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 (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?