protected and will be destroyed on entry, which would ultimately expose the heat shield, is that possible? or a failure to separate would mean LOC? (I understand Soyuz was designed where one part of the entry module will separate due to structural failure from reentry heating if the pyrotechnic separation fails) -
#282 by Surfdaddy on 30 May, 2022 18:59
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One difference between Dragon and Starliner is that Starliner is still using the "shed parts all throughout the mission" philosophy whereas Dragon is trying to shed as little as possible - heat shields, manuevering thrusters, etc.
I would expect that going forward, more and more rockets and spacecraft will trend toward reusable and retaining more of the spacecraft. -
#283 by markbike528cbx on 30 May, 2022 20:08
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Not a great example but the Shuttle TPS, heat shield was exposed 100% to deep space (in the typical "upside down" attitude). Debris impacts from foam etc seem to be paramount. The wiki article doesn't even mention in-space meteor(oids). https://en.wikipedia.org/wiki/Space_Shuttle_thermal_protection_system
And, from all that has been said here, NASA would be extremely nervous if the Dragon or Starliner heat shields had not been covered by their respective trunk and service module until just before reentry. The chance of damage to the heat shield due to micro-meteoroids or orbital debris is too great, in their opinion. They would rather see the heat shield entirely covered by something until the last moment.
<snipped the embedded image, which is not allowed at NSF>
I’m sure this has been hashed and rehashed, but this picture vividly illustrates why I would never fly on this spacecraft. Yes, all space vehicles have a number of deployments that must go perfectly for any crew to survive. But the engineering design decision to return to land, necessitating the requirement to make the heat shield completely detachable, is I believe, a bridge too far. Especially when you consider a very heavy service module also must be jettisoned minutes before that heat shield is needed to protect the occupants from a now-unavoidable fiery plunge into the exponentially dense lower atmosphere.
I will casually point out that Soyuz has been doing this very thing (dropping the heat shield shortly before touchdown on land) successfully since the 1960s. Shenzou has been doing the same thing since the early 2000s.
It is nothing new, it is fully understood and most definitely NOT an engineering bridge too far. Neither is jettisoning a service module btw, which has been done by Gemini, Apollo, Soyuz and Shenzou. And both cargo Dragon and Crew Dragon do something very similar: they jettison their trunks to expose their heat shields.
Ablative heat shield requirements seem to revolve around bond-line heating or cooling. No mention is made of meteoroid or debris impacts in the attached report.
"In space, the temperature requirements for the ablator were limited to a cold temperature of -260" F and a hot temperature of 250 F."
"Additional Block I1 design requirements included a reduction in the ablator temperature extremes in space to +-150" F "
"The temperature extremes without a coating were of the order of +-250" F; and tests had shown that, because of the difference in coefficients of thermal expansion between the ablator and stainless-steel substructure, the ablator cracked if soaked at a temperature less than -170" F."
[Apollo 4] "This attitude cold soaked the thick ablator on the side opposite the hatch and achieved the maximum thermal gradient around the CM heat shield." -----no indication of the minimum temperature achieved, but presumably less that the -150F limit.
I have the attached report because I was wondering why Apollo 13 didn't ditch a useless, deadweight, and possibly still dangerous (hypergolics) Service Module. My take is that they wanted the thermal stabilization of the heat shield that the Service Module provided.
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#284 by Steven Pietrobon on 31 May, 2022 06:42
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Note that Dragon 2 also needs to jettison external covers to deploy its parachutes.
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#285 by tea monster on 31 May, 2022 06:52
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I'm not a Boeing engineer, but I'm assuming the crew would have a much bumpier landing if the bottom shield failed to separate, but that the parachutes would land it relatively safely - injuries possibly, but no deaths.
Now on the other hand, if for whatever reason the forward shield fails to separate, that's going to be nasty. -
#286 by TrevorMonty on 31 May, 2022 07:19
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The seats would have crumple zones as backup, kick in if G forces were to high.
I'm not a Boeing engineer, but I'm assuming the crew would have a much bumpier landing if the bottom shield failed to separate, but that the parachutes would land it relatively safely - injuries possibly, but no deaths.
Now on the other hand, if for whatever reason the forward shield fails to separate, that's going to be nasty.
Sent from my SM-A528B using Tapatalk
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#287 by edzieba on 31 May, 2022 07:25
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The drogue mortars aren't mortars for nothing, the covers are designed to be frangible and to break off from the force of deployment as a contingency (with the snaking drogue lines also aiding as cutters for the main 'chute section). Likewise with the base heatshield, if it hangs up the landing bag deployment gas generators have sufficient capacity to shove the heatshield off whilst still inflating the bags. Neither is ideal, some chutes or bags may be lost in the process, and it turns a comfortable landing to a survivable one, but that's what all that extra margin is there for.
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#288 by Joffan on 31 May, 2022 18:19
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I have to say that the hatch-opening process could use some work with an ergonomics eye. On OFT-2 it took about 5 minutes with one guy kneeling right on the edge of the mobile access platform and reaching quite a long way across a gap to the capsule (and up the slope of capsule side). It looked both awkward and not particularly safe for that worker.
That gap will also need work I think for extracting any astronauts feeling the effects of Earth gravity after prolonged zero-g. I do recall a bit of an update cycle on the same process for Crew Dragon, but this seems like an good time to start the update for Starliner.
https://youtube.com/watch?v=b38sm4h2iWA&t=8260s -
#289 by SWGlassPit on 31 May, 2022 18:20
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Not a great example but the Shuttle TPS, heat shield was exposed 100% to deep space (in the typical "upside down" attitude). Debris impacts from foam etc seem to be paramount. The wiki article doesn't even mention in-space meteor(oids). https://en.wikipedia.org/wiki/Space_Shuttle_thermal_protection_system
Ablative heat shield requirements seem to revolve around bond-line heating or cooling. No mention is made of meteoroid or debris impacts in the attached report.
"In space, the temperature requirements for the ablator were limited to a cold temperature of -260" F and a hot temperature of 250 F."
"Additional Block I1 design requirements included a reduction in the ablator temperature extremes in space to +-150" F "
"The temperature extremes without a coating were of the order of +-250" F; and tests had shown that, because of the difference in coefficients of thermal expansion between the ablator and stainless-steel substructure, the ablator cracked if soaked at a temperature less than -170" F."
[Apollo 4] "This attitude cold soaked the thick ablator on the side opposite the hatch and achieved the maximum thermal gradient around the CM heat shield." -----no indication of the minimum temperature achieved, but presumably less that the -150F limit.
I have the attached report because I was wondering why Apollo 13 didn't ditch a useless, deadweight, and possibly still dangerous (hypergolics) Service Module. My take is that they wanted the thermal stabilization of the heat shield that the Service Module provided.
MMOD guy here. Worth noting that shuttle tile and Apollo style ablator have VERY different responses to MMOD impacts.
Shuttle tile tends to crater but otherwise absorb impacts. This makes sense, since shuttle tiles are basically very sparse glass foam. Not much exists to transfer the shock wave of impact through the material.
Ablators like Avcoat (which is what was used on Apollo) tend to shatter, creating much larger damage for the same size impact. -
#290 by Asteroza on 31 May, 2022 23:02
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So at this stage, what are the big homework items from OFT-2 before the next flight?
OMAC thruster issues is the obvious one...
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#291 by gemmy0I on 01 Jun, 2022 02:34
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Suppose somehow the base heat shield fails to separate, is that a LOC event? or just a harder but survivable landing? (I understand that the the heat shield covering parachutes failing to separate is most likely a LOC event)
It would likely be a "harder but survivable" landing; that is the case for Soyuz (and I would assume Shenzhou) in similar situations. For Soyuz/Shenzhou, a failure of the base heat shield to separate means that the landing retro-rockets are not exposed to provide the final "cushion" before touchdown, which is broadly similar to the role of the airbags for Starliner.Quote from: mnAbout the trunk or service module, suppose that fails to separate, it is obviously not protected and will be destroyed on entry, which would ultimately expose the heat shield, is that possible? or a failure to separate would mean LOC? (I understand Soyuz was designed where one part of the entry module will separate due to structural failure from reentry heating if the pyrotechnic separation fails)
(bolding mine)
Small note: Soyuz wasn't designed to ensure its service module's attachment points would "burn off" safely during reentry after a failed separation. Rather, that was discovered the hard way in practice, on the Soyuz 5 mission in 1969. As designed, a failure of the service module to separate was considered a guaranteed LOC event. As recounted in this article about the Soyuz 5 incident:Quote from: AmericaSpace articleWhen they realised what had happened—or, more accurately, what had not happened—several flight controllers buried their faces in their hands. One officer removed his cap, dropped three rubles into it, and passed it along the line; within minutes, it had filled with coins for Volynov’s young family. The cosmonaut was effectively plummeting back to Earth, nose-first, with the least-protected section of his craft exposed to the greatest thermal stress. Moreover, he was exposed to G forces in excess of nine times their normal terrestrial load. Against such overwhelming odds, it seemed that Boris Volynov’s fate was sealed.
As the article (very much worth a read in its entirety!) goes on to detail, the subsequent re-entry was extremely violent and damaged the capsule in ways that would cause further complications (the parachute lines got tangled and one of the landing retro-rockets failed to fire). The survival of the spacecraft and its pilot was very much "by accident" and to the surprise of the designers; they had no expectation beforehand that the more limited application of heat-shielding ablator on the front of the descent module would be sufficient to buy the crew enough time to wait out the burning-off of the service module connectors.
It seems that in the aftermath of the incident, the Soviet/Russian space program may have attempted to retroactively consider Soyuz's empirically-demonstrated resilience to this failure mode to be a designed failsafe. When it happened again nearly 40 years later on the Soyuz TMA-10 mission, Roscosmos neglected to disclose the occurrence to NASA (as no US or USOS-partner astronauts happened to be on board to witness it), on which account NASA was reportedly furious when it happened again on the next flight, Soyuz TMA-11 (which had Peggy Whitson onboard as well as South Korea's first astronaut). That led to more detailed engineering studies jointly by NASA and Roscosmos which concluded, at length, that the service module could in fact be "counted on" with some reliability to burn off and detach before the forward heat shielding ablated to the point of an unsurvivable failure.
As for Starliner (and Dragon)...
Based on the hard-learned lessons of Soyuz, I would imagine that Boeing and SpaceX's engineers put at least some effort into evaluating how their capsules would fare under similar failure modes. However, it's not clear that Starliner and Dragon's designs would be as "favorable" under such a situation as Soyuz's happened to be.
Dragon's trunk, for instance, is attached to the main capsule via attachment hardpoints located on the base heatshield surface, as shown in this picture (picture sourced from this Everyday Astronaut article). Unlike Soyuz, whose service-module attachment points clamp onto the outside of the descent module and thus protrude into the hot reentry airstream, Dragon's attachment points would be "protected" by the trunk's walls. So there's a good chance Dragon's trunk would not "burn off" and detach like Soyuz's (not until well after the crew module's forward heat shielding had failed, at least) - making this a near-certain LOC scenario. This is likely why Crew Dragon changed the separation timeline to detach the trunk before making the deorbit burn, leaving it in a stable orbit where it will deorbit within a few weeks (but buying time to diagnose the problem if it fails to separate) - versus Dragon 1, which detached the trunk after the burn to avoid leaving debris in orbit.
Unlike Dragon (but like every other capsule historically), Starliner doesn't have the luxury of detaching its service module before the reentry burn, because the OMACS thrusters on the SM itself are responsible for doing that burn. Based on pictures like this one (sourced from here), it looks like Starliner uses an attachment system similar to Dragon, i.e. with hardpoints on the base heat shield itself rather than external "claws" like Soyuz. So I suspect it would also be unlikely to burn off cleanly in a face-first reentry. The OMACS/RCS doghouses would maybe be able to "catch some hot air" and burn during reentry since they'd probably protrude out into the plasma stream, but whether that would create enough aerodynamic force to rip the service module free from any failed disconnection points is a more complicated question - never mind what such a "tearing free" would do to the structural integrity of the crew module and the base heat shield. In any case, it's clearly not a scenario with any significant chance of survival, meaning that Boeing would've had to treat it as a single point of failure in their analysis. (In fairness, I suspect Dragon'sheat shieldtrunk* separation is similarly considered a single point of failure from a safety certification standpoint - as there's likely little that could be done to troubleshoot a separation failure even while the capsule remains in a stable orbit. The likeliest recourse would be to return to ISS, but there may not be delta-v for that at that point in the mission.)
*Edited 6/3 to correct a typo (Dragon has a separable trunk, not a separable heat shield, and this is indeed what I meant in the original writing). Thanks kdhilliard for bringing this to my attention via PM! -
#292 by markbike528cbx on 01 Jun, 2022 05:16
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That is why I mentioned that the Shuttle may not be the best example vs Starliner ablator.Not a great example but the Shuttle TPS, heat shield was exposed 100% to deep space (in the typical "upside down" attitude). Debris impacts from foam etc seem to be paramount. The wiki article doesn't even mention in-space meteor(oids). https://en.wikipedia.org/wiki/Space_Shuttle_thermal_protection_system
Ablative heat shield requirements seem to revolve around bond-line heating or cooling. No mention is made of meteoroid or debris impacts in the attached report.
"In space, the temperature requirements for the ablator were limited to a cold temperature of -260" F and a hot temperature of 250 F."
"Additional Block I1 design requirements included a reduction in the ablator temperature extremes in space to +-150" F "
"The temperature extremes without a coating were of the order of +-250" F; and tests had shown that, because of the difference in coefficients of thermal expansion between the ablator and stainless-steel substructure, the ablator cracked if soaked at a temperature less than -170" F."
[Apollo 4] "This attitude cold soaked the thick ablator on the side opposite the hatch and achieved the maximum thermal gradient around the CM heat shield." -----no indication of the minimum temperature achieved, but presumably less that the -150F limit.
I have the attached report because I was wondering why Apollo 13 didn't ditch a useless, deadweight, and possibly still dangerous (hypergolics) Service Module. My take is that they wanted the thermal stabilization of the heat shield that the Service Module provided.
MMOD guy here. Worth noting that shuttle tile and Apollo style ablator have VERY different responses to MMOD impacts.
Shuttle tile tends to crater but otherwise absorb impacts. This makes sense, since shuttle tiles are basically very sparse glass foam. Not much exists to transfer the shock wave of impact through the material.
Ablators like Avcoat (which is what was used on Apollo) tend to shatter, creating much larger damage for the same size impact.
If meteoroid impacts have a large effect, why were they not even mentioned in the overview technical report I previously attached?
I'd bet that Avcoat etc. are more susceptible to shattering when cold. Most things are (roses, bananas etc at liquid N2 temps).
I'd appreciate an answer or guesstimate (or a real source) on why Apollo 13 didn't ditch the SM at some point earlier than it did. -Anybody?
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#293 by Eric Hedman on 01 Jun, 2022 05:23
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I'd appreciate an answer or guesstimate (or a real source) on why Apollo 13 didn't ditch the SM at some point earlier than it did. -Anybody?
I think they were still getting power from one of the fuel cells in it. If I remember they wanted to make sure the capsule wasn't drained of power before atmospheric entry. -
#294 by Steven Pietrobon on 01 Jun, 2022 05:29
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I'd appreciate an answer or guesstimate (or a real source) on why Apollo 13 didn't ditch the SM at some point earlier than it did. -Anybody?
It was to protect the heatshield from the cold of space and meteoroid impacts. This was one of the reason's why the CSM/LM was put in BBQ mode (a slow axial rotation, with the rotating stack side facing the Sun) on the way to the Moon and back. There was concern that if the heatshield became too cold, it would crack. There was also a concern that propellant lines would freeze.
https://www.universetoday.com/145636/even-more-things-that-saved-apollo-13-part-1-the-barbecue-roll/ -
#295 by Eric Hedman on 01 Jun, 2022 05:35
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This article has the best picture I've seen of the damaged service module shortly after it was jettisoned.
https://arstechnica.com/science/2020/04/heres-an-unprecedented-look-at-apollo-13s-damaged-service-module/ -
#296 by Eric Hedman on 01 Jun, 2022 05:39
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Here's a decent synopsis of what happened to the service module on a NASA page:
https://nssdc.gsfc.nasa.gov/planetary/lunar/ap13acc.html -
#297 by Ike17055 on 02 Jun, 2022 00:28
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<snipped the embedded image, which is not allowed at NSF>
I’m sure this has been hashed and rehashed, but this picture vividly illustrates why I would never fly on this spacecraft. Yes, all space vehicles have a number of deployments that must go perfectly for any crew to survive. But the engineering design decision to return to land, necessitating the requirement to make the heat shield completely detachable, is I believe, a bridge too far. Especially when you consider a very heavy service module also must be jettisoned minutes before that heat shield is needed to protect the occupants from a now-unavoidable fiery plunge into the exponentially dense lower atmosphere.
I will casually point out that Soyuz has been doing this very thing (dropping the heat shield shortly before touchdown on land) successfully since the 1960s. Shenzou has been doing the same thing since the early 2000s.
It is nothing new, it is fully understood and most definitely NOT an engineering bridge too far. Neither is jettisoning a service module btw, which has been done by Gemini, Apollo, Soyuz and Shenzou. And both cargo Dragon and Crew Dragon do something very similar: they jettison their trunks to expose their heat shields.
good thoughts. the amount of concern trolling on this thread is remarkable. the fact is that every flight for every vehicle is brim-filled with critical events, including numerous separation events that are absolutely essential to safe and successful flight execution from launch to landing- and even afterwards. Focusing on one event as all-important or the sole "weakness" or lone possible showstopper is tunnel-visioned and heaps undue concern on a small portion of the risk envelope. -
#298 by yg1968 on 02 Jun, 2022 04:27
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So at this stage, what are the big homework items from OFT-2 before the next flight?
OMAC thruster issues is the obvious one...
During the post-landing press conference, Steve Stich mentioned all of the issues at the beginning of the press conference: OMAC, RCS and coolant issues are the ones that I remember. He didn't say which ones required more work and unfortunately nobody asked him either. -
#299 by Lee Jay on 02 Jun, 2022 12:58
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Nonsense and a rubbish argument. The earth will never be flat, it's an impossibility so it's just wrong to claim it is, but in years to come we could see an issue with the starliner system ( I think it's a fine engineering solution btw) which renders the engineering questionable. So yes his opinion is valid and can't be dismissed out of hand.Opinions can't be wrong Jim. They are his/her opinions and while you might not agree , he/she is allowed to have them and they don't always have to match yours.
A too-often held wrong statement. Opinions can be wrong if they are about objective reality. Lots of people have wrong "opinions" - flat Earth, creationism, COVID vaccines have microchips in them, etc.
You're only entitled to have opinions about subjective subjects, not objective ones.
What I said is true - opinions are for subjective things only (favorite color, this food better than that food, art, and so on). Objective reality is based on data. Mother Nature doesn't care if you think you have a right to your own "opinion" or not.