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With Ariane 6 launch site selected, CNES (Centre National d’Etudes Spatiales, the French space agency) was aiming to freeze the design of the new rocket in May 24, 2013. A few days after and that same month (May 30, 2013), Europe’s Air & Space Academy “urged to halt work on the Ariane 6 design” and among reasons brought forth, in their view, the wrong choice for the first two stages propellant:

The academy is urging the agencies to stop work on the Ariane 6 they approved in November with a view to beginning full development in 2014. The academy-favored rocket would use liquid propulsion instead of solid, and would face four more years of preparatory work before moving to full development in 2018.

On May 8, 2013 Europe’s Air & Space Academy sent a letter to the ESA director general Jean-Jacques Dordain, in which it urged ESA to “urgently reopen the configuration studies” for Ariane 6.

          Some of the Ariane 6 concepts under investigation

               Some of the Ariane 6 concepts under investigation (Source: ESA)

Among other objections to the proposed Ariane 6 design, Air & Space Academy gave the following reason for not using solid propellants as being “the wrong choice”:

Two solid-fueled stages topped by a cryogenic upper stage is a dead-end design that does not permit the flexibility needed in a rocket that will serve as Europe’s main launcher for several decades.

This however seems rather argumentative. Granted, solid- versus liquid-fueled rockets debate among engineers is as old as the spage age, but besides most current launch vehicle designs using liquid and/or cryogenic rocket stages, is there any non-disputable evidence the choice of one is better over the choice of another?

ESA aims to develop a per launch cheaper, simpler design launch vehicle with Ariane 6, and the use of solid propellants seems rather obvious to me, which just so happens is also a firm belief of the ESA launcher director Antonio Fabrizi. ESA estimates a per launch cost of around €70 million, something the academy disagrees on, but still quotes significant savings compared to Ariane 5:

The academy estimates that today’s Ariane 5 ECA rocket costs 145 million euros per launch. The solid-fueled Ariane 6 will likely cost 98 million per launch, at least at the outset. It will be difficult to cut costs to arrive at the announced target.

Right. So the cost savings are still there and they are substantial regardless of whose quotes we believe. So this “solid- versus liquid-fueled rockets” isn't as intuitive as I would've hoped, after reading so many articles on the Ariane 6 dispute.

Let's see, if we can get some additional perspective, though. On July 12, 2013, it was announced that Johann-Dietrich Woerner, chairman of the German Aerospace Center (DLR), said the German government remains in favor of continued development of the current Ariane 5 heavy-lift rocket, with possible evolutions including environmentally acceptable new fuels in place of the vehicle’s current solid-rocket boosters. He goes on to say:

... solid propellant carries other disadvantages, including the fact that a solid-fueled second stage adds vibration risks to sensitive satellite payloads and also pollutes the upper atmosphere.

The statement regarding "vibration" again seems a bit argumentative, but is that even true? And while the environmental concerns seem a bit more fair, is that really such a problem, and why mention upper atmosphere pollution only?

So, to recap, my questions are:

  • Are there any other technical reasons for favoring liquid propellants over solid ones, besides above mentioned reasons?
  • Can the argument that the solid-fueled second stage adds vibration risk be substantiated with hard facts for a design-stage rocket?
  • What kind of upper atmosphere pollution are we talking of, if solid propellants are chosen over liquid ones, that don't even warrant mentioning launch site and lower atmosphere pollution?

In essence, I'm looking for answers that will provide more insight into this age old engineering debate on “solid- versus liquid-fueled rockets”. Ideally, the ESA / CNES design choice for the Ariane 6 should be discussed, and if possible, avoid or completely dismiss / invalidate politically motivated arguments, such as:

Woerner said that for this kind of scale economy to work, all the Ariane 6 solid-fueled boosters would need to be made in the same place. Given today’s European industrial landscape, he said, that would mean in Italy.

This quote just smells of local industry favoritism and should, without substantial evidence that "liquid" is the way to go in favor of "solid", be easy enough to dismiss as such. Or is there more to it than meets the eye?

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    $\begingroup$ That's a doozy of a question! It's obviously detailed a well researched. But it also seems like something that would benefit from a self-answer. It's hard to tell for sure, but it sounds like what I sometimes call a "stump-the-chumps" question. Basically, these are questions in which the asker already has a good idea about what the answer is. Have you considered breaking out the more answery portions of the question and writing an actual answer instead? $\endgroup$ – Jon Ericson Aug 30 '13 at 0:51
  • $\begingroup$ @JonEricson - I'm looking for answers from the engineering point of view. I've already somewhat dismissed that "not that cheap as you think" argument, and the timeframe 4 years shorter to go with "solids" re-emphasises it. The remaining arguments against "solids" are then either political motivation (not so much interested in), or pure engineering, if at all any. I could simply ask "solids vs liquids, pro et contra", but a good example case as this one should IMO discourage biased answers. In case I don't get an answer in a reasonable timeframe yes, I'll try to research it further myself. $\endgroup$ – TildalWave Aug 30 '13 at 1:11
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Okay, here's the general break down in Solids vs Liquids debate, with a bit of help from Wikipedia, ESA, and NASA:

Solids- Solid fueled rockets must be manufactured in a controlled environment. If not constructed properly, they can have impurities resulting in uncontrolled expansions (Explosions). They are safer generally speaking than Liquids, and don't require any special precautions. The on pad requirements are simpler. Most model rockets are solid fuel, and in fact, all they really need is a source of ignition, plus verification of the equipment. They do not have the ability to throttle on demand, although they can be built in such a way to throttle according to a pre-determined rate. They can be stopped at any time by ejecting the solid core. The rocket requirements around the engine are rather loose, and there aren't really any moving parts, aside from a usually throttleable nozzle and fins for atmospheric use.

Liquids Most liquid rockets are either cryogenic or toxic, requiring at least some component to be loaded at the rocket pad. They are throttleable upon demand, allow for more flexibility in the event of an issue, and can be stopped and restarted. Usually it is not safe to be near a fueled liquid rocket, usually requiring evacuation of the pad prior to fueling. This fueling requires extra work in the last minutes. Liquid rockets are generally speaking more complex, but usually offer better ISPs than solids.

Okay, so when you are wanting to launch a rocket to orbit, why would you want one vs the other?

Solid- Usually slightly cheaper and easier to use, usually less massive overall. Could allow for quicker re-use of the pad.

Liquid More flexible in the event of an issue, allows for a broader range of missions.

Using solid rockets for the first stages could improve the capabilities, as the Space Shuttle demonstrated, but the upper stage should be liquid or at least Hyrbid, or else you won't get the accuracy that is desired.

Vibration Liquid fueled rockets burn at a very continuous rate, and thus have relatively few vibrations. Slight impurities in the solid fuel can cause the burn rate to vary, causing a higher level of vibrations. In particular, trying to get fine course adjustments with a solid fueled rocket is very difficult, causing extra vibrations in the process.

Toxicity Most liquid rockets exhaust is either water or other natural chemicals, resulting in minimum damage from their exhaust. Solid Fueled rockets have slightly more exotic exhaust, which will cause more damage than water, but still are relatively benign. Bottom line here, it depends on which type of fuel you use, there are good and bad options for the environment among each. Liquids tend to be more extreme in the good/bad than solids, which tend to be more consistently middle-of-the-road.

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  • $\begingroup$ Great, but would you feel confident in drawing any conclusions or recommendations regarding the mentioned solid fueled first stage proposal for Ariane 6 and its opposition? My understanding is ESA proposed the use of two different first stage configurations depending on attempted orbits and payload requirements, and the upper stage is cryo, so the first stage burns ought to be pretty standard with not much course / throttle correction going on in the lower atmosphere, vibration could be taken care of with dumpers, and there's pollutants in both cases anyway. 4 years more is a lot of time... $\endgroup$ – TildalWave Sep 18 '13 at 11:01
  • $\begingroup$ It's hard to judge conclusions without seeing a lot of technical details, but I don't think it would make much of a difference really. There doesn't seem to be a compelling argument one way or another, if you ask me. Which is better, Android or iPhone? $\endgroup$ – PearsonArtPhoto Sep 18 '13 at 16:51
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    $\begingroup$ Well it's obviously the one I have. :P Anyway, I wanted if you could think of ways to apply this solids vs liquids to some sort of conclusion that would directly apply to the nature of the question. I.e. those three sub-questions I listed. As a conclusion tho, not being able to support any in particular over the other sounds like a conclusion to me, especially given the 4+ more years to develop Ariane 6 with a liquid first stage than sticking with current proposal to go with solids. They could probably design Ariane 7 that would have a liquid first stage during the extra time period. ;) $\endgroup$ – TildalWave Sep 18 '13 at 16:59
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This is a pretty interesting question in retrospect now.

When it was asked in 2013, booster re-usablity was just a gleam in SpaceX's eye. The first Falcon 9 attempt to land a booster wasn't until 2015. But clearly SpaceX's pricing was already a problem for ESA, the Falcon 9 is roughly equivalent to the Ariane 5 in launch capacity, but have always been priced at about 1/3 an Ariane 5 price. So one question is, how did SpaceX have such a massive cost advantage before it even mastered re-usability?

One important reason for the SpaceX's cost advantage was liquid fueled rocket engines. SpaceX designed the Falcon 9 with nine identical Merlin engines, and one minor variant, the second stage vacuum Merlin. They launched twenty two Falcon 9 rockets from 2014-2016 without re-using any. That required building over 70 Merlin engines a year. Essentially they turned what was previously a hand built process into an assembly line with all the advantages that brings. Higher assembly productivity, standardized parts can be purchased/built less expensively than custom parts, a regimented process that produces uniform products, and the ability to improve the process and the product over time.

That improvement clearly happened in the end product. The first Merlin engine (for the Falcon 1) only produced 100,000 lbs of thrust, the current (FT) version generates 210,000 lbs.

Solid rocket engines have no moving parts, are simple to make and difficult to improve. Even if SpaceX had settled on nine solid rockets for it's first stage, there would have been much less opportunity to improve them or lower their costs.

The other obvious answer that wasn't addressed by the other respondents is booster re-usability. Not throwing away your boosters every launch is a potentially huge, Elon Musk once claimed the fuel costs for a Falcon 9 launch was only $200,000, or about 3% of the launch price.

Solid rocket boosters can't be re-used easily, if at all. The Space Shuttle solid rocket boosters had to be entirely re-furbished after each flight. They landed in salt water at a good rate of speed, which wasn't good for re-use. Ultimately the cost of refurbishing was not much less than the cost of the new boosters.

ESA understood both the opportunities of re-usablity and manufacturing volume. In fact their commitment to large rocket engines may have cost them the ability to do re-usablity. According to wikipedia (using that as a link because original source is in french).

https://en.wikipedia.org/wiki/Ariane_6#cite_note-FSReusableLiquidMethaneStage-36

"Con-current with the Liquid fly-back booster research in the late 90s and early 00s CNES along with Russia concluded studies indicating that reusing the first stage was economically unviable as manufacturing ten rockets a year was cheaper and more feasible than recovery, refurbishment and loss of performance caused by reusability.[36] It was suggested that with Arianespace launch cadence of 12 flights per year that an engine that could be reused a dozen times would produce a demand for only one engine per year making supporting an ongoing engine manufacturing supply chain unviable."

Using only one engine per launch means twelve launches a year doesn't develop nearly the same manufacturing volume to improve engine costs and performance. Using solid rocket boosters to complement means you can only fly back the core booster, so re-usability would only save a much smaller part of your launch stack costs.

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  • $\begingroup$ Thanks for digging this up. I didn't know CNES looked into reusability back then. $\endgroup$ – Hobbes Feb 10 '18 at 8:53
  • $\begingroup$ Another interesting point, in retrospect, is that they can potentially replace the one big engine with several small, something they seem to do at some point down the line with the Prometheus engines. $\endgroup$ – Eth Sep 27 '18 at 16:29
  • $\begingroup$ @eth Yes, I think more engines is the future for pretty much every launch provider. The N1 was too far ahead of it's time and it's development too shy of resources to succeed. But with modern CAD and computer modeling using large groups of liquid fuel engines isn't just feasible, it's pretty much optimal. The higher manufacturing volume reduces costs, and the ability to deeply throttle down to a single engine out of a group is the key to supporting inexpensive re-use through RTLS and barge landings. $\endgroup$ – SafeFastExpressive Nov 17 '18 at 19:16
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I think you already know the answer, it is not really a technical issue, it is (mainly) political. The Germans have experience with liquid engines (they make major parts for the liquid main engine of Ariane 5) and the Italians have experience with solid fuel (they make part of the solid boosters for Ariane 5 and the majority of the solid fueled Vega-rocket). There are others involved in the different engines but that is the basics. The French (although also involved in the liquid main engine) probably have a preference for solid fuel because there is a direct relationship between the solid boosters of the Ariane 5 and the French submarine-based nuclear missiles.

Quick comments on your recap:

  • A rocket stage with liquid engines can be tested before launch to make sure it works. Just fuel it up again and launch it. SpaceX fires up the engines on the launch pad for a couple of seconds as a final test a couple of days before launch. Can’t do that with solids...

  • Yes solid rockets vibrate more but that’s not really a problem unless you are launching people. Plenty of mostly solid fueled rockets launching satellites in the world. The suggested NASA-rocket for launching people using a solid booster from the shuttle as first stage apparently had serious problems with vibrations.

  • Hydrochloric acid and aluminum compounds are the pollution from solid fuel. So on the grounds of environmental concerns maybe one should stick to liquid oxygen + liquid hydrogen, only water vapor as exhaust. And it only takes water and electricity to make the stuff.

To finish. The real problem for Ariane 6 (and 5) is that none of them can compete with SpaceX. Europe should really try to bypass the political inefficiency the same way as NASA is trying to do, with real free market competition.

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