[EMAIL PROTECTED]">Delta-v is a combination of ISP and mass fraction of course... it's not a primary variable. Actually dry mass is every bit important as ISP but possibly not quite as much of a black magic.Randall Clague wrote:We're on the same page. What I'm asking you is, to which capability
number shall we apply Moore's Law? Altitude? Delta-v? Mass
fraction? Total impulse? Each will yield a different answer.
To disagree with Ian a bit, ISTM that Delta V is the factor that can be
increased by actual engineering;
[EMAIL PROTECTED]">Limits, yes. But to achieve that you have to have good combustion, which is a lot to do with injectors and such like, and that isn't always so easy; there's a certain amount of black magic. Qn: What percentage of theoretical maximum are you getting right now?Isp is a matter of propellant choice
and making the engine last long enough for your burn (or your reuse
cycle), and has pretty well known limits for any given propellant
chemistry.
[EMAIL PROTECTED]">So does ISP. It's not that one is better than the other, it's more like, which is harder to get 'good enough'? Perhaps we don't know- the point is to try to guess which is the most constraining factor and work at that, once that is solved, then build the rest of the rocket around it. But of course the whole rocket has to meet its budget...Altitude increases wildly with relatively modest increases
in Delta V. Mass fraction is good, but translates very directly into
Delta V; in fact, it's Delta V that gets you anywhere.
[EMAIL PROTECTED]">I don't think that the mass of the engine is going to be so very large. Even H2/LOX engines manage 70:1 and they're heavy. I think the British Black Knight engine was about that ratio- stainless steel isn't noted for it's lightweight construction, I don't think they were optimising for low weight so much, but you may want to optimise more if you are going for SSTO or SSTE. Still, engine mass below a certain point isn't the issue- tankage and other issues tend to dominate, or are atleast as large- at that point delta-v is influenced more by ISP.A huge mass
fraction with peroxide monopropellant still won't get you to orbit, and
an Isp equivalent to 10 km/s effective exhaust velocity won't help you
if the engine is so heavy you only have a 30% mass fraction.
[EMAIL PROTECTED]">Delta-v is the target. But it's about as far away from any part of the design as you can get. So many decisions affect it.Assuming KISS II had a Delta V of around 400 m/s (subsonic plus gravity
losses for that 6500' flight -- and it's a guesstimate), will KISS III
be capable of 800 m/s? I'm thinking not, since AIUI the only change is
constant pressure instead of blowdown -- but even if doubling takes two
years instead of one, Delta V is the parameter by which to measure,
That's not quite the point I'm making. If you come up with a budget for each variable- tankage mass, engine thrust/weight ratio, ISP, guidance mass, intertankage mass etc. if you miss the budget by 1%, how hard is it to bring it up? Some things are hard, adding ISP is 'easy' at low efficiencies but gets much harder; tankage mass is easy at 20%, below 0.5% perhaps nobody knows how to do this. The Space Shuttle manages maybe 97% ISP efficiency; my suspicion is they did an enormous amount of work to get it that high. Presumably you're looking for a design with the least amount of work in it (ideally).
[EMAIL PROTECTED]">That's not the first question. The timeline comes out of the project plan that comes out of the design. Do you have a design for 13 km/s?since a Delta V of about 13 km/s will get you pretty much anywhere in
the Solar System (assuming you don't mind waiting for slingshot
maneuvers). The goal is clear; the question is, when can ERPS get
there?
