STS Shuttle discussion thread

[Green Baron]

... 2 of 5 orbiters lost, 14 dead astronauts, over complicated to the edge of manageability, questionable design (tile system, seals between booster stages), needed fundamental checks after each mission, expensive beyond a question why. Didn't even remotely fulfill its initial missions or even reach a state of regular flights as was envisioned at the kickoff.

But nice to watch. The best thing about it was the construction of the ISS.

I'm sure i soon get what i have coming :-)

[PB666]

1 minute ago, Green Baron said:

... 2 of 5 orbiters lost, 14 dead astronauts, over complicated to the edge of manageability, questionable design (tile system, seals between booster stages), needed fundamental checks after each mission, expensive beyond a question why. Didn't even remotely fulfill its initial missions or even reach a state of regular flights as was envisioned at the kickoff.

 

Mostly true, while it did not fulfill its full envisioned missions it did fulfill other missions.

[kerbiloid]

38 minutes ago, PB666 said:

Lets see what happened in the wake of the shuttle . . . . .

NASA has no manned launch system, the manned launch system that is under contract is rampant with cost overruns, has not anywhere near being tested, it only useful a limited number of manned operations, its incredibly expensive, to be launched on the most expensive launch system that exist, alot of QC issues in that system.

Meanwhile, no in space repair capability (except ISS and almost nothing is in its elliptical). No ability to assemble platforms in space, no ability to do manned science missions outside of the ISS.

And also 7 years after the fact no gateways, no manned missions to celestials, no replacement of shuttle with more evolved systems, just vaporware.

One basket for all eggs.

[tater]

Forget Challenger, that's the Shuttle would could save in a counterfactual by simply putting off the launch tile the weather warmed up.

The constant tile damage due to shed foam is a fundamental design flaw. Note that other LVs with expanding urethane foam insulation do not share this failure mode---since there is nothing critical to destroy below the foam.

[sevenperforce]

4 minutes ago, tater said:

Forget Challenger, that's the Shuttle would could save in a counterfactual by simply putting off the launch tile the weather warmed up.

The constant tile damage due to shed foam is a fundamental design flaw. Note that other LVs with expanding urethane foam insulation do not share this failure mode---since there is nothing critical to destroy below the foam.

In theory, having a crew cabin which is an escape pod lifeboat with its own single-use ballistic-entry heat shield could obviate a large portion of the problem. Foam strikes could still result in LOV but they would have to strike the crew cabin directly in order to cause LOC...and at that point it would be a LOC during launch anyway.

[tater]

4 minutes ago, sevenperforce said:

In theory, having a crew cabin which is an escape pod lifeboat with its own single-use ballistic-entry heat shield could obviate a large portion of the problem. Foam strikes could still result in LOV but they would have to strike the crew cabin directly in order to cause LOC...and at that point it would be a LOC during launch anyway.

True, but such a pod likely impacts payload capacity negatively. Still a plus, though.

A modern analog might be a BFS mixed crew/cargo version. My question about crew size was getting at this. If 10 is about as many as you might want, then the crew portion could be designed as a lifeboat at the very top, with the clamshell cargo bay behind. Much if it will have TPS anyway, it just needs to wrap around (assuming a stable, capsule-like reentry profile).

[sevenperforce]

10 hours ago, PB666 said:

That is bull _____. The 12 strut failed (dont talk here if you are not going to read the report)

The last person who told me that I hadn't read the reports I'd read was a creationist.

10 hours ago, PB666 said:

THe report basically says this, we looked at everything we could look at, from the orbiters components and child components, to the tank to the engines, no fault could be found in any of the systems other than the damage that the SRB caused, its seems political, but tracing the burn marks on the wings, the tiles on the core, timing of gas releases, . . . . .the tempest was moving very rapidly from one state to the next within essentially 1 second the 12 broke it forced the ET into the orbiter it slammed into the tank and then hit the orbiter and at that point everything disintegrated.

What, exactly, are you trying to prove here?

While swinging wildly, the crippled SRB struck the right wing of the orbiter. We know it struck the right wing of the orbiter because the tiles recovered from the seafloor show the impact point. However, we also know that it did not rip the wing apart, because the right wing is visible in the footage, tumbling out of the smoke cloud.

Did the SRB shear the wing off completely or simply weaken the structure so that it severed along the vehicle fuselage? We do not know. However, we do know this is the only place that the SRB struck the orbiter, and the subsequent destruction of the rest of the orbiter was due to aerodynamic forces, not due to direct impact by the SRB.

[PakledHostage]

5 minutes ago, Green Baron said:

... 2 of 5 orbiters lost, 14 dead astronauts, over complicated to the edge of manageability, questionable design (tile system, seals between booster stages), needed fundamental checks after each mission, expensive beyond a question why. Didn't even remotely fulfill its initial missions or even reach a state of regular flights as was envisioned at the kickoff.

But that's just it. Is the glass half full or half empty? A lot of the criticism of the Shuttle program seems to be motivated by people's political ideology, by SpaceX tribalism, by people's belief that the Shuttle's very existence held us back from achieving some "Buck Rogers" vision of the future, etc. But those are all far from objective perspectives and the fervent certitude of those shuttle program haters is tedious. Sure the Shuttle design and program were far from perfect, but don't throw the baby out with the bath water... We learned a lot from the Shuttle program, both good and bad. Appreciate the upsides, learn from the mistakes, then move on and make it better next time. That's a large part of what real world engineering is all about.

[sevenperforce]

4 minutes ago, tater said:

True, but such a pod likely impacts payload capacity negatively. Still a plus, though.

A modern analog might be a BFS mixed crew/cargo version. My question about crew size was getting at this. If 10 is about as many as you might want, then the crew portion could be designed as a lifeboat at the very top, with the clamshell cargo bay behind. Much if it will have TPS anyway, it just needs to wrap around (assuming a stable, capsule-like reentry profile).

AFAIK, SpaceX has no plans to launch both crew and cargo on the same BFR flights, other than what can be flatpacked and accessible from the crew cabin.

Admittedly, it doesn't make sense to send an LAS-equipped ship all the way to Mars. If you're on Mars, LAS doesn't help you much.

If they ever launch people on the BFR, I would hope they build a short-duration crew-only version with a jettisonable crew cabin. Even if the upper stage engines can spool up and fire quickly enough to abort away from a first-stage RUD, this does nothing whatsoever to help in the event of an upper-stage malfunction.

And yes, any such aux system will broadly impact payload capacity. But that's the price you pay for safety.

10 minutes ago, PakledHostage said:

But that's just it. Is the glass half full or half empty? A lot of the criticism of the Shuttle program seems to be motivated by people's political ideology, by SpaceX tribalism, by people's belief that the Shuttle's very existence held us back from achieving some "Buck Rogers" vision of the future, etc. But those are all far from objective perspectives and the fervent certitude of those shuttle program haters is tedious. Sure the Shuttle design and program were far from perfect, but don't throw the baby out with the bath water... We learned a lot from the Shuttle program, both good and bad. Appreciate the upsides, learn from the mistakes, then move on and make it better next time. That's a large part of what real world engineering is all about.

I don't see blind criticism of the whole thing. I see a lot of people saying that the Shuttle should have been treated as an X project, with its lessons-learned integrated into a better system.

[Green Baron]

12 minutes ago, PakledHostage said:

...Is the glass half full or half empty?  [...]  Appreciate the upsides, learn from the mistakes, then move on and make it better next time. That's a large part of what real world engineering is all about.

Well, 2 complete failures in 13x missions is bad. For a human rated vehicle it is abysmal. And it wasn't necessary to learn that way, both causes where known before they ended in catastrophes and simply ignored (which i use as another word for "not regarded an immediate danger").

Edit: personal opionion, of course ...

Edited January 17, 2018 by Green Baron

[PB666]

36 minutes ago, tater said:

Forget Challenger, that's the Shuttle would could save in a counterfactual by simply putting off the launch tile the weather warmed up.

The constant tile damage due to shed foam is a fundamental design flaw. Note that other LVs with expanding urethane foam insulation do not share this failure mode---since there is nothing critical to destroy below the foam.

It was a fundemental design flaw in the ET, not in the overall design, I will address this elsewhere.

[PakledHostage]

22 minutes ago, tater said:

Forget Challenger, that's the Shuttle would could save in a counterfactual by simply putting off the launch tile the weather warmed up.

We went over this yesterday: The Shuttle (and any rocket, for that matter) must operate on razor thin margins or it couldn't do what it needs to do. Operate it outside of those razor thin margins and you'll run into problems. This is progressively more true of any high performance machine. 

In a more mundane example, the parameters for every takeoff of every commercial airliner flight anywhere are pre-calculated by the airline's dispatchers and flight performance engineers and then provided to the pilots as part of their flight plan. The takeoff performance analysis factors in things like runway surface conditions, density altitude, wind speed, fuel and payload weight, etc. If the pilots deviate from the plan and operate the flight outside of the margins that they are provided, there's a good chance that things will end badly. That's not a design flaw on the part of transport category aircraft. That's the nature of the business when you are operating high performance equipment.

 

[PB666]

22 minutes ago, PakledHostage said:

 fervent certitude of those shuttle program haters is tedious.

worse than tedious it hatred wrapped in revisionist thinking and generally bad logic.

[Green Baron]

@PakledHostage, i think the pilots do the calculations for everything concerning the aircraft and flight themselves, according to the flight manual, orders of their employer, and other regulations like safety advisories not yet incorporated in manuals and procedures, notams, etc.. Before all they have full responsibility for the flight and nobody else.

Especially take off run calculations, weight and balance, fuel consumption, weather, choice of alternate airports, decide if the flight takes place or not in case of mishaps or errors, etc. is pilot's job and nobody else, the latter after having talked to the back office of course.

When it comes to provisions, order a hotel in case of redirection or deviation, right for overflights etc. pp. then back office comes into play.

Edited January 17, 2018 by Green Baron

[PB666]

13 minutes ago, Green Baron said:

Well, 2 complete failures in 13x missions is bad. For a human rated vehicle it is abysmal. And it wasn't necessary to learn that way, both causes where known before they ended in catastrophes and simply ignored (which i use as another word for "not regarded an immediate danger").

Edit: personal opionion, of course ...

Given the number of people place in space its par with the next best launch system, albiet soviet/russian design. But I don't know what world you live in, I live in the world that exists, and in that world putting 10³ people in space with the loss of 10¹ is in the realm of what space flight's risk are. Could have it been better. . . . .see other posts.

But before you condemn the design the issue that you need to parse out is how many astronauts did the politicians kill and how many did the design and safety engineers kill. The challenger was a two-fold management failure.
It was the Nixon's White house office of budget and Management that decided to go with the SRBs to save development costs (which is silly for a high use vehicle) and it was the NASA admin that decided to launch outside of the SRBs operation window. You can't really pin those 7 deaths on the shuttles design,  it was if the managers went into the crew cabin and threw into a grenade and cold heartedly left. You cannot blame the pilot of a 747 for crashing a plane if the manager of the airport doesn't inform him that there is a giant hole in the runway, can you?

 

[Green Baron]

8 minutes ago, PB666 said:

The challenger was a two-fold management failure.

I share your opinion, i just didn't want to step on forbidden ground ;-)

Quote

You cannot blame the pilot of a 747 for crashing a plane if the manager of the airport doesn't inform him that there is a giant hole in the runway, can you?

Nope, you can't, for that exists the system of NOTAMs, that inform the pilots about these annoying mishaps.

"Tower, this is xyz123, there's a burning runway light !"

"I hope there are more than one runway lights burning ..."

"Sorry, i mean, it smokes and is on flames."

Edited January 17, 2018 by Green Baron

[PakledHostage]

10 minutes ago, Green Baron said:

@PakledHostage, i think the pilots do the calculations for everything concerning the aircraft and flight themselves, according to the flight manual, orders of their employer, and other regulations like safety advisories not yet incorporated in manuals and procedures, notams, etc.. Before all they have full responsibility for the flight and nobody else.

 No.

[Starman4308]

16 minutes ago, PakledHostage said:

But that's just it. Is the glass half full or half empty? A lot of the criticism of the Shuttle program seems to be motivated by people's political ideology, by SpaceX tribalism, by people's belief that the Shuttle's very existence held us back from achieving some "Buck Rogers" vision of the future, etc. But those are all far from objective perspectives and the fervent certitude of those shuttle program haters is tedious. Sure the Shuttle design and program were far from perfect, but don't throw the baby out with the bath water... We learned a lot from the Shuttle program, both good and bad. Appreciate the upsides, learn from the mistakes, then move on and make it better next time. That's a large part of what real world engineering is all about.

Let's put this into perspective.

The overall cost of the Shuttle program is estimated at $196 billion (2011). This gives us a per-launch cost of $1.45 billion. For each launch, they could put 27500 kg of payload into orbit, 7 crew, and 68,585 kg of dry mass.

Per kilogram of payload put into a permanent orbit, that's $52,727 per kilogram.

Per kilogram of payload and dry mass (excluding only the 10 tons of SSME), that's $16,863 per kilogram... and I suspect a good fraction of that dry mass was not very useful for extended in-orbit capabilities as a "mini space station".

 

Compare that to the 20,520 kg payload and $153 million cost of an Atlas 551. $7,456 per kilogram placed into a permanent orbit.

Compare that to the estimated $405-653 million to put a CST-100 Starliner or Crew Dragon into orbit.

To match the max payload and dry mass of a Space Shuttle, you're talking about $641M for cargo and $653M max for the crew. Even under ludicrously favorable assumptions such as "every kilogram of not-SSME is equal to a kilogram of payload placed in permanent LEO" and "every crewmember put into orbit was really needed in orbit even for launching a commsat", the Space Shuttle was an overly expensive HLV... and as a crew launch vehicle, it was an unsafe disaster.

 

While I would like to fondly reminisce on something that got cancelled before I got interested in spaceflight, and talk about some of the fun missions it carried out, I've been stuck trying to continually point out (and not necessarily to you) that it was, as point of fact, a trainwreck from day 1.

Incidentally, the link above, https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170008895.pdf , is a good read on the commercial launch services program.

[Green Baron]

5 minutes ago, PakledHostage said:

 No.

I learned something :-)

[PB666]

2 minutes ago, Starman4308 said:

Let's put this into perspective.

The overall cost of the Shuttle program is estimated at $196 billion (2011). This gives us a per-launch cost of $1.45 billion. For each launch, they could put 27500 kg of payload into orbit, 7 crew, and 68,585 kg of dry mass.

Part of that cost was created by stupid management decisions and impositions by the government on the program, decisions that were best left to engineers, not politicians and political bean counters. Lucky for spaceX they only have to report their performance and capability to the government, and, simple-mindedly, the government can choses a contractor of the lower costs. When politicians and management start overthinking issues the problems and cost go out the roof. Elon (his hand on his wallet) has to decide whats in the best interest, he has a vested interest in learning, Spiro Agnew has no vested interest in learning what NASA needs, he was just a corrupt politician that was forced to resign, and then his boss was forced to resign and that all went into the stupidities in the space-shuttle ET and SRBs.
 

[GoSlash27]

PB666,

13 minutes ago, PB666 said:

(wall of invective, specious reasoning, ad hominem attacks)

"Here in the real world", the shuttle program was retired because NASA deemed it too unsafe to fly. Period, full- stop.
Furthermore, I've grown tired of your unwillingness to maintain a respectful tone when everyone else here is doing their best to be civil and polite despite their disagreements.
 Do me a favor and please refrain from addressing me anymore until you decide to do so in a civilized manner.

Please and thank you,
-Slashy

[tater]

1 hour ago, PakledHostage said:

But that's just it. Is the glass half full or half empty? A lot of the criticism of the Shuttle program seems to be motivated by people's political ideology, by SpaceX tribalism, by people's belief that the Shuttle's very existence held us back from achieving some "Buck Rogers" vision of the future, etc. But those are all far from objective perspectives and the fervent certitude of those shuttle program haters is tedious. Sure the Shuttle design and program were far from perfect, but don't throw the baby out with the bath water... We learned a lot from the Shuttle program, both good and bad. Appreciate the upsides, learn from the mistakes, then move on and make it better next time. That's a large part of what real world engineering is all about.

Shuttle was an amazing vehicle. I agree with much of this post, in particular learning from mistakes. I think that they should have made Shuttle 2.0, and very early in the program, frankly, after they understood the operational problems that grossly slowed vehicle turn around from the way the program was envisioned.

52 minutes ago, PakledHostage said:

We went over this yesterday: The Shuttle (and any rocket, for that matter) must operate on razor thin margins or it couldn't do what it needs to do. Operate it outside of those razor thin margins and you'll run into problems. This is progressively more true of any high performance machine. 

In a more mundane example, the parameters for every takeoff of every commercial airliner flight anywhere are pre-calculated by the airline's dispatchers and flight performance engineers and then provided to the pilots as part of their flight plan. The takeoff performance analysis factors in things like runway surface conditions, density altitude, wind speed, fuel and payload weight, etc. If the pilots deviate from the plan and operate the flight outside of the margins that they are provided, there's a good chance that things will end badly. That's not a design flaw on the part of transport category aircraft. That's the nature of the business when you are operating high performance equipment.

 

Razor thin margins are the order of the day given payload mass fractions that are possible. This is a crew vehicle, however, and the simplest math is that crew safety costs non-crew payload mass fraction, so the best solution is to separate things (as my mother always used to say "things are replaceable"), and people, who are NOT.

Comparing to airliners makes no sense given that airliners are many orders of magnitude safer than Shuttle. As a result, their safety margin calculations any given day result in them being only 1 million times safer than Shuttle, rather than 1.1 million times safer (or whatever, I'm making up those numbers, but airliners are an incredibly safe mode of transport). What if every single aircraft flight had sheets of ice come off the nose and sometimes impacting the wing or engines? What if 1 in 135 of those ingestions caused a LOV incident. Would anyone fly like that, or would they redesign the nose so it didn't ice up---even if the plane could only hold half as many people as a result?

Edited January 17, 2018 by tater

[Starman4308]

From the cargo end, why send up replaceable cargo on an expensive man rated vehicle? Better to send that up on something cheaper, even if it doesn't reach the extreme safety requirements of a manned vehicle... and send up the astronauts separately. 

[tater]

I had a love-hate relationship with Shuttle from day one. It was AWESOME to see. I still get goosebumps watching Shuttle launch videos. I've met many Shuttle astronauts, heck, I've driven a bunch of them in my car, had meals with them, etc., and I heard many stories about what it was like aboard. In a sense, I still love Shuttle. But I hate it, too, partially for reasons @PakledHostage has mentioned about some people above. Because of the opportunity cost to NASA. Because it wasn't the holistic system first envisioned, just one part, without any of the other parts (tug, ferry, etc). I hate that it got bloated. I hate that the design was frozen at a point where it was not economical... never the "pickup truck to space" it was pitched as.

I also realize that due to the nature of NASA (and government in general), it could not be evolved in the way it really needed to. It was simply too expensive to change it in a significant way, and likely risk aversion played a role here as well. Any new vehicle faced an STS-1 moment with people riding up something utterly untested, and they chose the devil they knew. I get it, but that doesn't mean I have to like it. As someone who was a space nut in the 70s, and languished waiting on my 2001 future during the 80s, then 90s, and up until the present, I can get a little... opinionated regarding Shuttle. That's not gonna change without a counterfactual history happening .

Edited January 17, 2018 by tater

[PB666]

11 hours ago, Starman4308 said:

For example, while the proximal cause for the Challenger disaster was a management issue of ignoring the engineers, the proximal cause is just one element of the disaster.

Lets break this down into its components and then solve the problem. But instead of looking just at challenger lets look also at columbia.

1. What caused the incidents
2. What caused the loss of orbiter
3. What caused the loss of life
4. Cost of system and cost of system failure.

 

1. The logic here is that component failure creates risk that propagates through the system, as we see the shuttle challenger that once a serious problem in one part of the system occurs, it can at a point in time evolve rapidly.

Design issues. Shuttle was designed to be the primary replacement to the Apollo mission
The plan was drafted in 1969 (49 years ago) to be a system of reuseable spacecraft (Phase A). The primary intended use was to support a space station and ferry 4 crewmembers back and forth to space with about 10 kT of payload. Early in the projects evolution it was realized that the project would be difficult to fund, but the DoD stepped forward offered support indicating it could be used as part of its operations, also.
 

Quote

The shuttle supporters answered that given enough launches, a reusable system would have lower overall costs than disposable rockets. If dividing total program costs over a given number of launches, a high shuttle launch rate would result in lower per-launch costs. This in turn would make the shuttle cost competitive with or superior to expendable launchers. Some theoretical studies mentioned 55 shuttle launches per year, however the final design chosen would not support that launch rate. In particular the maximum external tank production rate was limited to 24 tanks per year at NASA's Michoud Assembly Facility.

The combined space station and Air Force payload requirements weren't sufficient to reach desired shuttle launch rates. Therefore, the plan was for all future U.S. space launches—space station, Air Force, commercial satellites, and scientific research—to use only the space shuttle. Most other expendable boosters would be phased out. Wikipedia (space shuttle design process)

At the onset we see we have a problem in the infrastructure, even before the shuttle is built, that if ground services are not extended, then there would definitely be backlogs and slowdowns. Again this was not the shuttles fault, congress authorized the shuttle but failed to authorize the support structures. - Political problem one. This caused backlogs in the launch and this would eventually push one launch into another shuttles launch forcing the second to move its window to the 'edge' (challenger). 

Quote

The reusable booster was eventually abandoned due to several factors: high price (combined with limited funding), technical complexity, and development risk. Instead, a partially (not fully) reusable design was selected, where an external propellent tank was discarded for each launch, and the booster rockets and shuttle orbiter were refurbished for reuse.

This was 1971 and they did not have access to all the facilities (computers guidance satellites) and materials we have today. But at some point in time this could have evolved . . . it didn't (Political management issue).
 

Quote

Because the space agency needed outside support, the Defense Department (DoD) and the National Reconnaissance Office (NRO) gained primary control over the design process. For example, NASA planned a 40 feet-long and 12 feet-wide cargo bay, but NRO specified a 60 feet by 15 feet bay because it expected future intelligence satellites to become larger. When Faget again proposed a 12 feet-wide payload bay, the military almost immediately insisted on retaining the 15-feet width.

So we see why the cargo bay was extended. The wider bay did benefit the Hubble program and the ISS. But we can see how the NRO has leveraged itself into the process.

Quote

While NASA would likely have chosen liquid boosters had it complete control over the design, the Office of Management and Budget insisted on less expensive solid boosters due to their lower projected development costs.^{[3]:416–423[7]} While a liquid-fueled booster design provided better performance, lower per-flight costs, less environmental impact and less developmental risk, solid boosters were seen as requiring less funding to develop at a time when the Shuttle program had many different elements competing for limited development funds

So now we have for a civilian Launch system the Air Force, NRO, and the office of managment and budget getting their fingers into really what is an engineering issue. This was poor decision making, it you are building a sytem to be used 1000s of times, you pay the extra development costs. - Management issue
Rather than go with the widely tested Rocketdyne F-1 type rocket the decision was made to go with a virtually untested form of booster.

Quote

"faulty design unacceptably sensitive to a number of factors" of the SRB joints compounded by unusually cold weather the morning of the flight.^[11][12] The commission found that the large rubber "O-rings" in SRB joints were not effective at low temperatures like those of the January 1986 morning of the accident (36 °F (2.2 °C)). A cold-compromised joint in the right SRB failed at launch and eventually allowed hot gases from within that rocket booster to sear a hole into the adjacent main external fuel tank and also weaken the lower strut holding the SRB to the external tank.

It should be noted that despite the SRBs inherent design flaws and limitations is a very similar version has been chosen for use on SLS. - management problem is ongoing.
Can Nasa/Admin/Congress be trusted to select manned launch vehicles at all?

b. the external fuel tank and the cause of the Columbia disaster.
The second issue was the insulation and Ice issue on the ET. The ET's issue are of a different nature. The shuttles very high ISP (a good thing) was provisioned by liquid gasesof molecular  hydrogen and oxygen. The original design was to have H2 and O2 on board the orbiter, but this compromised the payload capability. So it was moved to the external tank. The ET is now one tank, its 3 tanks, 2 tanks inside the visible ET. This is attached to the orbiter. The ET design was all about getting the percent of weight in tank relative to fuel to the lowest possible, conservation of gases was not a major theme at the time (seventies and gas was 20 cents a gallon). 

Most of what tater has said about the tank is correct with regard to an inherent flaw, but as we will see its not a fundamental flaw as much as it is a design oversight.
So first, let us look at the design. For the cost of the SR-25s to be justified they need to provide the majority of acceleration, they are one of the few vehicles were the engines operate from launch to orbit, of the 9000 dV required to reach orbit, the SSME provide about 3/4ths of the acceleration, not powerful enough to provide lift off and vertical momentum, once these are provisioned the SSME carry the orbiter up. And pretty much as an isolated system they have only had two incidences on was qualified as an abort to orbit at a lower orbit 51-F. 

There are two areas of the ET design that bring questions. The first is for fuel loading, when you add cryogenic gases (which I have done so many many times) you have an outgassing that is much larger in volume than the gas you put into the system. The gas hits the edge of the vehicle and it boils and at first the gas comes back warm but quickly evolves to a very cold gas as the entire system cools. So as the gas reaches the air you get nuclei (that which forms snow) and it gets over everthing, your gloves, the sides . . . . .Ice. As the wind blows the 'snow' can melt and refreeze into chunks of ice which can fall and hit the orbiter or the struts on the SRBs during flight, both are bad. So the solution is to move the fills on the slope of the ET that faces away from these, if Ice forms here and falls it falls away from the axis of the ET-Obiter interconnect plane.

The second issue is ice that forms around the top of the foam and the foam itself. There are two issues. The first one is this what is the limit of foam that can be placed on the ET that in flight would never strike the shuttle. If we look at the tank about 80% of the ET can be covered with foam and never risk hitting the orbiter. So it doesn't necessarily involve covering all of the foam, just some of the foam, or likewise it means moving some of the foam inside the shuttle. THere are problem with doing this and maintaining function.
As an aside we are talking about significant vehicle evolution something that should have occurred in the 90's and 00's

i. That the SLWT was the only tank rated to reach ISS with PL and crew, which means weight is added.
ii. That the ET may have to be reshaped slightly to improve the insulation and reduce ice.

OK so this problem as discussed in other posts is that having a 6-11 capacity is not required, in the worst possible situtation if you had 4 crew on the shuttle 3 ISS could evacuate to soyuz and 3 to shuttle resulting in 4 to 7 crew compliment. Updated computers to govern almost all aspects of flight and lower crew numbers and reduction in weight could have allowed the ET to add weight and safely mediate the ice issue on the orbiter side. But the shuttle really does not need to go to the ISS, it need only to reach an orbit where things can be assembled and the under their own power move to a higher orbit. Orbiters, for instance now can be made with carbon-fiber.

So now having the weight we need where do we apply it. To mitigate the ice issue you need a barrier between the foamed tank and unprotected part. This can be done by bolting into the shell of the ET a section of carbon fiber about 3 or four inches wide (the insulation on a typical N2 tank is about an inch and half). And on top of this you would have foam but at the terminal you would have Al-Li composite. The internal struts likewise would have CF intermediates as they penetrate. The new more ovoid cross section tank would have insulation between the liquid storage tanks and the ET wall. In the wall you would, toward the top have a very simple serpentine wire heater that kept the metal above the melting temperature of water. This is only needed at the top below which was not needed as the ice would not likely impact the orbiter below a certain distance.

These are relatively minor design changes, overall the tank is the same, you have an additional 4 inches material on the orbiter side inside the tank, the fuel tanks are slightly shifted and more elliptical, the insulation could be done by an aerogel or other light weight insulation, about the same mass as the foam, all you need is the carbon-fiber expansion brackets that run the length of the tank on both sides and the intertank heat-transfer blocks built into the struts. Again you would never see these.

2. Preventing orbiter loss.
Even if we fail to consider loss-of-life, since the orbiter is a reusable vehicle, economically we want to depreciate it over its life (which in accounting should provides the capital for its future replacement). This means that as we use an orbiter its function provides a cost basis to provide for its own replacement through earning and savings. In government talk some aspect of each mission is cycled back into the fund to replace the capital equipment that makes the mission possible and this way costs are divided over time. In business risk of failure is an added insurance cost which increases cost and we don't want to do this. 

Under two circumstances, as mentioned the loss of orbiter is result of launch damage. That revolves around two issues. First, the loss of the challenger is blamed entirely on the SRB, badly designed for manned space flight and badly implemented. The SRB should not be used and oddly the OMB should have forseen the trouble with the SRB and opted for more developed LFRB. As stated repeated the type of failure that the Morton-Thiokol 4 sealed SRB produces is unacceptable and its launch restrictions are unacceptable in manned space flight, and oddly they are being used on the SLS. There is no way to attenuate a faulty SRB in flight,  .Management oversight of SRB use should be rigorous to a fault. 
 
The second issue is shuttle-tiling system and again we have two situations, 35000 tiles needed to be replaced, but 35000 of the tiles damaged were not critical, but one or two on the columbia were and the orbiter disintegrated. Again this is an issue of evolution from 1975 to 1983 the learning curve was pretty steep, this does not mean that learning stops. The shuttles heat resistant skin could have evolved overing both lower weight and increased resilience to damage. Above we see that the ET could have been improved to prevent damage.
Again we see a circumstance that while in flight damage was noted, the damage could have been surveyed at ISS and crew members could have stayed on the station along with orbiter while a resolution to the problem was found. Again a shuttle could bring back eleven and the orbiter could have been moved to another location while solutions were found. But once again we had a management failure.

Orbiters will not last forever and eventually flight stresses will overcome them, so its almost better to retire and evolve orbiters after 10 or so flights rather than to take risk with older orbiters.

3. Preventing loss of life, this almost entirely shows itself to be a management issue. Don't fly the any craft outside of the design tolerances of its composite parts. Again if a tile can withstand a force of impact of 200 lbs at a foot, then you don't expose the tile to 2 pounds traveling at 100 mph. Every part has a tolerance so its simple enough to decide, does this situation create an excessive deviation from design parameters, then the simple answer is don't launch. The SRB problem and ET problems can be mitigated by component evolution. The question is that if it can evolve should it have evolve.
So the concept to design phase was 1968 to 1975ish. That was 40 to 50 years ago, how much has your phone changed in 40 to 50 years, what about your car, your house. Everything you do has changed in the last 50 years.
We cannot argue that fixing the shuttle's structure is not a good thing. The first flights of the shuttle were a pioneering engineering feat but then who travels around in Connestoga wagons now-a-days. We would be all easting at Lake Tahoe DIY take out  . . .Donner,  party of 5.

The loss of life problem is tied to the fact that it should have evolved more rapidly. The replacements should have been notably more evolved than the first, the shuttles should have been retired after 10 flights under the condition that new shuttles were evolving toward some futuristic goal. And every major component of the complex system should have been treated as its own launch vehicle and the critiques that go with a more simple analysis. One of those goals would be greater safety. The shuttles ET facilities and assembly facilities should have been doubled in capacity and again we should have hired contractors to help get those LFBs to RTLS once their mission was complete.

So then to the loss of life question, by what the shuttle did and by 1968-1980 standards the shuttle was pretty fantastic system. But then if we argue, as Tater so aptly put, a system that repeatedly experiencing the same life threatening safety issue since day 1, then we should not look at the system in say 2000 with the same enthusiasm as we do in 1985. Of course we don't, who should? But the problem is that the fed is neither a scientist or an engineer, almost all appointments are political and that in-and-of-itself is a design compromise. But that's the nature of the beast, its up to people and organizations to insist on that evolution, its not a fault of the shuttle that it did not evolve to become more safe, its a fault of the people who were responsible for caring for the STS.

4. Cost of mission and cost of failures.

The cost of failures cannot be underestimated. Both instances were intangible liabilities on the shuttle and space program in general. Not only they lead to cost overruns and costly delays. Challenger cost the program about 2 man-hour years. But the challengers failure can be rooted into a decision made by OMB 10 years previously and it was a bad decision. So of OMB is an oversight watchdog, who then watches the watchdog? Again the engineers on the program should have been more emphatic, because ultimately they will be harmed.

The cost of missions is debatable. The problem in the design and use we have to consider also alternatives and their repertoire of functions.
So there are metrics.
You have a vector state on an elliptical in space, you need to send 3 men to and from that vector, do you need a shuttle . . . .no.
You have a vector state on an elliptical in space, you need to send supplies do you need manned space flight . . . no.
You have a vector state on an ellipitical in space and you need need to send supplies, 3 men, and retrieve experiments and waste and return to earth do you need a shuttle, it might be useful.
You have a vector state on an elliptical in space and you need to add a structure to it and you need men to help do this . . .its a little more useful, but computers have replaced this function (e.g. biglelow pod).

Over time the cost function of the shuttle has increased because the alternatives either have increased performance or decreased cost . . . this is the way the system should evolve.
You have a concept state in space and you want to build on that concept (usually something bulky are heavy) do you need a shuttle
a. you can insert a reference structure in space without a shuttle.
b. manned extensions of that space require a facility, shuttle was the only provider. The current replacement is vaporware.
c. manning of the extension does not require shuttle

You have a valuable asset in space that needs tending or collection and it is not in the plane of the ISS, you need the function of the shuttle.

So that we can see that benefit to cost ratio of the shuttle has declined over time, but its still present, there is still benefit to some  of its functions that are not (or have not been) replaced.

Some of NASA's new projects are talking about throwin 30 billion dollars into space, but its mostly vapor ware and the plans change every 6 months but there is little hardware to show for it.
There is also a cost associated with any progressive society for not evolving or advancing, there is an intangible benefit to the non-redundant functions of the shuttle, and that is why it should not have been cancelled until its suitable replacements had been completed. As it looks to me, these replacements are not near-future, they are only waving hands. And if by some miracle they do appear . . . .SpaceX.