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When testing this mod with Ferram, I noticed that my wings break off very quickly when turning. A closer look at the Spaceplane Plus wings showed that the Centre of Lift seems to be at the wing tip, as shown in the picture below. This applies to all Spaceplane Plus wings in Ferram. The default parts with Ferram as well as the Spaceplane Plus parts without Ferram seem to work well. The position of the centre of lift seems to be the cause of wings breaking off early, since an equal lift force results in a much higher moment. For example, when applying a force at the wing tip of the Delta Wing, the wing usually breaks off between 1000 and 1200kN, while the wing can sustain 2500kN of Force at the centre of the wing. These parts are amazing, so I hope this will be fixed soon. p

Simplified, an aircraft flying horizontally in KSP can accelerate until the engine thrust equals the drag. The Basic Jet has a higher thrust than the Turbo Jet below ~230m/s as seen on my 'Engine thrust relative to speed' diagram. An aircraft is usually slower at lower altitudes due to the higher air density resulting in a higher drag. So I assume your aircraft is faster with Basic Jets because it is slower than 230m/s below 10km.

I've had the most success with my hybrid first stage design. It uses Basic Jet Engines to get to an altitude of around 20-25km and then uses rocket engines to climb to around 100km. The second stage is disconnected once above 70km and has enough time to get into orbit before the first stage gets below 23km and despawns (tested). The first stage then uses parachutes and the remaining jet fuel to make a soft landing in water. Basic Jet Engines work better then Turbo Jets at the low speeds and altitude needed for this stage. Each Basic Jet can lift around 10-13 tonnes of rocket. A detailed explanation why can be found here [Forum Link] A simple version can be built once the Aerodynamics node is unlocked in the Technology Tree but the principle can also be used to build much larger and more complex rockets. In this example, the first stage of this rocket costs 27054.4√ and 22885.31√ or around 85% can be recovered. It would also be possible to make it fully re-usable by flying the first stage back to the space centre, refueling it with docking ports or the Kerbal Attachment System and using a crane to attach the next payload to the top.

Sure einsteiner, I put it on my Dropbox to download, created in Microsoft Excel 2013. [Link]

Pecan is correct, a plane using Basic Jets is pretty limited above 250m/s and 10km altitude. However, they are very useful when used as first stage engines for a ballistic rockets. A single Basic Jet can accelerate a 13 tonne rocket to around 250m/s vertical speed and, when using 2 radial air intakes, an altitude of almost 30km before stalling. In my test, it only used around 25 units of fuel to do so, which makes it extreme efficient compared to normal rockets. A second stage using conventional rockets should then be able to bring the payload efficiently into orbit. Compared to that, a Turbo Jet can only lift around 11 tonnes of rocket and uses a lot more fuel at low altitudes. While it would be theoretically able to accelerate to a higher speed due to the increasing thrust at higher speeds, vertically ascending rockets usually reach the stalling altitude before this comes into effect. Finding added to the original post

I've replaced the "Indicated ISP relative to altitude" part with a new "Fuel flow relative to altitude" graph, since there seems to be no direct connection between the indicated ISP and fuel flow. @Justin Kerbice, Thank you for the information. I assume those are the values KSP uses to create the shape of those graphs above. It would be interesting to know what the exact formulas are though. This would enable players to calculate, at what speed an altitude they need to fly their SSTO in jet mode without the engine stalling.

The in-game information as well as the current KSP wiki article on jet engines give some indication on the performance of jet engines in KSP. However they are usually not enough when building optimized SSTO's. This is why I started analysing the vanilla engine components. So far, I've made the following discoveries: [Graphs in Imgur album] Findings on thrust and speed: The maximum thrust of an engine is always relative to the current air speed. The Basic Jet works well at low speeds but is unsuitable for fast aircrafts, even if they fly at high altitude with minimum drag. Due to it's high thrust at low speeds, the basic jet can be used as a first stage engine for a ballistic rocket. Both the Turbo Jet as well as the RAPIER engine in jet mode peak at surface speeds of 1000m/s and work well beyond that. (Note: I've cut off the graph at 1500m/s, however, the curve would continue beyond that. The turbo jet will reach 0kN 2400m/s, while the rapier reaches 0kN at 2200m/s. Findings on thrust and time: All jet engines have long start up times, which makes them difficult to use in VTOLs, since they need a short response time. According to my analysis, the Turbo Jet is able to increase it's thrust slightly faster then the Basic Jet despite a lower peak. (Measurements taken at fixed ground location) Findings on fuel flow and altitude: Jet engines are very efficient compared to rocket engines. The fuel flow is only relative to altitude. That means that a fully spooled up Turbo Jet at maximum throttle at a certain altitude always uses the same amout of fuel per second, no matter how much thrust it produces. For those flying on Laythe: The graph below was made using data from Kerbin. I assume the ISP is actually calculated relative to air pressure instead of altitude above sea level. However, I haven't tested this so far. [For comparison] Compared to the other jet engines, the basic jet is very efficient when flying below 5000m. Above that, Turbo Jet and RAPIER engines are usually much more efficient since they're able to fly much faster due to the reduced drag at higher altitudes. The indicated ISP is also relative to altitude as seen here. However, there seems to be no direct connection between fuel flow, actual ISP and indicated ISP. Findings on intake air flow, speed and air pressure:The intake air flow is relative to air pressure an speed. The graphs above show the intake air flow relative to speed at Kerbin sea level. (= 1 atm) The approximate formula of these is are as follow. They shouln't deviate more then 0.5% from the actual intake air flow : (Q = intake air flow, v = speed) - Ram Air Intake: Q = 0.096 * v + 58.7 - Circular Intake: Q = 0.078 * v + 47.0 - Engine Nacelle: Q = 0.049 * v + 29.3 - XM-G50 Radial Air Intake: 0.039 * v +23.5 Due to the angle of attack, the movement speed will usually differ from the speed at the air intakes. The true intake speed can be viewed by right clicking the air intake and subtracting 100 from the indicated Air Speed. These values can be multiplied by the current air pressure measured by the PresMat Barometer to get the intake air flow at higher altitudes. (Stock part, [wiki page]) I hope this information will help some of you to better understand the different jet engines in KSP and to fly SSTOs at even higher speeds before switching to rocket power.