The Debate of Solid vs Liquid

[Racescort666]

So I came across this article: https://www.popularmechanics.com/space/rockets/a19724715/liquid-solid-rocket-fuel-spacex-orbital-atk-blue-origin/ That claims there is a debate between solid rockets vs liquid rockets. It reads an awful lot like an Orbital ATK sales pitch if you ask me but I will leave that to the reader.

[YNM]

The problems with solids is that they're freakin' heavy.

[tater]

Solid rockets are fireworks, not real rockets.

There is no rivalry, solids have lousy I_sp.

[Racescort666]

5 minutes ago, tater said:

Solid rockets are fireworks, not real rockets.

There is no rivalry, solids have lousy I_sp.

But “ISP per pound”...

wait, that is completely ridiculous.

[Bill Phil]

Modern solids are a far cry from simple fireworks. APCP can get up to 303.8 seconds of specific impulse. Not too shabby, although that's probably vac ISP.

I don't see a reason for a rivalry. Whatever is better at fitting the design requirements is what you should do. If that would be solids, then use them. If liquids are better, use them. If a combination is better, use it. They use solids for high energy trajectories in space. 

I quite like the Titan SRBs. Those things can thrust vector for steering.

5 minutes ago, Racescort666 said:

But “ISP per pound”...

wait, that is completely ridiculous.

Well... ISP (in seconds) is thrust per pound (or newton) of propellant expended... But ISP per pound doesn't really mean anything...

[Racescort666]

2 minutes ago, Bill Phil said:

Modern solids are a far cry from simple fireworks. APCP can get up to 303.8 seconds of specific impulse. Not too shabby, although that's probably vac ISP.

I don't see a reason for a rivalry. Whatever is better at fitting the design requirements is what you should do. If that would be solids, then use them. If liquids are better, use them. If a combination is better, use it. They use solids for high energy trajectories in space. 

I quite like the Titan SRBs. Those things can thrust vector for steering.

Well... ISP (in seconds) is thrust per pound (or newton) of propellant expended... But ISP per pound doesn't really mean anything...

My problem with the article is that it is manufacturing drama when there is none.

As you pointed out, for specific circumstances one is objectively better than another and as a result we have converging design philosophies for rockets depending on their purpose. When it makes sense to use SRMs, do so and no engineer will argue with you. The same can not be said for politicians and journalists.

Also “ISP per pound” was a direct quote from the article. I had a hard time finishing the article after that.

[PB666]

22 minutes ago, Racescort666 said:

My problem with the article is that it is manufacturing drama when there is none.

As you pointed out, for specific circumstances one is objectively better than another and as a result we have converging design philosophies for rockets depending on their purpose. When it makes sense to use SRMs, do so and no engineer will argue with you. The same can not be said for politicians and journalists.

Also “ISP per pound” was a direct quote from the article. I had a hard time finishing the article after that.

The difference principle difference between a solid rocket at and a liquid rocket is that solids are good for boosting and tossing. For fine flight control one requires liquid rockets. The SFRBs on the shuttle provide most of the thrust, but the liquid engines provide most of the dV, but also provide a means of tempering the thrust near MaxQ. Solid only rockets are dangerous for human space flight because of g-load limitations of biological organisms.

Solids - Boost and weight tossing.
Liquids - acceleration x time and control.

If you thought about it like this . .if a solid rocket is free, and all you have to do is pay for structural connection to the core then it cost almost nothing to the core to boost the core up and then toss the SFRB.

But as it turns out SFRB thrusters might be cheaper, but not cheaper then RTLS liq boosters that can be reused. So if you pretend the RTLS liq booster is free, then there is no reason ever to use an SFRB.

 

[DerekL1963]

28 minutes ago, Racescort666 said:

My problem with the article is that it is manufacturing drama when there is none.

It's Popular Mechanics - it's not exactly written for discerning and educated reader.

[mikegarrison]

What solids really excel at is doing nothing. That's why they are great for military missiles. They can just sit there doing nothing for years and years and years and still ignite almost instantly. And yet, they are also quite safe and unlikely to explode while sitting around and doing nothing. They don't corrode your rocket, either.

[tater]

Yeah, They have incredible utility within a certain regime.

[RCgothic]

Large solids don't have any business being on a manned rocket. You can't shut them down.

Ullage motors, escape motors and sepratrons are a different matter. Highly reliable in those roles.

[Canopus]

1 minute ago, RCgothic said:

Large solids don't have any business being on a manned rocket. You can't shut them down.

In a scenario where you'd need to shutdown your boosters i doubt you'd have a much better time with liquid fuel rockets.

[PB666]

4 hours ago, mikegarrison said:

What solids really excel at is doing nothing. That's why they are great for military missiles. They can just sit there doing nothing for years and years and years and still ignite almost instantly. And yet, they are also quite safe and unlikely to explode while sitting around and doing nothing. They don't corrode your rocket, either.

The charges for main guns of a battleship make great fodder when the main guns get hit. . . . . .the vulnerability of using solid explosives anywhere. 

[p1t1o]

Meh. They both have clear-cut advantages and disadvantages, you select whatever suits the program.

Had a glance at the article, I dont really like it. It reads as a breakdown of the differences between liquid and solid - which is fine - but then frames it as some kind of controvercy? Whereas Im pretty sure orbital ATK know exactly what they're doing?

 

6 minutes ago, PB666 said:

The charges for main guns of a battleship make great fodder when the main guns get hit. . . . . .the vulnerability of using solid explosives anywhere. 

Its not like tanks of stored fuel+oxidiser are invulnerable...

 

[Canopus]

1 hour ago, PB666 said:

The charges for main guns of a battleship make great fodder when the main guns get hit. . . . . .the vulnerability of using solid explosives anywhere. 

Well if your ICBM silo is under artillery fire, it's over anyway.

[Scotius]

Any magazine storing large amount of volatile\explosive substances is a dangerous place. Several firework factories learned it hard way - with devastating consequences.

[tater]

5 hours ago, Canopus said:

In a scenario where you'd need to shutdown your boosters i doubt you'd have a much better time with liquid fuel rockets.

Ares 1 was always kooky. Part of the problem is variability in thrust, they're just not that smooth. Makes for a bumpy ride. Wasn't the blast radius of an exploding SRM larger than the abort radius of Orion?

[PB666]

4 hours ago, p1t1o said:

Meh. They both have clear-cut advantages and disadvantages, you select whatever suits the program.

Had a glance at the article, I dont really like it. It reads as a breakdown of the differences between liquid and solid - which is fine - but then frames it as some kind of controvercy? Whereas Im pretty sure orbital ATK know exactly what they're doing?

 

Its not like tanks of stored fuel+oxidiser are invulnerable...

 

But they are separate, BTW, I forgot to add monopropellants are equally vulnerable. So if you are worried about liquids igniting then you best have an electric propulsion system. Monopropellant is generally closer to your payload than the LfOx. 

[Bill Phil]

6 hours ago, RCgothic said:

Large solids don't have any business being on a manned rocket. You can't shut them down.

You can shut down large solids. Just not immediately... It takes time.

57 minutes ago, tater said:

Ares 1 was always kooky. Part of the problem is variability in thrust, they're just not that smooth. Makes for a bumpy ride. Wasn't the blast radius of an exploding SRM larger than the abort radius of Orion?

From a study conducted by the 45th Space Wing, it was determined that an abort 30 - 60 seconds after launch had a 100% chance of killing the crew. I'm glad we didn't fly that thing... solids as on crew vehicles are just insane. SLS may prove equally problematic, though... there's no real guarantee.

[DerekL1963]

1 hour ago, Bill Phil said:

You can shut down large solids. Just not immediately... It takes time.

A few hundred milliseconds, not really so different from large liquids.  In both cases, by the time you've reached zero net thrust - the manned capsule has long since fired it's LES and departed the neighborhood.

[sevenperforce]

7 hours ago, Canopus said:

In a scenario where you'd need to shutdown your boosters i doubt you'd have a much better time with liquid fuel rockets.

Uh, you would have a MUCH better time with LF rockets. Cutting the engines, waiting for clearance, and then unzipping the sides of an LRB is much safer than firing the FTS on a solid.

[Canopus]

41 minutes ago, sevenperforce said:

Uh, you would have a MUCH better time with LF rockets. Cutting the engines, waiting for clearance, and then unzipping the sides of an LRB is much safer than firing the FTS on a solid.

You wouldn't wait for the malfunctioning launch vehicle to shut its engines down before you engage the Launch abort motors.

[DerekL1963]

1 hour ago, sevenperforce said:

Uh, you would have a MUCH better time with LF rockets. Cutting the engines, waiting for clearance, and then unzipping the sides of an LRB is much safer than firing the FTS on a solid.

Canopus is correct, you cut the engines and fire the LES at practically the same time.  (This is why LES have such murderously high t/w ratios and produce high G forces on the occupants.)  Clearance is then a product of lower (and eventually zero) acceleration of the booster due to loss of thrust and acceleration (of the capsule) by the LES.

As far as your statement goes, you're not thinking through the process - if you have adequate clearance, then by definition it's safe to fire the destruct system regardless of whether the booster is solid or liquid.

[sevenperforce]

54 minutes ago, DerekL1963 said:

Canopus is correct, you cut the engines and fire the LES at practically the same time.  (This is why LES have such murderously high t/w ratios and produce high G forces on the occupants.)  Clearance is then a product of lower (and eventually zero) acceleration of the booster due to loss of thrust and acceleration (of the capsule) by the LES.

As far as your statement goes, you're not thinking through the process - if you have adequate clearance, then by definition it's safe to fire the destruct system regardless of whether the booster is solid or liquid.

You mistake my point. Cutting the engines on a liquid booster immediately kills thrust on the firing stage, giving you just a little extra cushion as your LES carries you away. You can then unzip the booster for range safety once the capsule is well clear. An SRB engine cannot be immediately turned off without FTSing the whole stage, and you cannot FTS the whole stage until the LES pulls the capsule clear, which means it's possible to have SRB-to-capsule impact.

Consider something like Ares I, where you have an SRB carrying a liquid upper stage carrying a capsule. If you had a CRS-7-style structural failure of the upper stage at Max-Q and you didn't have clean separation between the capsule's boost shroud and the upper stage, the still-firing SRB could push its way through the debris and impact the capsule from underneath. That's not possible with a liquid booster, because you can simply kill its engines.

[Canopus]

21 minutes ago, sevenperforce said:

You mistake my point. Cutting the engines on a liquid booster immediately kills thrust on the firing stage, giving you just a little extra cushion as your LES carries you away. You can then unzip the booster for range safety once the capsule is well clear. An SRB engine cannot be immediately turned off without FTSing the whole stage, and you cannot FTS the whole stage until the LES pulls the capsule clear, which means it's possible to have SRB-to-capsule impact.

Consider something like Ares I, where you have an SRB carrying a liquid upper stage carrying a capsule. If you had a CRS-7-style structural failure of the upper stage at Max-Q and you didn't have clean separation between the capsule's boost shroud and the upper stage, the still-firing SRB could push its way through the debris and impact the capsule from underneath. That's not possible with a liquid booster, because you can simply kill its engines.

I don't think they can be sure to still have control over the booster in such a severe event. So even in case of a liquid booster the launch escape system would most likely be designed to escape from a stage running at full power.