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I've been building a lot of SSTOs lately. I'm fairly far into a career-mode game and it was annoying me that I didn't have a cheap way to get Kerbals into orbit and back down again, but while I have a good portion of the tech tree unlocked it wasn't complete yet and I didn't have access to the R.A.P.I.E.R. engines. Not to be deterred, I built the Grasshopper, a spaceplane based around a pair of Whiplash engines for atmospheric flight and a Toroidal Aerospike for space. The thing turned out fairly well, able to easily transport 4 Kerbals up to 250km orbits (which is where I keep my Kerbin space station) and back down again and, when pushed to its limits, can get up to a 500km circular orbit (although once up there I had trouble getting back down again, needing to use my RCS thrusters for that last bit of delta-V that I needed to get back into the atmosphere.) Today, after completing a successful mission with the Grasshopper (several, actually; I had four different Kerbals from four different rescue contracts on my station and brought them all down at once) I took a look at my new contracts and discovered that I'd been offered one to test the R.A.P.I.E.R. engine, effectively unlocking it for the duration of the contract without needing to invest the science points to get it normally. I immediately upgraded the Grasshopper with the new technology, replacing its Whiplash engines with R.A.P.I.E.R.s, ditching the Aerospike (putting a second shielded docking port in its place) and rebalancing my fuel tanks to better suit the new configuration. The new craft turned out to be slightly lighter than the old one (but also more expensive) and I immediately took it to the runway, got it into orbit and discovered that it performed... well... pretty much exactly as well as the old one. It could get to a 500km circular orbit but didn't have enough oxidizer to come back down again. The new design did have some advantages, with that second docking port meaning that I could attach a NERV tug to the back for long-range journeys and still have it dock with a station (where before I could only have it docked with one thing at a time) and I managed to fit a science container in where I used to have a small LiquidFuel tank, but it was also substantially harder to fly due to the R.A.P.I.E.R.'s poor low-speed performance and, as I said, more expensive. That got me thinking about the various SSTO designs I'd seen for this game. Nearly all of them are R.A.P.I.E.R.-based (with the occasional NERV engine for when you absolutely positively have to take a pair of wings and a set of air-breathing engines with you to Eeloo,) to the point where I don't think that a lot of builders are even considering the possibility of alternate designs. The R.A.P.I.E.R. is certainly convenient, being a jet engine and a rocket in a single package, but it has significant disadvantages in both modes, with its odd thrust curve requiring an exacting (and often tediously long) ascent profile in atmosphere and just being generally inefficient in vacuum. On the other hand, the Whiplash (and even the Panther) are still able to get a craft into the upper atmosphere with a significant fraction of its orbital velocity, are cheaper, get unlocked earlier and offer some significant advantages for in-atmosphere flying (with the Whiplash's smoother performance curve and the Panther's dry mode offering a very long cruise-time for when your re-entry falls wide of the KSC.) So what do you think? Is it time to re-examine the R.A.P.I.E.R.'s status as the be-all and end-all of SSTO engines?