Great video. Much of the high level design of anything comes down to finding the trade offs one can live with. Thomas Sowell quote: “There are no solutions, there are only trade-offs; and you try to get the best trade-off you can get, that's all you can hope for.”
This Channel is like finding a gold nugget, specially on this age of clickbait and people who dont know what they are talking about. Thanks for the info. Liked and suscribed
You know about the SS-520-5? It's a modified sounding rocket with a third stage designed to put payloads into orbit. It can put 4kg into LEO and is the smallest orbital rocket ever built, only 9.5m tall.
It seems like the delta4 and atlas5 suffer from un ideal staging due to the only available hydrolox engine(RL10) being so small and low thrust. If there was a medium thrust hydrolox engine still around that 2nd stage could be appropriately made bigger maybe doubling the leo payload capacity
As attractive as that seems, it probably wouldn't work very well. Delta IV Heavy already struggles to get off the pad with it's max capacity, and with a larger upper stage it would most likely be restricted to GTO, GEO and deep space payloads, which both already excel at.
FWIW, the current version of stable diffusion web produces terrible results when given the prompt "centaur holding a falcon". Nothing recognizable as a centaur.
Given the advantages of larger numbers of smaller engines that you mentioned, and also given that it’s the approach being used by most new rocket companies with large boosters (e.g. Superheavy), why do you think Saturn V went with only five massive engines in the first stage?
That's an interesting question, and I don't know the real answer. My *guess* is that it's because NASA was buying those engines, and buying fewer engines tends to be more economical in that case. It is interesting that they made that choice; AFAIK there was nothing that suggested it would be simple to create such an engine, and it turned out to be very difficult.
@@EagerSpaceI think that part of the answer is that the design was already underway and maybe substantially complete, because the military had been designing it when the bombs they needed to loft were extremely heavy. So given the deadline of 1969, it may have been their only real choice for high thrust. Also maybe they felt that production and QA on five engines was easier than 35.
Hydrolox boosters: Is one limitation the difficulty of pumping enough hydrogen to generate the mass flow to lift off the pad? The discussion is always around tanking the hydrogen not pumping it. Loosely, Delta V heavy (DVH) and Falcon heavy (FH) are the same size but at lift off DVH weighs half as much and has half the LEO capability. "Volume" still needs to be pumped even when weight is more important. Is simply pumping enough hydrogen to lift twice the payload impractical even without the large tank problem? ed: -Atlas- >>>---> _Delta_
Interesting question, and I don't really know the answer. If we look at the RS-25, the high pressure hydrogen turbopump is about 69,000 horsepower, and the high pressure oxygen turbopump is 25,000 horsepower, so it takes 2.7 times the power to get the hydrogen in there. And that's at an oxygen/hydrogen mixture ratio of 6:1, so the engine is pumping 6 times as much oxygen by mass. And of course the design needs four turbopumps. It's thrust/weight is in the 50s, and the RS-68 is in the same ballpark, so quite a bit less than the RD-180 which is in the 70s. According to Musk, Raptor supposedly generates 100,000 horsepower total in the turbopumps which is pretty much the same as the RS-25 if you add up all 4 turbopumps there. Raptor's about 20% more thrust than the RS-25 at this point, so you could conclude that it's more efficient in terms of pump power, but the musk number is from 2018, so who knows that it is now.
A interesting question is can SLS could be re-design to fly with metalox engines in the main stage? I make some calculations and if you suppose that would have the same size because of the higher density would weigh around 2350 tons (500 tons of methane, 1750 oxygen and 100 dry mass), that obviously will require more than engines, let's say 7 raptor, but after made deltaV calculations I find that for the current mission would delivery with 370m/s less than currently is capable the hidrolox version, is a way to optimize that? Well are three, first add more rocket boosters, but that would make a 5300 rocket for 4 SRB, second use vacuum optimized nozzle, and if you make that change it can match the current version, and the third method is using a bigger upper stage, and for that you find that can have better performance than current versions can do.
I'm too lazy to do the volume calculations right now, but generally speaking a hydrolox stage is about double the volume of either a kerolox or a metholox stage, so the core stage is just too darn big to run methalox. Doing so essentially turns it into something like super heavy, with a large amount of engines, and then you don't need the SRBs, and it's no longer the same rocket. The problem with raptors in this application is that they are optimized for sea level and give away some Isp because of that. The RS-25 is a vacuum engine with a nozzle that just barely works for sea level applications, and it's designed that way because with the shuttle profile, it spends far more time in vacuum than in atmosphere. So, shrink down the core stage to a reasonable size for methalox and use an engine like the vacuum raptor, and I think you'll get something comparable.
@@EagerSpace you cannot shrink the core without reducing seriously payload delivered to the moon, that is because of the lower specific impulse of methane-LOx combination, even with a 2350 tons methalox core and a 260 tons hidrolox upper stage the payload delivered to the moon would be only 70 tons, witch is more than 45, (I think that would be maximum capacity for the bigger version of the current SLS) , but we are talking about a 4100 tons rocket not about one around 2700. A shrinked SLS core would be like New Glenn(but with only 4 engines) with two large SRB attached, which would be enough for sending a 27 tons Orion to the moon, but only for that kind of payload . So wouldn't be SLS anymore.
@@theOrionsarms Eric is talking about size (i.e, volume), not weight, so the fact that it would weigh 4100 tonnes instead of 2700 is irrelevant. A 2350 tonne methalox core would be approximately the same size as the current SLS core, and if that gets 70 tonnes to TLI then there's clearly room to shrink it. By my math, a ~1200 tonne methalox core using RVacs, along with the EUS and BOLE boosters like SLS Block 2, would also get ~46 tonnes of payload to TLI. The total mass would increase to ~2900 tonnes, but the core would only need to be about half as large, reducing it's diameter from 8.4m to around 6m.
@@theOrionsarms I'm happy to keep discussing this but I'd like to go deeper into the numbers and the assumptions you are making. The SLS core stage is about 85 tons empty and has a gross mass of about 1073 tons, so it has a mass ratio of about 12.6 and the natural log of that is 2.53. Super heavy is likely around 180 tons empty and carries 3600 tons of propellant, so it has a mass ratio of 21 and the natural log of that is 3.04. That gives super heavy a mass ratio advantage of 3.04 / 2.53 = 1.20 The RS-25 specific impulse is 452, the sea level raptor is about 348 in vacuum, so the ratio is about 1.29. That makes SLS look better, but the thrust/weight of the SLS core is about 0.7, so you would really want to compare a version of super heavy with a similar thrust / weight ratio. Doing that is going to improve the mass ratio of super heavy. It's going to end up as a wash, within a few percentage points.
@@EagerSpace I don't compared SLS with super heavy, I assumed a SLS core filled with methalox , and I used a 430sec for hidrolox core and 340sec for methalox one and for dry masses was 90tons empty for hidrolox and 100 for methalox, and for the small upper stage I figured that hidrolox would deliver 8500m/s deltaV and methalox 9044ms, so in a sense you are right that a methalox stage can deliver more deltaV, but is another factor that need to be put in equation, contribution of the boosters to total deltaV, for hidrolox one is 2085m/s and for methalox one is 1218m/s that's because methalox stage is more heavy, obviously the numbers are changing if you use a bigger upper stage, but for sea level raptor still aren't much better than hidrolox one with the same bigger upper stage, really good results are if you combine a vacuum style raptor with 360sec average specific impulse and a even bigger hidrolox upper stage like I said for a 260tons one payload capacity is around 70tons.
Can you just put a heads up to show people where you mislabeled the Atlas V and Delta IV, you showed a Delta IV and mislabeled it as an Atlas V. Sorry I just don't want people to get confused. RL10 prices are coming down too, they were expensive in the early 2000s because a lot less were being produced after the retirement of Titan IV. They have been making a lot more of them recently so price has come down, and AJR has been making efforts to reduce the price.
Thanks for the catch. In the Build Your Own Rocket video, the Atlas V and Delta IV medium sisters though it would be funny if they switched their pictures. They deeply regret their behavior
