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> The highest performing rocket propellants in common use are hydrogen and oxygen, whose product is water. Citation needed. Hydrolox has the highest Isp, but f

> The highest performing rocket propellants in common use are hydrogen and oxygen, whose product is water. Citation needed. Hydrolox has the highest Isp, but for a boost stage, that alone at all costs is not the most important quality. Hydrolox engines, tanks and piping all needs to be much larger and heavier than other fuels, and in practice, this eats the gains that the higher Isp buys you and more. It's not an accident that basically all the new engine designs, by multiple different companies, have tested hydrolox and then switched to methalox.

Robotbeat 8y ago
Oh, that is a valid point. Dry mass is just as important as exhaust velocity (Isp). However, I should point out that it's easier to make progress in improving dry mass than it is to improve Isp, so as materials and structural technology improves, hydrolox continues to look better. Additionally: operating oxygen-rich causes the bulk density of hydrolox to improve dramatically. This is relevant for first stages, and if reusable launch vehicles become so cheap that propellant cost becomes a major constraint, then I should point out that liquid oxygen is practically free: less than $100 per ton. For first stages, you're right (unless you get clever and are able to solve the engineering challenges of running oxygen-rich). For upper stages, hydrolox again looks very attractive (because a first-order optimization shows that it's energetically favorable to use higher Isp on the upper stage than the lower stage if you need to make a choice... of course, commonality of stages is also desirable at lower flight rates).
- Tuna-Fish 8y ago
  > so as materials and structural technology improves, hydrolox continues to look better. The opposite is true. The relevant problem of hydrogen is that it readily leaks through thin walls. This means that as methalox rockets move to ever thinner composite tanks made of better materials, hydrolox really cannot do better than the current state of the art. > operating oxygen-rich causes the bulk density of hydrolox to improve dramatically. True. But it also makes your exhaust into a hot, oxidizing torch, meaning you need much more durable (and heavier!) engine bells. > For upper stages, hydrolox again looks very attractive The other problem is that it must be cooled much deeper, and takes much less energy to boil off, making long term storage a hard problem. This makes hydrolox less interesting for any upper stages/manouvering engines that do anything more complex than deliver cargo to earth orbit. Once you add long coast times, it gets really bad -- to the point that the SLS moon missions actually get lower efficiency from their upper stages than they would get from kerolox, once you account for the extra hydrogen they have to ferry up just to let it boil away. > commonality of stages is also desirable at lower flight rates Commonality of stages is desirable at all flight rates. Paying for the highest possible efficiency gadget for every task is one of the reasons old space has such huge problems competing in cost with the new entries. Just having a little more thrust and burning a little more fuel is more cost-efficient than having another engine manufacturing line. Methalox engines will probably power almost everything newly designed going forward. Not because they are the best solution for anything, but because they are 90% there at everything, while lacking the worst problems of everything. Their fuel is easy to contain and stable for long-term storage, they have no carbon build up in the exhaust, their exhaust is reasonably cool and not heavily corroding, and they have enough thrust and enough of Isp that these advantages will outweigh a minor efficiency loss in almost all situations. The winds of the industry have shifted, and now that methane has been proven, no new hydrolox engines are being developed, with basically everyone choosing methalox for new designs instead. JSC Kuznetsov, SpaceX, BO, ESA and Landspace all independently made the same choice. Others have chosen to use engines developed by some of them. The end result is that the only major rocket operators still developing a future rocket with non-methalox engines are: - Roscosmos (The Angara rocket family development was started before methalox was seen as viable. There are future plans for switching propellants, but afaik no funding for them currently.) - Arianespace (The Ariane 6 development was started before methalox was proven. Ariane NEXT is going to run on methane.) - NASA (SLS was mandated to use existing engine designs by the senate, because the senators' home states needed the money.)
  - Robotbeat 8y ago
    I'm very well aware of industry trends. Your points about hydrogen leaking are not actually accurate. This doesn't negate the advantages at all, and as vehicles get larger, the advantages become even more pronounced. Oxygen-rich is used on Russian engines, which are some of the best engines ever made (although they haven't improved on them much in the last couple decades). Commonality of stages is more important at lower flight rates since you have to pay for amortization of the greater development costs. I like methane/oxygen. It is a very good choice. But I also don't buy into the hydrolox phobia that has become popular. Hydrolox is still a very good choice. (I think that SpaceX and Blue Origin are completely right for picking methane/oxygen for BFR and New Glenn; I'm speaking even more long-term...)

Robotbeat 8y ago

Oh, that is a valid point. Dry mass is just as important as exhaust velocity (Isp). However, I should point out that it's easier to make progress in improving dry mass than it is to improve Isp, so as materials and structural technology improves, hydrolox continues to look better. Additionally: operating oxygen-rich causes the bulk density of hydrolox to improve dramatically. This is relevant for first stages, and if reusable launch vehicles become so cheap that propellant cost becomes a major constraint, then I should point out that liquid oxygen is practically free: less than $100 per ton. For first stages, you're right (unless you get clever and are able to solve the engineering challenges of running oxygen-rich). For upper stages, hydrolox again looks very attractive (because a first-order optimization shows that it's energetically favorable to use higher Isp on the upper stage than the lower stage if you need to make a choice... of course, commonality of stages is also desirable at lower flight rates).

Tuna-Fish 8y ago
> so as materials and structural technology improves, hydrolox continues to look better. The opposite is true. The relevant problem of hydrogen is that it readily leaks through thin walls. This means that as methalox rockets move to ever thinner composite tanks made of better materials, hydrolox really cannot do better than the current state of the art. > operating oxygen-rich causes the bulk density of hydrolox to improve dramatically. True. But it also makes your exhaust into a hot, oxidizing torch, meaning you need much more durable (and heavier!) engine bells. > For upper stages, hydrolox again looks very attractive The other problem is that it must be cooled much deeper, and takes much less energy to boil off, making long term storage a hard problem. This makes hydrolox less interesting for any upper stages/manouvering engines that do anything more complex than deliver cargo to earth orbit. Once you add long coast times, it gets really bad -- to the point that the SLS moon missions actually get lower efficiency from their upper stages than they would get from kerolox, once you account for the extra hydrogen they have to ferry up just to let it boil away. > commonality of stages is also desirable at lower flight rates Commonality of stages is desirable at all flight rates. Paying for the highest possible efficiency gadget for every task is one of the reasons old space has such huge problems competing in cost with the new entries. Just having a little more thrust and burning a little more fuel is more cost-efficient than having another engine manufacturing line. Methalox engines will probably power almost everything newly designed going forward. Not because they are the best solution for anything, but because they are 90% there at everything, while lacking the worst problems of everything. Their fuel is easy to contain and stable for long-term storage, they have no carbon build up in the exhaust, their exhaust is reasonably cool and not heavily corroding, and they have enough thrust and enough of Isp that these advantages will outweigh a minor efficiency loss in almost all situations. The winds of the industry have shifted, and now that methane has been proven, no new hydrolox engines are being developed, with basically everyone choosing methalox for new designs instead. JSC Kuznetsov, SpaceX, BO, ESA and Landspace all independently made the same choice. Others have chosen to use engines developed by some of them. The end result is that the only major rocket operators still developing a future rocket with non-methalox engines are: - Roscosmos (The Angara rocket family development was started before methalox was seen as viable. There are future plans for switching propellants, but afaik no funding for them currently.) - Arianespace (The Ariane 6 development was started before methalox was proven. Ariane NEXT is going to run on methane.) - NASA (SLS was mandated to use existing engine designs by the senate, because the senators' home states needed the money.)
- Robotbeat 8y ago
  I'm very well aware of industry trends. Your points about hydrogen leaking are not actually accurate. This doesn't negate the advantages at all, and as vehicles get larger, the advantages become even more pronounced. Oxygen-rich is used on Russian engines, which are some of the best engines ever made (although they haven't improved on them much in the last couple decades). Commonality of stages is more important at lower flight rates since you have to pay for amortization of the greater development costs. I like methane/oxygen. It is a very good choice. But I also don't buy into the hydrolox phobia that has become popular. Hydrolox is still a very good choice. (I think that SpaceX and Blue Origin are completely right for picking methane/oxygen for BFR and New Glenn; I'm speaking even more long-term...)