Author Topic: Question about SRBs and the S-IC  (Read 7055 times)

Offline VincentMcConnell

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Question about SRBs and the S-IC
« on: June 17, 2012, 12:17:41 PM »
Quote
Each Shuttle SRB had a liftoff thrust of 12 MN; the F-1, only 6.77 MN.
-Kaq9

I knew the thrust was comparable, but double? Why do we say the Saturn V was the most powerful rocket to leave the ground if the shuttle has a stronger SRB stack?
Thanks for any and all answers.
"It looks better now, Al. What change did you make?"
"I just hit it on the top with my hammer."

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Offline Peter B

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Re: Question about SRBs and the S-IC
« Reply #1 on: June 17, 2012, 12:25:08 PM »
Quote
Each Shuttle SRB had a liftoff thrust of 12 MN; the F-1, only 6.77 MN.
-Kaq9

I knew the thrust was comparable, but double? Why do we say the Saturn V was the most powerful rocket to leave the ground if the shuttle has a stronger SRB stack?
Thanks for any and all answers.
Five F-1s per S-1C.

Two SRBs per Shuttle.

Offline VincentMcConnell

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Re: Question about SRBs and the S-IC
« Reply #2 on: June 17, 2012, 12:26:42 PM »
Oh... Duh... Silly me. I forgot that the entire assembly wasn't the F1. The five F1's make up the assembly. Got it. Thanks!
"It looks better now, Al. What change did you make?"
"I just hit it on the top with my hammer."

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Offline Jason Thompson

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Re: Question about SRBs and the S-IC
« Reply #3 on: June 17, 2012, 12:27:38 PM »
For a number of reasons. First, the combination of two SRBs and three SSMEs still comes in below the combined thrust of the of five F-1 engines used in the Saturn V first stage. Second, the SSMEs and the SRBS are pretty much all the shuttle has. Once they're gone it's shot its bolt and only has the OAMS system for small course corrections. The Saturn V still has the second and third stages to be added to its total power after the five F-1 engines have shut down.

The N-1 rocket generated more liftoff thrust even than the Saturn V, at about 10 million pounds, but because the whole thing used kerosene/LOX for fuel it was overall a lot less powerful than the Saturn V.
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Offline ka9q

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Re: Question about SRBs and the S-IC
« Reply #4 on: June 22, 2012, 06:20:26 AM »
Right. My point in comparing the F-1 to a SRB was to compare the thrust per engine and to show that single solid rocket motors can be made with much more thrust than single liquid rocket motors.

The main drawback to solid rockets vs liquid rockets is their much lower specific impulse. The propellant doesn't contain as much chemical energy per kilogram as the modern liquid propellants, especially liquid hydrogen and oxygen.

Solids have a few other problems. They emit chlorine compounds that may damage the ozone layer. Their exhaust contains solid particles (e.g., aluminum oxide) that, in space, could constitute hazardous orbital debris. They have to be handled fully loaded, increasing weight and hazards. They cannot be shut down or throttled once ignited, though they can produce a predetermined thrust-vs-time curve. And when they fail, they tend to do so suddenly and catastrophically.

But working against all these drawbacks is the simplicity and low cost of solid rocket motors and the fact that they can be built with much more thrust than an individual liquid rocket engine. And at launch, high thrust is often everything. That's why they're so popular as boosters, with upper stages usually burning more energetic liquid propellants.


Offline Bob B.

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Re: Question about SRBs and the S-IC
« Reply #5 on: June 22, 2012, 11:49:34 AM »
They cannot be shut down or throttled once ignited, though they can produce a predetermined thrust-vs-time curve. And when they fail, they tend to do so suddenly and catastrophically.

I'm by no means an expert on solid rocket motors, but it's my understanding that, although they may fail suddenly, they don't necessarily do so without warning.  It may be possible to sense a imminent failure and actuate the emergency escape system before a catastrophe occurs.  It is also my understanding that once a solid motor casing ruptures and the pressure is relieved, the motor essentially shuts down.  On the other hand, liquids propellants can ignite into a massive fire ball upon engine failure.  Again, I don't have any personal experience with this, this is just what I've read.

But working against all these drawbacks is the simplicity and low cost of solid rocket motors and the fact that they can be built with much more thrust than an individual liquid rocket engine. And at launch, high thrust is often everything. That's why they're so popular as boosters, with upper stages usually burning more energetic liquid propellants.

Solid propellants also have a high density impulse, meaning high impulse per unit volume.

The high thrust of solid motors can be used to offset much of their ISP disadvantage.  For instance, if a solid fueled booster has a higher thrust-to-weight at liftoff than a liquid fueled booster, then less of the thrust is used to overcome gravity.  Say, for example, that a liquid fueled booster has a T/W ratio of 1.25 at liftoff, then 80% of the thrust is used simply to cancel the pull of Earth gravity.  Using a more powerful solid booster we might be able to increase the T/W ratio to, say, 1.5.  In this case only 67% of the thrust is canceling Earth's gravity.  This can make the solid fueled booster effectively more efficient than the liquid fueled booster despite the fact it has a lower ISP.

Offline cjameshuff

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Re: Question about SRBs and the S-IC
« Reply #6 on: June 22, 2012, 01:27:27 PM »
I'm by no means an expert on solid rocket motors, but it's my understanding that, although they may fail suddenly, they don't necessarily do so without warning.  It may be possible to sense a imminent failure and actuate the emergency escape system before a catastrophe occurs.  It is also my understanding that once a solid motor casing ruptures and the pressure is relieved, the motor essentially shuts down.  On the other hand, liquids propellants can ignite into a massive fire ball upon engine failure.  Again, I don't have any personal experience with this, this is just what I've read.

There may be signs of imminent failure, and there may not be. A fragment breaking loose and sharply increasing the surface area or blocking the central channel or a casing failure will cause a very sudden explosion.

Burn rate may drop steeply, but it doesn't stop burning. There's been quite a few failures that resulted in a spectacular shower of flaming chunks of fuel. This was one of the objections to Ares I, at some points during launch a failure would lead to the capsule's parachutes opening in the debris pattern, with essentially zero chance of surviving due to heating from the burning fuel around it.

Liquid engines are far more likely to shut down cleanly, and don't need to be blown up in the process. The Falcon 9 can even continue on its mission after experiencing an engine outage, as the Saturn V was capable of, something that is simply impossible with solids. If there is a catastrophic failure, the fireball is far more localized due to the fuel consisting of fluids that rapidly disperse...no large footprint of flaming debris. The fuel is also poorly mixed, and will not burn all at once or even completely.

Liquid engines can also be tested prior to launch, or abort without leaving the pad. The recent SpaceX Dragon flight shut down on the pad moments after starting up due to abnormal pressure readings from one engine. After changing a valve, they launched and did a successful mission to the ISS. If it had used solids, the vehicle would have been lost either due to immediate explosion or by self destruction by the range safety systems. In a manned vehicle, you'd be comparing a similar delay with the crew arriving days late, to a failure with the crew violently being blasted away from an exploding vehicle via emergency rockets and hopefully clearing the debris pattern.


Solid propellants also have a high density impulse, meaning high impulse per unit volume.

Not hugely more so than LOX/RP-1, except at the smallest scales.


The high thrust of solid motors can be used to offset much of their ISP disadvantage.  For instance, if a solid fueled booster has a higher thrust-to-weight at liftoff than a liquid fueled booster, then less of the thrust is used to overcome gravity.  Say, for example, that a liquid fueled booster has a T/W ratio of 1.25 at liftoff, then 80% of the thrust is used simply to cancel the pull of Earth gravity.  Using a more powerful solid booster we might be able to increase the T/W ratio to, say, 1.5.  In this case only 67% of the thrust is canceling Earth's gravity.  This can make the solid fueled booster effectively more efficient than the liquid fueled booster despite the fact it has a lower ISP.

This only applies briefly, while the vehicle is initially accelerating vertically, and requires accepting the safety issues and limitations, environmental issues, etc of solid rockets. And even as strap-on boosters, in real world rockets they have proven to be lower performance than liquids...liquid boosters were considered as a way of increasing the payload and safety of the Shuttle.

They are simple in concept, but complex and hazardous to fabricate and handle, inflexible in adapting to different missions, and do not scale up well...note the several tons of tuned mass vibration dampers that Ares I needed to keep from killing its crew or shaking itself apart. Solids aren't nearly as simple and cheap as they're advertised to be.

Offline Bob B.

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Re: Question about SRBs and the S-IC
« Reply #7 on: June 22, 2012, 03:58:32 PM »
Not hugely more so than LOX/RP-1, except at the smallest scales.

I'm not sure I agree with you on that one.  From the numbers I've worked with, solids have a density impulse about 60% greater than LOX/RP-1 and about 45% higher than hypergols.

This only applies briefly, while the vehicle is initially accelerating vertically

High thrust-to-weight ratios can reduce the gravity loss by several hundred meters/second, so it's certainly not insignificant.  The effect is greatest during the brief period when the rocket is moving vertically, but it continues even after the vehicle pitches over.  Of course, high T/W ratios have their downside as well, such as higher acceleration and aerodynamic loads.

Offline ka9q

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Re: Question about SRBs and the S-IC
« Reply #8 on: June 22, 2012, 05:07:21 PM »
The Challenger accident was actually a very unusual failure mode for a solid rocket. I can't think of another solid rocket failure that wasn't sudden and catastrophic, with absolutely no warning. The case simply ruptures, and within milliseconds you have a big ball of rapidly expanding, rapidly burning propellant where your launcher used to be.

The frequent use of steel casings in solid rocket boosters shows that Isp just isn't that critical in a booster; high thrust is what you need to overcome gravity loss in vertical flight. Filament-wound SRBs were developed for use at VAFB, but were never flown.

I'd have to check on the density impulse figures; I'm sure they're more compact than LH2, but RP1 and the hypergols are a lot denser. I suppose solids are still cheaper and simpler than liquids, which is why they're so popular. But they always make me nervous, especially when humans ride them. Good riddance to the Ares I, which would not have been survivable in the event of a first stage failure.

Offline ka9q

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Re: Question about SRBs and the S-IC
« Reply #9 on: June 22, 2012, 05:09:31 PM »
Don't solids have additional disadvantages, like high vibration from combustion instabilities? I seem to remember that was a serious problem in the Ares I that was never solved before cancellation.

Offline ka9q

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Re: Question about SRBs and the S-IC
« Reply #10 on: June 22, 2012, 05:22:06 PM »
Back to the original question...why did the F-1 have such a poor specific impulse? Was it simply due to its use of a gas generator cycle, or did other compromises have to be made to get such a high thrust liquid-fueled engine? I know its combustion chamber pressure was remarkably low, only 70 bar -- about that of a CO2 fire extinguisher at 29C. The RD-180 chamber pressure is 266.8 bar; the SSME is 206.4 bar.

The Delta IV's RS-68, which also uses a gas generator, is only 97 bar so it would seem that low chamber pressures are associated with gas generators. It also has a vacuum Isp of only 410 sec, low for a LH2 engine; the SSME is 452.3 sec in vacuum. Why are gas generators associated with low chamber pressures? Are higher pressures possible, but too expensive in propellant given their inefficient use of propellants?


Offline cjameshuff

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Re: Question about SRBs and the S-IC
« Reply #11 on: June 22, 2012, 07:01:58 PM »
I'd have to check on the density impulse figures; I'm sure they're more compact than LH2, but RP1 and the hypergols are a lot denser. I suppose solids are still cheaper and simpler than liquids, which is why they're so popular. But they always make me nervous, especially when humans ride them. Good riddance to the Ares I, which would not have been survivable in the event of a first stage failure.

They aren't *that* popular. Large solids only really see use as strap on boosters for a handful of large launchers and a couple small-payload, many-stage orbital launchers. The rockets that use them haven't been notably cheaper as a result. The rockets that *have* been relatively cheap have used liquid boosters or no boosters at all...Soyuz, Proton, Long March, more recently the Falcon 9...


Don't solids have additional disadvantages, like high vibration from combustion instabilities? I seem to remember that was a serious problem in the Ares I that was never solved before cancellation.

As I mentioned, they added tuned mass dampers to compensate, taking a big chunk out of the available second stage mass. It's not known if this would have fixed it, as they never got around to flying a 5-segment booster...Ares I-X was a stock Shuttle booster past its expiration date with a mass simulator second stage and Atlas V avionics. (And still managed to cost half a billion dollars to launch.)

Vibration means you need a stronger (thus heavier) second stage as well, and increases odds of undesirable interactions with your control systems or payload. Using solids increases costs of the other parts of your system, and adds other costs as well...transport and storage costs for the huge, heavy blocks of highly flammable rocket fuel, disposal of that fuel when it exceeds its rated shelf life, etc.

As for the F1, as a first stage engine it needed thrust more than Isp, there was no experience building engines of that size, and there was little time to spend on optimizing it. And gas generator engines run at relatively low pressure because increasing chamber pressure in a gas generator engine increases required pumping power to force fuel into the engine, which increases pumping losses...you're burning more fuel to run those pumps. Staged combustion engines recover most of the pump power and so don't experience such a loss from high chamber pressure.

Offline Bob B.

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Re: Question about SRBs and the S-IC
« Reply #12 on: June 22, 2012, 07:15:18 PM »
Back to the original question...why did the F-1 have such a poor specific impulse?

The F-1 specific impulse was typical of a gas generator engine buring LOX/RP-1.  Here are some other examples:

MA-5 = 259 s (boosters)
RS-27 = 264 s
RD-107 = 257 s

Quote
The Delta IV's RS-68, which also uses a gas generator, is only 97 bar so it would seem that low chamber pressures are associated with gas generators.

The RS-68 has the highest chamber pressure I've seen in a large gas generator engine.  Yes, gas generators have low chamber pressures compared to staged combustion.

Quote
Why are gas generators associated with low chamber pressures? Are higher pressures possible, but too expensive in propellant given their inefficient use of propellants?

I thought we discussed this not too long ago in another thread.  There is a point where the marginal return gained by a higher chamber pressure is less than the loss resulting from the greater propellant flow needed to drive the turbopumps at the higher discharge pressure.

Offline cjameshuff

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Re: Question about SRBs and the S-IC
« Reply #13 on: June 22, 2012, 07:55:03 PM »
The RS-68 has the highest chamber pressure I've seen in a large gas generator engine.

The Merlin 1D, a LOX/RP-1 engine, also has a chamber pressure of 97 bar, and a vacuum Isp of 310 s.
And only about a fifth the thrust of a RS-68, so might not qualify as "large".

Offline Bob B.

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Re: Question about SRBs and the S-IC
« Reply #14 on: June 22, 2012, 09:37:41 PM »
The Merlin 1D, a LOX/RP-1 engine, also has a chamber pressure of 97 bar, and a vacuum Isp of 310 s.
And only about a fifth the thrust of a RS-68, so might not qualify as "large".

I didn't realize the Merlin had that high a pressure.  The highest chamber pressure I've ever heard about in a gas generator is 110 atm, which is the European Vulcain (Ariane 5 core stage).  The Vulcain has a sea level thrust of 900 kN (202,000 lbf).  That's much more powerful than I remembered, so I have to renege on my previous comment and move the Vulcain to the top spot on the list.