ShermanPVK

I get what all of you are saying, especially about the "Skipper.," I have also used the LVT-45 as a second-stage engine for heavy 1.25m payloads, but I still think that engines specifically designed to be balanced for second-stage propulsion would be a very good addition. Really, the 1.25m rockets are the ones that I most feel need this kind of engine. When I mentioned increasing the power of launch engines, I was really just talking about the LVT-30/45 which really struggle to lift another LVT-45 powered stage without boosters. Certainly not the Mainsail or the 3.75m engines. I am quite content with the parts we have for heavy payloads. About my comments on lander engines, I did not intend to say that iSP was a direct result of the size of the rocket bell, merely that the rocket bell size was something that lander and second stage engines do not have in common. I do also understand that the "Poodle" and 909 use a different kind of fuel, but what I was really saying was that the engines are extremely and unnecessarily overpowered as landing engines, but don't really have the thrust for insertion to LKO with payloads heavier than capsules and there's really quite a jump between the "Poodle" and "Skipper". To clarify, I think it's weird that one of the lowest iSP classes of engine in the real Apollo program has the highest iSP available for chemical rockets in KSP (even though I understand that they use different resources). I think that while the parts we have now are most certainly adequate at least, there is an area where two or three new engines based on a kind of real engine would fit quite nicely and not having them has been my biggest pet-peeve about designing launch vehicles in KSP for a long time.

The Issue: Currently, in KSP, we mostly just have launch engines and lander engines. We really lack dedicated second-stage engines for orbital insertion and transfer burns. This pretty much leaves us with the choice between putting another launch engine on the second stage and making the staging itself almost useless if not counterproductive, adding so many boosters that the launch core reaches orbit, or using one of the lander engines for orbital insertion. The latter is the most similar to how real launches work and is likely to be what new players will do. In KSP, it works well enough for low-efficiency orbital insertion at high altitude, but is generally impractical for realistic, high efficiency LKO insertions due to the low thrust. Dozens of my launch attempts have failed due to not having enough second stage thrust. I also don't think it makes much sense that we should use lander engines for orbital insertion and transfer burns, or even that they should be good for it. Lander engines are designed to fill their role (landing small craft on low-gravity bodies) by sacrificing efficiency and thrust to have low mass and be short enough for the landing legs and ladder to touch the ground. Comparison to Real Rocketry: Stock KSP and Realism: I understand and support Squad's decision for stock Kerbal Space Program not to closely follow realistic physics and engineering, but this is still a rocketry game and it is important to consider how rocketry really works when making design decisions. Considering the relatively recent decision to implement re-entry heating and sensible aerodynamics, I'm assuming that there is motivation within Squad to ensure that KSP is not too dissimilar in its mechanics to reality. Scale: KSP is, of course, at a much smaller scale than reality, making it much less difficult in general to play. It does, however, introduce a problem. While there is a much greater margin for error on Delta-v, there is not any more time or space to get things done. High rates of acceleration are needed to get from a low sub-orbital trajectory to low orbit regardless of how much extra Delta-v the stage has. Lander engines simply don't have the thrust to propel decently heavy craft to low orbits. Lander Engines: We have three main lander engines in KSP: the LV-909, the "Poodle," and the 48-7S. The 48-7S is a tiny, low efficiency engine capable of powering most Mun landers, ascent stages, or combined lander-ascenders, but is not suitable for other uses in craft of significant mass. The LV-909 and "Poodle" are not only far beyond suitable for almost any Mun landing and ascent, but are also the most efficient chemical engines in KSP at an iSP of 390s despite having comparatively tiny engine bells. They are second only to the LV-N in their usefulness for high energy transfers. In reality, the LEM descent engine had an iSP of only 310, lower even than the old configuration for the Mainsail. It was required to give the lander more Delta-v than is typically used for the entire Munar descent, ascent, and return transfer. The ascent stage engine had the same low iSP and was used for a similarly demanding task. The LV-909 and the "Poodle" have much greater efficiency than is sensible for landing and ascent purposes. Second-Stage Engines: Real orbital launch systems generally use engines specifically designed to be used for the second stage. These engines are characterized by very high specific impulse, low-moderate thrust, and long engine bells. Notable examples include the RL-10 and the J-2, both having iSPs of over 450s. The RL-10 has a mass of just under 280 kg and a thrust of about 110 kilonewtons. The J-2 has a mass of just under 1800 kg and a thrust of over 1 meganewton. The closest things to engines of this kind in KSP are the launch engines: "Skipper" and KR-2L. Both have much higher thrust for their size and lower specific impulse than the LV-909. The use of the latter as a second stage engine is problematic as it produces thrust-to-weight ratios that often destroy payloads with g-forces. In addition, neither engine is suitable for 1.25m craft. This effectively leaves Kerbal Space Program without a major class of engine. Suggestion: New Engines: I think that the addition of dedicated second stage engines would be a major improvement to stock Kerbal Space Program. Basing their performance too closely on real life analogues, however, would break the stock pattern without being better at their jobs than the existing lander engines. Realistic thrust would not be enough to solve the problem of LKO insertion caused by the game's scale. What I am proposing is a set of long, moderately heavy engines with higher thrust and somewhat higher efficiency than the existing lander engines but not enough thrust to be used as launch engines. To me, it seems like a great way of both improving gameplay and making KSP a bit more realistic. Implementation: I have noted potential issues with this idea. There may not be enough of a contrast between the power of the launch engines and the power required for good Kerbal second-stage engines. This may be solved by slightly increasing the power of the launch engines. I always thought that they were not quite powerful enough. Particularly, the LVT-30 & 45, the most powerful engines of the 1.25m size, often seem to have trouble achieving an acceptable thrust-to-weight ratio on relatively small rockets and often cannot lift a second stage and payload enough to achieve orbit without boosters. The 2.5m second-stage engine may be very similar in thrust to the "Skipper", which is just barely powerful enough to use as a launch engine for small 2.5m craft. Adding a 3.75m second-stage engine makes the usefulness of the KR-2L much more limited, so a rebalance of it may be a good idea. Finally, as I mentioned before, the landing engines are significantly overpowered for the job of landing. With new dedicated engines filling the role the lander engines were used for, reducing their iSPs to more reasonable levels for their job would not impede gameplay. Public Release: This is the subject that I am most unsure about. Personally, I won't feel that the game is complete until we have second-stage engines, so I'd like to see them in version 1.0, but I don't know if it would be best to add a whole new class of engine along with everything else that will change in version 1.0. It may be better to know what the community thinks of the major rebalance before deciding whether or not to implement it in the retail release, so I wonder if it would be best to release it as an official mod or something like that to find out if it would work.

You could do it yourself pretty easily by copying modules to the cargo/drop bay .cfg files from a fuel tank's .cfg using notepad. Once you start to try, it's pretty self explanatory. Just remember to keep the LF & O quantities in the right proportions and you shouldn't have a problem with changing how much fuel the parts hold.