The Debate of Solid vs Liquid

[sevenperforce]

1 hour ago, p1t1o said:

Spoiler

 

 

Ok so some push-back, and on examination, perhaps I should not have typed "WAY" in all-caps, twice 

But solids *are* the cheap option. And I wasnt using KSP as my yardstick either.

Its not merely about price-per kilogram of fuel, its about how much it cost to get your payload into the desired orbit. How can anyone play KSP and presume that cost can be derived from a single value like price per unit of fuel?

Solid might have poor Isp, and they might be more expensive dollar-per-kilo (although I had a lot of difficulty confirming just exactly how expensive the average SRB is, nor is it particularly easy to find quoted prices on bulk amounts of what are essentially explosives....I may be on a few more watchlists) but they are cheap in terms of dollar-per-unit-thrust.

And some more things:

You really cannot discount the simplicity of their construction. Nor can you ignore the development costs for solids, which are certainly much less than that of liquid engines and does have an impact on the cost of launches even well after development is complete. Rocket scientists gots to gets paid.

Kerosene, one of the safer liquid fuels, is easily stored...only compared to things like cryogenic liquids or toxic fuels like nitric acid. Kerosene is still a hazardous liquid that requires specialised site safety measures. 

Producing liquid oxygen on site is also quite handy, but this isnt exactly cheap. The fact is it is produced on site because it is such a hassle to work with - its dangerous enough that constructing a specialised factory is preferable to storage or transport. And a liquid oxygen factory is not a safe place, requiring expertise and oversight. Do not ignore how dangerous LOx can be. Solid fuel is explosive. LOx can make your clothes explosive. LOx can make concrete flammable. Not that these are likely hazards - because of safety measures...which come at a cost.

Solid fuel *is* dangerous,  but for example, it wont spill or flow through gaps, it doesnt form clouds of explosive vapour, it wont flow into watercourses, it cant be easily inhaled or exposed to skin or eyes. Solid boosters can be locked up in a climate controlled shed and largely ignored until needed, transport costs for hazardous liquids as compared to solids are higher. None of these things individually are huge deal breakers but they all add up.

Some people say "But they cant be shut down" - I say, for a rocket, this is often an advantage. A liquid engine can fail, and the rocket fall back to the pad - failures in the first 30 seconds are the most feared events. Light off a solid and there is very little on gods green Earth that will stop it from at least going very far away. Solids are extremely reliable, whilst liquid engines require complex active control measures and have many failure modes. Plus others and I, have pointed out that it is far from impossible to shut down an SRB. I will note though that safety concerns change dramatically when the vehicle is crewed, but then this changes many significant factors and man-rating a craft is hugely expensive for these reasons.

Some people say "solids cant be tested, only fired" - true, but then an SRB has 2 components: the fuel and the casing, it hardly needs testing. A liquid engine on the other hand can have hundred of components, many with very fine dimensions and tolerances, made of many different materials, often involving huge differences in temperature. They MUST be tested in case it EXPLODES. An SRB is far more predictable in nature.

Many of the above things are important factors in insurance policies which is a large part of the price of a space launch. Sometimes a rocket could even be more expensive, but if it gets you a lower insurance premium based on reliability and component failure rates....

Like, do you want the liquid upper stage on your one-off, very expensive, important satellite? Or do you want the very simple, solid-state booster that has worked on hundreds of launches. Even if it means a slightly larger lower stage, the solid is often the right choice.

 

 

I will admit though that after some research, the difference in cost is - as far as I could tell due to difficulties mentioned above -  not as much as I had thought.

Dont make the mistake either, of assuming that I am trying to say that solids are better/cheaper in all situations. Im only giving a rationale for why they are ever used at all.

 

Manned and unmanned flight must be treated seperated. Could multiple SRBs be shutoff? Of course, certainly. Can they be shut off whilst saving an intact payload? Thats much more difficult - but it is not exactly easy with liquid engines either.

But multiple SRBs *can* be prevented from running out of control, though it may be easier/simpler to engineer the flight so that their maximum splashdown footprint is all within a safe area.

All very good points. I think there is very significant concern about the use of solids for manned flight because of the shutdown issue. Yes, you can design an SRB with a thrust termination system, but liquids are easier to shut off by their very nature. All liquid rocket engines can have commanded shutdown; the SRB's thrust termination system is hardware that must be added in (which also means added weight and volume).

If the Shuttle SRBs, for example, had been equipped with thrust termination systems, it would have added weight and reduced payload and performance. Moreover, it would have had to have been VERY carefully designed in order to prevent termination blow impingement on the tank or orbiter.

[p1t1o]

4 minutes ago, sevenperforce said:

All very good points. I think there is very significant concern about the use of solids for manned flight because of the shutdown issue. Yes, you can design an SRB with a thrust termination system, but liquids are easier to shut off by their very nature. All liquid rocket engines can have commanded shutdown; the SRB's thrust termination system is hardware that must be added in (which also means added weight and volume).

If the Shuttle SRBs, for example, had been equipped with thrust termination systems, it would have added weight and reduced payload and performance. Moreover, it would have had to have been VERY carefully designed in order to prevent termination blow impingement on the tank or orbiter.

Totally agree.

[GoSlash27]

2 hours ago, kerbiloid said:

Can two parallel SRB of SpaceShuttle, SLS or another rocket simultaneously?

Sure. The problem with the shuttle is that you can't arrange the ports so the vents don't impinge something delicate and/ or flammable. Not a problem with SLS.

 Not saying that they should or will use this system, just correcting the common misconception that solids can't be shut off.

Best,

-Slashy

[Pand5461]

16 hours ago, DerekL1963 said:

Kerosene is a highly volatile material than can be set off by a tiny spark. 

This can go the level "sugar powder is flammable, too".

Kerosene is safer because it does not contain oxidizer.

Kerosene is cheaper because of mass production and competitive market.

Liquid oxygen is fairly cheap as well, again because it has many industrial uses.

Solid fuels and hypergolics are hardly used outside military and space tech, so their production doesn't have to be cost-efficient. Not to mention they are toxic and volatile (high-explosive-type of volatile, not flammable-type), so handling adds even more cost.

Also, solid rockets arrive fueled at assembly site, so all assembly operations must be performed with fully-fueled boosters, hence the need of heavy and expensive equipment in the assembly building. And additional safety measures.

2 hours ago, p1t1o said:

But solids *are* the cheap option.

Looks like, it's size-dependent.

Handling bigger SRBs becomes progressively more dangerous, so safety measures rise the operational costs.

And, given worse specific impulse, mass of a solid rocket launcher would increase faster with the mass of payload, than the mass of liquid-fueled launcher.

 

Another thing I've dug in the internets, is that solid rocket burn time is determined primarily by its diameter. So, elongating an SRB increases thrust rather than burn time. Given that Space Shuttle/SLS SRB segments have the maximum diameter to transport them by railroad and burn for mere two minutes, all-solid rockets may be unsuitable for delicate payloads because of fast g-force buildup.

 

[wumpus]

30 minutes ago, Pand5461 said:

Another thing I've dug in the internets, is that solid rocket burn time is determined primarily by its diameter. So, elongating an SRB increases thrust rather than burn time. Given that Space Shuttle/SLS SRB segments have the maximum diameter to transport them by railroad and burn for mere two minutes, all-solid rockets may be unsuitable for delicate payloads because of fast g-force buildup.

As far as I know, "all solid" rockets use multiple stages.  Pegasus uses 3, the tiny solid Japanese orbital rocket used 3-4, the peacekeeper missile used 3 (+ hypergolic navigation/MIRV), minotaurs seem to use at least 4 stages (wiki is weak on specific minutemen data), typically solid.

The same argument can be used for liquid, although to a lesser degree.  Generally speaking, SRB's lower Isp means more stages and g-force buildup is less of an issue.  Granted, you can avoid gravity losses much easier by starting with high g-forces, which makes the buildup that much worse, but low g-forces don't seem to be driving launch vehicle development.

[DerekL1963]

1 hour ago, Pand5461 said:

Kerosene is cheaper because of mass production and competitive market.

Avgas and commercial grade kerosene are mass produced in a competitive market.  RP-1 is neither.  It's a specialized product produced in (relatively) small quantities.

 

1 hour ago, GoSlash27 said:

The problem with the shuttle is that you can't arrange the ports so the vents don't impinge something delicate and/ or flammable. Not a problem with SLS.

NASA didn't seem to have a problem doing so.  Thrust termination (for the Shuttle) was dropped because a propulsion system powerful enough to get the orbiter away from the stack was too heavy, not because of plume impingement.

[sevenperforce]

15 minutes ago, DerekL1963 said:

2 hours ago, GoSlash27 said:

The problem with the shuttle is that you can't arrange the ports so the vents don't impinge something delicate and/ or flammable. Not a problem with SLS.

NASA didn't seem to have a problem doing so.  Thrust termination (for the Shuttle) was dropped because a propulsion system powerful enough to get the orbiter away from the stack was too heavy, not because of plume impingement.

If the SRBs had been equipped with TTS, then there would have been a chance that they could have cut Challenger's SRBs off, jettisoned them, dropped the tank, and prayed. Probably still would have broken apart due to aerodynamics, but there would have been a chance.

And there were definitely other parts of the ascent envelope prior to booster cutoff that would have allowed benign abort without an active LES.

[DerekL1963]

5 minutes ago, sevenperforce said:

If the SRBs had been equipped with TTS, then there would have been a chance that they could have cut Challenger's SRBs off, jettisoned them, dropped the tank, and prayed. 

Utter nonsense.  There wouldn't have been thrust termination in the first place unless the Orbiter could power itself away from the stack and maintain control.

 

7 minutes ago, sevenperforce said:

And there were definitely other parts of the ascent envelope prior to booster cutoff that would have allowed benign abort without an active LES.

Nope.  There are no benign abort modes prior to SRB burnout.  None, zero, zip, nada.

[GoSlash27]

(Shuttle stuff)

All fair points, but all problems inherent to the shuttle's unique design rather than any flaw in SRBs themselves.

Not advocating the use of srbs for manned flight or arguing against it. Just correcting an incorrect assertion

Best,

-Slashy

[sevenperforce]

28 minutes ago, DerekL1963 said:

There are no benign abort modes prior to SRB burnout.  None, zero, zip, nada.

...because there is no SRB TTS.

I do not have quite the engineering and analysis chops to say that zeroing the thrust on the Challenger SRBs at T+70 would have definitely saved the lives of the crew, but I think we can be pretty darn certain that it could not possibly have been worse than STS-51-L.

[Capt. Hunt]

49 minutes ago, sevenperforce said:

If the SRBs had been equipped with TTS, then there would have been a chance that they could have cut Challenger's SRBs off, jettisoned them, dropped the tank, and prayed. Probably still would have broken apart due to aerodynamics, but there would have been a chance.

And there were definitely other parts of the ascent envelope prior to booster cutoff that would have allowed benign abort without an active LES.

Nope, an RTLS abort was the first in flight abort option (aside from ejecting on the first 4 flights), and they couldn't do that until SRB burnout.

9 minutes ago, sevenperforce said:

...because there is no SRB TTS.

I do not have quite the engineering and analysis chops to say that zeroing the thrust on the Challenger SRBs at T+70 would have definitely saved the lives of the crew, but I think we can be pretty darn certain that it could not possibly have been worse than STS-51-L.

By then it was probably too late, the plume had been eating away at the skin of the ET since right after SRB ignition.  IIRC, they didn't even see any indication of a problem during the launch itself, they didn't spot the plume until they went over the tapes after the fact.  The only way to save Challenger would have been to not launch at those temperatures.

Edited April 13, 2018 by Capt. Hunt

[sevenperforce]

22 minutes ago, Capt. Hunt said:

Quote

I do not have quite the engineering and analysis chops to say that zeroing the thrust on the Challenger SRBs at T+70 would have definitely saved the lives of the crew, but I think we can be pretty darn certain that it could not possibly have been worse than STS-51-L.

By then it was probably too late, the plume had been eating away at the skin of the ET since right after SRB ignition. 

IIRC, plume impingement was primarily on the lower SRB holding strut. That strut eventually melted away enough that it failed, and the still-firing SRB then rotated around its remaining two attachment point, striking the ET and rupturing it. As the stack started to spin out of control, the SRB ripped free and impacted the right wing of the orbiter and the whole stack ripped itself apart with the two SRBs flying off merrily until FTS trigger.

Quote

IIRC, they didn't even see any indication of a problem during the launch itself, they didn't spot the plume until they went over the tapes after the fact.  The only way to save Challenger would have been to not launch at those temperatures.

If the SRBs had a thrust termination option, then they would have evaluated automatic shutdown protocols. The stack was already experiencing thrust shortfall, pressure fluctuations, and pointing problems before T+70; automatically-commanded shutdown of the SRBs based on telemetry would have at least been an evaluated possibility.

And the question of whether other possible failures could have also been prevented by TTS is still open.

Edited April 13, 2018 by sevenperforce

[Bill Phil]

1 hour ago, DerekL1963 said:

Avgas and commercial grade kerosene are mass produced in a competitive market.  RP-1 is neither.  It's a specialized product produced in (relatively) small quantities.

From what I can find, jet fuel (not sure which particular fuel...) and RP-1 can be refined in the same refineries. What really drives up price is the high quality input stock that comes from a small number of oil fields, and of course the small market.

[DerekL1963]

3 hours ago, sevenperforce said:

I do not have quite the engineering and analysis chops to say that zeroing the thrust on the Challenger SRBs at T+70 would have definitely saved the lives of the crew, but I think we can be pretty darn certain that it could not possibly have been worse than STS-51-L.

0.o  It doesn't take any serious engineering and analysis chops to realize that throwing an uncontrolled Orbiter into the airstream at T+70 wouldn't be significantly different than doing so at T+73.  (The Challenger wasn't destroyed due to explosive forces, as there was no explosion.  It was ripped apart by aerodynamic forces when it ended up broadside to the airflow.)  That's why NASA abandoned thrust termination in the first place - there was no practical way to provide a system that could maintain attitude control and thrust the Orbiter away from the stack.

 

3 hours ago, sevenperforce said:

IIRC, plume impingement was primarily on the lower SRB holding strut. That strut eventually melted away enough that it failed, and the still-firing SRB then rotated around its remaining two attachment point, striking the ET and rupturing it. As the stack started to spin out of control, the SRB ripped free and impacted the right wing of the orbiter and the whole stack ripped itself apart with the two SRBs flying off merrily until FTS trigger.

Let's just say that you should review the Wikipedia article on the Challenger accident and leave it at that.

[sevenperforce]

36 minutes ago, DerekL1963 said:

0.o  It doesn't take any serious engineering and analysis chops to realize that throwing an uncontrolled Orbiter into the airstream at T+70 wouldn't be significantly different than doing so at T+73.  (The Challenger wasn't destroyed due to explosive forces, as there was no explosion.  It was ripped apart by aerodynamic forces when it ended up broadside to the airflow.)  That's why NASA abandoned thrust termination in the first place - there was no practical way to provide a system that could maintain attitude control and thrust the Orbiter away from the stack.

Let's just say that you should review the Wikipedia article on the Challenger accident and leave it at that.

I have. Although a leak began in the hydrogen tank at T+64.66, it was the failure of the aft right strut at T+72.284 which led to the immediate breakup of the orbiter. The ET did not experience structural failure until T+73.124.

Had the SRBs included a TTS option, then automated systems would have been in place to trigger TTS. Loss of right SRB chamber pressure at T+59 could have been one of the triggers, five to six seconds before the ET began leaking and a full thirteen seconds before the eventual structural failure of the aft right strut.

Had a TTS been commanded prior to (or even just after) the formation of a leak in the ET, without jettisoning the SRBs, it is not inconceivable that the stack could have maintained aerodynamic stability (it was relatively draggy in the back and had the heavy oxygen tank up front) long enough to perform a controlled pitch-forward using the SSMEs and then jettison tank and SRBs together, allowing the orbiter to enter the airstream prograde, rather than uncontrolled and at a high AoA with one wing sheared off.

Obviously no such contingency was planned, as they had no safe way of shutting down the SRBs in the event of an SRB problem.

[GoSlash27]

All,
I'm afraid this whole discussion about the Challenger is wandering off the topic of this thread. Yes, SRBs were involved in the accident, but that was a very special case which clearly does not apply to all designs... especially not any current or planned designs.
 Surely arguing about the details and precise timeline of that tragedy isn't pertinent here?

Best,
-Slashy

 

 

[StrandedonEarth]

34 minutes ago, GoSlash27 said:

that was a very special case which clearly does not apply to all designs... especially not any current or planned designs.

Actually, it is quite pertinent, as the SLS is using essentially the same SRB's (same segments, just one more; no recovery system) on a rocket that is expected to be manned. The major difference is that the crew is above the tankage (where they should be) instead of beside it.

[Canopus]

4 minutes ago, StrandedonEarth said:

Actually, it is quite pertinent, as the SLS is using essentially the same SRB's (same segments, just one more; no recovery system) on a rocket that is expected to be manned. The major difference is that the crew is above the tankage (where they should be) instead of beside it.

Problem with the shuttle weren't the boosters, they worked in 134 out of 135 launches, but the fact that the whole configuration didn't allow for any kind of Abort while they were running. Thinking the shuttle was reliable enough to not even consider any kind of LES was madness and the risk wouldn't have been much lower with liquid boosters, i'm sure.

[Bill Phil]

25 minutes ago, StrandedonEarth said:

Actually, it is quite pertinent, as the SLS is using essentially the same SRB's (same segments, just one more; no recovery system) on a rocket that is expected to be manned. The major difference is that the crew is above the tankage (where they should be) instead of beside it.

The 5-segment SRBs SLS will use are not the Shuttle SRBs with an extra segment. They look similar, sure, but they are not. They've been redesigned. They're not the same at all. Completely different, save for propellant used.

So long as the LES can pull the capsule away far and fast enough there's not too big of an issue. The problem with Shuttle was being next to the stack.

If the SRBs prove to be an issue during a launch they'll abort and the crew should be pulled away safely. 

12 minutes ago, Canopus said:

Problem with the shuttle weren't the boosters, they worked in 134 out of 135 launches, but the fact that the whole configuration didn't allow for any kind of Abort while they were running. Thinking the shuttle was reliable enough to not even consider any kind of LES was madness and the risk wouldn't have been much lower with liquid boosters, i'm sure.

The boosters showed problems before the first launch, but they used them anyways. Before the Challenger disaster, the SRBs were essentially failing in the exact same way, with the only difference being them not below their certified temperature range before launch, and thus the mission was able to continue unimpeded. This happened 7 times out of 9 launches in 1985. If they had been affected in the same way in one of those launches, the disaster could've happened earlier in the program, potentially as early as STS-2. Thankfully that design problem was eventually addressed, but had that disaster not occurred, imagine the same failure during one of the "death star" missions. Hydrolox in the cargo bay. And that was scheduled for the same year as STS-51-L.

No matter what, spaceflight must be handled responsibly. Design compromises can easily lead to failures that can cause deaths. 

Edited April 13, 2018 by Bill Phil

[DerekL1963]

34 minutes ago, Bill Phil said:

The boosters showed problems before the first launch, but they used them anyways. Before the Challenger disaster, the SRBs were essentially failing in the exact same way, with the only difference being them not below their certified temperature range before launch, and thus the mission was able to continue unimpeded.

Actually, the worst pre-Challenger failure occurred at temperatures well within the specified operating range.  The cold contributed to the accident, but it did not cause the accident.

That's why, in addition to joint heaters they added stiffening pins to the redesigned joint, because the basic problem was the joint opening up when the case pressurized at ignition.

[GoSlash27]

2 hours ago, StrandedonEarth said:

Actually, it is quite pertinent, as the SLS is using essentially the same SRB's  on a rocket that is expected to be manned.

 Only inasfar as it happens to be the same SRB... sorta. The fault in the design of the SRB itself was corrected and it hasn't failed since. Also, the only reason a burn- through cost the LOCV was because of the unique design of the launch vehicle, which is no longer in service.

 That's where any pertinence to this subject ends. I'd just like to avoid a pedantic discussion of the minutae of Challenger further derailing this discussion. What went wrong there has no bearing on the serviceability of SRBs in manned spaceflight going forward. If people want to rehash that subject, there's a thread for it.

All I'm sayin'

-Slashy

Edited April 14, 2018 by GoSlash27

[Bill Phil]

8 minutes ago, DerekL1963 said:

Actually, the worst pre-Challenger failure occurred at temperatures well within the specified operating range.  The cold contributed to the accident, but it did not cause the accident.

That's what I said. The SRBs were essentially failing but they were not below their certified temperature range before launch. Even so, the SRBs were failing all the same. It just so happened to not be a mission LOCV failure until Challenger. The cold exacerbated an already existing issue.

[DerekL1963]

49 minutes ago, Bill Phil said:

That's what I said.

Yeah, so you did.  I misread, mea culpa.

[VaPaL]

For anybody that doesn't know/remember  the accindent here in Brazil in 2003 (@1:20):

https://en.wikipedia.org/wiki/VLS-1_V03

VLS-1 was a solid fuel rocket only, it accident happened while people worked on the rocket, 21 one horrible deaths. The reports of how they died varied from instant death, to slow suffocation due to toxic fumes while burnig trapped inside the VAB.

No option is safer than the other, when things go wrong, they go wrong.

[YNM]

One things sure :

You can't load solids on liquid engines, and you can't put liquids in an SRB casing.

 

It's just whichever you grew up with.