What place do solid rocket fuels have in spaceflight?

[BringBackSuperHeavies!]

I don’t understand. I can see why people use solid Rocket motors for kick stages, but for any other application I don’t think they have a proper place in spaceflight. No need to add six strap on solid boosters, just make a decent size liquid fueled rocket to carry big payloads. Liqiud rockets have more payload for their size, and liquid rocket aren’t actually that much more expensive to launch or add than solid rockets, if at all. You spend several million dollars on some low ISP solid booster. Maybe just add some more cheap liqiud engines and that can give you better performance for your money (bang for your buck). Plus, you can barely even throttle the things and you can’t stop them until they run out of fuel. Use a comparably priced liquid engine that has deep throttle capabilities (actually not all liquid engines can stop then restart)! Solid rockets just provide a service that liquids can provide for a similiar or even less price.

[laszlo]

I don’t understand. I can see why people use solid Rocket motors for kick stages, but for any other application I don’t think they have a proper place in spaceflight. No need to add six strap on solid boosters, just make a decent size liquid fueled rocket to carry big payloads. Liqiud rockets have more payload for their size, and liquid rocket aren’t actually that much more expensive to launch or add than solid rockets, if at all. You spend several million dollars on some low ISP solid booster. Maybe just add some more cheap liqiud engines and that can give you better performance for your money (bang for your buck). Plus, you can barely even throttle the things and you can’t stop them until they run out of fuel. Use a comparably priced liquid engine that has deep throttle capabilities (actually not all liquid engines can stop then restart)! Solid rockets just provide a service that liquids can provide for a similiar or even less price.

They're easier to design, build and maintain- no separate tanks and plumbing for fuel and oxidizer. No leaks. No pumps. No drive systems for pumps. Ignition is a well-known process that works without ullage,  pressurization or additional ignition fluids. Once loaded they can stay ready to use for extended (months, years) periods of time. Can be shipped cross-country fully loaded. No boil-off. No spills. Generally less sensitive to mechanical impact. Fueling not a part of the launch procedure so faster launch times. No need to maintain a tank farm and plumbing. No line of hundreds of trucks to refill the tank farm.

Higher energy density than many liquid stages, not just because of propellant density, but also because of the space between the pipes.

Combustion has been being stopped with enough precision to accurately deliver nuclear warheads since the early 1960's. Energy management techniques allow rockets to steer to specific velocity/position goals.

High I_SP doesn't necessarily make sense in all situations, especially first stage boosters. Higher bulk thrust, which is easier to accomplish with solids than liquids, is sometimes more important.

Yes, liquid-fueled boosters have their advantages, but so do solids. Which to use depends entirely on what the specific requirements are.

[deltaV]

Virtually all military rockets and missiles use solids, for good reasons such as solids always being ready to use. I'm guessing the defense contractors that often design orbital rockets have often used the tools they're familiar with and used solids even for use cases where a purely civilian designer would have been more likely to choose liquids. Also some orbital rockets are actually repurposed ICBMs in whole or in part.

It seems that solids can't compete with liquids in the new world of partial or full re-usability so if you wait a decade or so solids for orbital launch will probably be a niche player at most. (Except for uncompetitive launch vehicles that are kept alive for national security or political reasons, e.g. Ariane 6, SLS.)

[laszlo]

Virtually all military rockets and missiles use solids, for good reasons such as solids always being ready to use. I'm guessing the defense contractors that often design orbital rockets have often used the tools they're familiar with and used solids even for use cases where a purely civilian designer would have been more likely to choose liquids. Also some orbital rockets are actually repurposed ICBMs in whole or in part.

It seems that solids can't compete with liquids in the new world of partial or full re-usability so if you wait a decade or so solids for orbital launch will probably be a niche player at most. (Except for uncompetitive launch vehicles that are kept alive for national security or political reasons, e.g. Ariane 6, SLS.)

It isn't just because old dogs can't learn new tricks. Solids are a cost-effective, proven and reliable technology that allows an incremental increase in thrust for an existing booster without having to worry about a lot of esoteric engineering or complicated GSE.

[BringBackSuperHeavies!]

So liquids aren’t proven either? Not everyone uses solids. Not just SpaceX (you think Elon and his team didn’t do a detailed analysis of the trade offs?) but Russia, and ULA themselves. Solids are technically more reliable, but when it comes to abort scenarios they present a problem. Since a solid cannot be throttled much, this also presents something of a problem when trying to precise control a rocket’s ascent. And liquid engines are pretty darn reliable anyway. The SpaceX Merlin and some other have had very high success rates. As for bulk thrust, kerosene and or/methane can provide high bulk thrust while not compromising as much on efficiency. Also the core stage is liquid anyway, and that’s still less reliable and more complex. Likely the use of solids is mostly because big ICBM makers had control over rocket designs.

[Vahe231991]

I don’t understand. I can see why people use solid Rocket motors for kick stages, but for any other application I don’t think they have a proper place in spaceflight. No need to add six strap on solid boosters, just make a decent size liquid fueled rocket to carry big payloads. Liqiud rockets have more payload for their size, and liquid rocket aren’t actually that much more expensive to launch or add than solid rockets, if at all. You spend several million dollars on some low ISP solid booster. Maybe just add some more cheap liqiud engines and that can give you better performance for your money (bang for your buck). Plus, you can barely even throttle the things and you can’t stop them until they run out of fuel. Use a comparably priced liquid engine that has deep throttle capabilities (actually not all liquid engines can stop then restart)! Solid rockets just provide a service that liquids can provide for a similiar or even less price.

Solid fuels remain an appropriate propellant for rockets capable of launching small payloads to orbit, namely if three or more stages are used. The Minotaur rocket family uses solid fuels because it is derived from the Peacekeeper and Minuteman ICBMs, both of which were designed to use solid fuel propellant.

[laszlo]

So liquids aren’t proven either? Not everyone uses solids. Not just SpaceX (you think Elon and his team didn’t do a detailed analysis of the trade offs?) but Russia, and ULA themselves. Solids are technically more reliable, but when it comes to abort scenarios they present a problem. Since a solid cannot be throttled much, this also presents something of a problem when trying to precise control a rocket’s ascent. And liquid engines are pretty darn reliable anyway. The SpaceX Merlin and some other have had very high success rates. As for bulk thrust, kerosene and or/methane can provide high bulk thrust while not compromising as much on efficiency. Also the core stage is liquid anyway, and that’s still less reliable and more complex. Likely the use of solids is mostly because big ICBM makers had control over rocket designs.

No one said that liquids aren't proven or reliable, just that the engineering and construction is simpler for solids. If you believe that the best part is no part, then no tanks, no plumbing, no pumps, no fueling or draining systems, no ullage, no pressurization (autogenous or otherwise), etc. has some appeal.

Of course SpaceX did the trade studies and it's perfectly reasonable that they decided not to go with solids. They're aiming for a spacecraft that can be refueled on Mars and flown back to Earth. The infrastructure to make solid boosters is too complex for off-planet use for now. On the other hand, for launch providers who just need to add a few tons of thrust to an existing first stage, especially low launch rate expendables, solids are a valid solution.

I_SP/efficiency is not the only criterion for an engine. It's actually less important than energy density and high thrust for first stages. And yes, it's completely possible to build a liquid fuel high thrust booster, but it takes a lot more effort than for solids. For example, the Space Shuttle SRB contract  was let in 1973, work started in 1974, the first test was in 1977 and first orbital flight in 1981. Methalox Raptor development seems to have started around 2013 and 10 years on there is yet to be a successful orbital flight, even though SpaceX is a new-space fail early and often company with 21st century computer tech while Thiokol was using slide rule-based waterfall methodologies on a government contract. Don't get me wrong - this does not indicate anything wrong with SpaceX. It just shows how much more complicated getting that kind of thrust efficiently from reusable liquid engines is compared to solid boosters. That's the answer to your original question of what place do solid rocket fuels have in spaceflight. Solids are simple and cheap compared to liquids. if you can live with the solid's disadvantages and you don't need liquid's advantages, they have a place. Each booster design doesn't have to be perfect, just get the job done.

The reason that SLS uses SRBs is because the Shuttle did. SLS is supposed to keep old Shuttle workers employed and old Shuttle contractors in business. (I'm not getting into whether that's a good thing or not, it has nothing to do with liquid vs. solid.) The reason that Shuttle had SRBs is because Congress and Nixon did not want to fund a fully-reusable liquid-fueled first stage. Switching to solids was supposed to save the equivalent of nearly $15 billion in today's money in development costs. Plus, with the solids being fully reusable the operating cost was supposed to be low, too. ICBM's had nothing to do with the SLS choice of SRBs vs. liquid boosters.

So again, I'm not arguing that solids are always a better choice. I'm just answering your original question of what place do solid rocket fuels have in spaceflight.

[spacenut]

In the future, I think solids will go away, especially with all the reusable rockets coming on line.  There might still be some strap on monolithic solids to help boost rockets.  However, large solids like SLS will go away, probably within the next 10 years.  To expensive to keep throwing away.  Reuse is the future.  I think liquids will get things to and from orbit, then SEP, NEP, nuclear, or other propulsion will eventually be used in space only, with liquid shuttle rockets to and from the surface of planets and moons. 

[edkyle99]

Solids are technically more reliable, but when it comes to abort scenarios they present a problem. Since a solid cannot be throttled much, this also presents something of a problem when trying to precise control a rocket’s ascent.

In nearly 260 Falcon 9 flights there has only been one serious ascent abort (to orbit), and that was a Merlin 1C-powered vehicle.   Solids do have "throttle" profiles built into their grains.  Shuttle and Artemis SRBs thrust decreases for Max-Q, for example, and thrust decreases toward the end of their burns.  Meanwhile, there has been at least research into active throttling methods for solid motors.  There are long proven ways to precisely terminate solid thrust.

And liquid engines are pretty darn reliable anyway. The SpaceX Merlin and some other have had very high success rates.

Merlin 1D is the most reliable high thrust liquid rocket engine ever.  It is a very special machine and I don't see it as an argument for the norm.   Other liquid vehicles have more problems.  Electron just had a failed Stage 2 engine start.  Astra Rocket, Terran 1, LauncherOne, etc., all suffered failures just this year or last, and so, notably, did Super Heavy/Starship.  Raptor has not shown an inclination toward Merlin 1D reliablity.  SH has yet to even complete a static firing with all engines burning.

 - Ed Kyle

[Barley]

Merlin 1D is the most reliable high thrust liquid rocket engine ever.  It is a very special machine and I don't see it as an argument for the norm.

When people get to choose the best becomes the norm.  There's a reason you're not driving a model T.

Merlin is the norm.  It sets the bar.  If an alternative is not in the ball park it will be at best a political boondoggle while almost all launches use Merlin.

[DanClemmensen]

Merlin 1D is the most reliable high thrust liquid rocket engine ever.  It is a very special machine and I don't see it as an argument for the norm.

When people get to choose the best becomes the norm.  There's a reason you're not driving a model T.

Merlin is the norm.  It sets the bar.  If an alternative is not in the ball park it will be at best a political boondoggle while almost all launches use Merlin.

In 2023, approximately 90% of the world's payload mass launched to orbit uses the Merlin.

[Vahe231991]

Which solid rocket booster in the 1990s was used to assist space launch vehicles in carrying the most payload mass into orbit?

[gin455res]

I don’t understand. I can see why people use solid Rocket motors for kick stages, but for any other application I don’t think they have a proper place in spaceflight. No need to add six strap on solid boosters, just make a decent size liquid fueled rocket to carry big payloads. Liqiud rockets have more payload for their size, and liquid rocket aren’t actually that much more expensive to launch or add than solid rockets, if at all. You spend several million dollars on some low ISP solid booster. Maybe just add some more cheap liqiud engines and that can give you better performance for your money (bang for your buck). Plus, you can barely even throttle the things and you can’t stop them until they run out of fuel. Use a comparably priced liquid engine that has deep throttle capabilities (actually not all liquid engines can stop then restart)! Solid rockets just provide a service that liquids can provide for a similiar or even less price.

How about as a second-stage in a semi-reusable 3 stage launcher with reusable booster and upper stages?


Booster and upper stage are rtls. 

[edkyle99]

Merlin 1D is the most reliable high thrust liquid rocket engine ever.  It is a very special machine and I don't see it as an argument for the norm.

When people get to choose the best becomes the norm.  There's a reason you're not driving a model T.

Merlin is the norm.  It sets the bar.  If an alternative is not in the ball park it will be at best a political boondoggle while almost all launches use Merlin.

Merlin 1D is an ideal, but it is not the norm.  It is an outlier.  There have been 16 orbital launch failures world-wide since the start of 2022.  Only two of those were, or likely were, due to solid rocket problems.

 - Ed Kyle

[Robotbeat]

It's an outlier, except most launches in the world use Merlin 1D.

[StormtrooperJoe]

 I mean ICBMs are spacecraft, albeit only for for a very short period of time. Solid fuels are perfect for storeable ready to use rockets which means they will continue to be used in weaponry. There might be some sort of niche for satellites that can be launched at a moment's notice for the military, but even then as Firefly recently showed that's not necessarily needed.

[Proponent]

There have been 16 orbital launch failures world-wide since the start of 2022.  Only two of those were, or likely were, due to solid rocket problems.

Out of how many stages and boosters (SRBs or LRBs) of each type in total?

[edkyle99]

There have been 16 orbital launch failures world-wide since the start of 2022.  Only two of those were, or likely were, due to solid rocket problems.

Out of how many stages and boosters (SRBs or LRBs) of each type in total?

That answer will take some work, as there are many vehicles with both liquid and solid elements.  In terms of LVs that consist of a solid "core" stage, I roughly count 40 launches during 2022-23 so far, with four failures - but two of those failures involved liquid propellant systems (RCS or kick stages)!

 - Ed Kyle

[Barley]

There have been 16 orbital launch failures world-wide since the start of 2022.  Only two of those were, or likely were, due to solid rocket problems.

Out of how many stages and boosters (SRBs or LRBs) of each type in total?

That answer will take some work, as there are many vehicles with both liquid and solid elements.  In terms of LVs that consist of a solid "core" stage, I roughly could 40 launches during 2022-23 so far, with four failures - but two of those failures involved liquid propellant systems (RCS or kick stages)!

 - Ed Kyle

The answer is largely irrelevant.  It is only relevant if you make two incorrect assumptions.

There is a whole unreasonable expectation that prototypes should never fail.  How many prototypes you blow up is really only relevant if you have a program like Apollo where all launches were essentially prototypes.  A production run in low single digits is not a production run.

The lower cutoff for obvious incompetence. At what point do you stop counting amateurs that fail to get to orbit because they got the delta-V requirement wrong by orders of magnitude?  The Duffus to Einstein ratio is always disturbingly large and the cutoff is always arbitrary.

[edkyle99]

The lower cutoff for obvious incompetence. At what point do you stop counting amateurs that fail to get to orbit because they got the delta-V requirement wrong by orders of magnitude?  The Duffus to Einstein ratio is always disturbingly large and the cutoff is always arbitrary.

In terms of the solid motor failures during 2022-23, there aren't any beginners involved.  One was the 22nd Arianespace Vega (a Vega-C).  The other was (apparently, not yet confirmed a solid motor failure) the 10th Ceres-1, the first failure of the type.

Among liquids, yes there is a tilt toward new or relatively new entries.  The list includes ZQ-2 Y1, a Firefly Alpha, two Rocket 3.3 vehicles, two North Korean Chollima 1's, and the inaugural Terran 1 and RS-1 flights.  But the list also includes the 41st Electron, the sixth and final LauncherOne, and the surprising veteran upper stage failure on the inaugural JAXA H-3. 

In addition, the mostly solid SQX-1 and Epsilon rockets both suffered liquid RCS system leaks, Epsilon on its sixth flight and SQX-1 on its fourth.

 - Ed Kyle