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Burt Rutan talks rather passionately about the lack of innovation in space flight, but also mentions how fighter jet (maximum) speed performance has stalled:

In fact, from what little I know regarding fifth generation fighters, not only has the maximum speed stagnated, but it is degrading.

Traits such as stealth prohibit variable intake geometry and variable wing geometry, the maximum speed appears to never be used in reality, it is probably costly and creates other problems (we only have to look at the SR-71 here). Furthermore, other engine characteristics are probably more desirable, such as supercruise and good acceleration in the range where this is critical and actually useful.

However, in this question I'm asking what, approximately (mach 4, 5, 6?) would the maximum speed likely be, if maximum speed was allocated a similar budget and priority akin to that during the cold war era?

The reason I feel this question can be answered at all, is that I imagine there are some very real, practical problems with very high performance aircraft that require extraordinary solutions to overcome. An answer that identifies where these approximately are would thus go a long way towards a satisfactory answer.

Basically, the crux of this question is to identify if there have been any significant aerodynamic, engine, or other performance related advancements that would enable a fighter jet to travel significantly faster than e.g. the F-15 or MiG-25, while still retaining the same functionality as namely the same types of aircraft.

Similarly for a bomber, it is well-known that sustained mach 3 flight can be achieved by looking at e.g. the XB-70 or SR-71. Even with designs from the 1950s. What is realistic today?

Just to be clear: I'm not expecting an exact figure here, but if there are any studies that have looked into this, it would be a very interesting read. If there are no studies, perhaps someone would want to take the challenge and offer a somewhat comprehensive answer as a conjecture anyway. Any math/physics would be great.

I assume that in order to even begin to answer this question, it must be defined what a "fighter jet" is, must it have an air breathing engine, or can it also have a rocket engine, akin to the X-15? Since I'm asking the question, I will also take the liberty to arbitrarily define this here: The engine type is irrelevant, there may be more than one engine, of different types, of the same type. Whatever would make sense in the context of a fighter jet, or even a bomber. Furthermore, it must be a manned aircraft. If it can reach suborbital flight and beyond, like the X-15, it is still an aircraft (again, arbitrarily defined).

An answer that separates the air breathing and rocket engines as two different categories is fine.

This definition of making if fighter jet/bomber is mostly based on wanting to exclude unmanned experimental aircraft, even if hypersonic aircraft such as the X-43 probably serve as a good reference, I feel they have little root in reality, having no life support, no weapon systems, and perhaps no practical purpose whatsoever beyond data recording.

It may also seem like a pointless question, but this might be quite useful in terms of bridging the gap between aircraft and spacecraft, or any other application one might think of -- suborbital passenger jets, etc.

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    $\begingroup$ Unless you specify a range and payload capacity, and even a mission profile, you will never get anything more practical than a research prototype like x43. $\endgroup$ – user3528438 Feb 15 '18 at 23:42
  • $\begingroup$ You speak about maximum speed having "degraded" as though that was something to be lamented. Actually, as in all things, improving performance in one area must be balanced by compromises in others, and maximum speed, above Mach 1, is of vary minimal utility compared with other factors such as maneuverability, payload, stealth, range, endurance, etc.... All a max speed advantage gives you is the ability to run away from another aircraft that can't go as fast as you, and that only if you have enough fuel to use it. $\endgroup$ – Charles Bretana Feb 16 '18 at 0:18
  • $\begingroup$ @user3528438 I did specify it indirectly by giving specific examples such as the F-15 and MiG-25, while these have different payloads they have similar mission profiles, and exact numbers are not important here. I feel the type of answer applicable here is explained well enough in the question itself. $\endgroup$ – AlphaCentauri Feb 16 '18 at 1:17
  • $\begingroup$ @Charles Bretana it is probably a good thing in one context that it has degraded, as you point out, and negative in another. Developments in military aviation have a tendency to provide useful technology for other sectors too, in particular one example would be space related technology, as the companies that build aircraft has in the past been contracted to build space craft. The closer these technologies are, the more space flight would benefit from it. However, this is almost certainly not the only such case where this would be beneficial. $\endgroup$ – AlphaCentauri Feb 16 '18 at 1:20
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    $\begingroup$ We not only don't, we can't know. If speed was a high priority, much research would have been done in these decades, with the results we can't predict; new materials/fuels invented, etc. Even on social level, if it was indeed so important, people would (or might not!) put up with sonic booms, noise etc., more or less happily in different countries, with different workarounds in place... $\endgroup$ – Zeus Feb 16 '18 at 6:14
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Increasing the speed of an aircraft results in having to make trade-offs. Drag will increase as the square of the velocity, meaning a doubling of speed will take 4x the power to achieve. This problem is compounded by the fact that it is very difficult to design an engine (and an airframe) that performs really well at super sonic speeds, while still performing well at subsonic speeds.

All design decisions always end up in trade-offs. Increasing maneuverability generally requires doing things that will increase the drag, which means you will require more power to compensate (which then requires a larger engine, which will reduce your maneuverability due to the increased weight).

All of these decisions mean that, in the end, increasing the speed of a fighter inevitably requires the reduction of something else, either turning speed, or weapon payload, or range. While better technology has given us more powerful engines, and new materials and alloys allow us to create more maneuverable aircraft, we've actually seen the top speeds of aircraft go down compared to past fighter aircraft, even though we could create an aircraft as fast as anything from the past, with increased maneuverability and weapon payload in comparison.

These decisions are being made because by reducing the top speed of aircraft, they in return also get better maneuverability, so it's conscious decision to take the higher weapons payload, increased range, and higher maneuverability in place of a higher top speed because a very fast top speed is just not all that useful in warfare compared to everything else. Reconnaissance (which is what the SR-71 was designed for) is better handled by satellites, which can't be shot down in practice, and don't require flying over another country's airspace in a way that is likely to draw their ire. Bombers would rather carry a larger payload, and be fast enough to get to the target, but getting their significantly faster at the cost of a reduced payload won't buy you much. Same goes for fighters, they need to fly around somewhat quickly, but a pure speed advantage over an opponent isn't all that useful either (compared to just having a better fighter). So it's a combination of both physics and specifically what things the military values that just means that there's little value to purely increasing speed.

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    $\begingroup$ Thanks for the answer, but this mostly states the reasons why increased maximum speed is not a priority, I've already stated various reasons for this in the question itself, and it is not what the question is regarding. The fact that drag increases as a square of the velocity is true, however, the air density is also a factor there, and the aircraft can simply increase its service ceiling to mitigate this issue -- this is after all how spacecraft manage to reach orbital velocity, something which would not be possible at ground level. $\endgroup$ – AlphaCentauri Feb 16 '18 at 0:06
  • $\begingroup$ Increasing the service sealing only works when you carry your own oxidizer, at some point there isn't enough air to sustain lift and/or thrust in the engine. So you have to make trade offs, because we don't have aircraft that have optimum performance at all altitudes and speeds. You can make a plane that goes very fast, at the cost of something else. In the end, you're constrained by the military mission you specified above, which has to happen. A fighter jet isn't a fighter jet if it can't fight. $\endgroup$ – Tom K Jun 8 '18 at 1:48
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If we take the pinnacle of fast conventional aircraft development - the SR71 and XB70, we find that Mach 3 is about the practical limit for sustained flight, given current metallurgy. Above that, temperature becomes the limiting factor, as in friction temperature, with the aircraft skin heating up to 600-800 degrees F at a sustained Mach 3. We have yet to develop a material that can sustain higher temperatures while maintaining both the light weight and strength of titanium (SR71) and honeycomb stainless steel (XB70). A failure of the cockpit air conditioning system also means either slowing down considerably, or a well cooked aircrew, very quickly.

The air launched X15 sustained speeds in excess of Mach 6, but only for very short time periods, a few minutes... fuel was the limiting factor there. The X15 couldn't sustain Mach 6 long enough for temperature to exceed it's design, while the SR71 and XB70 could sustain Mach 3 for over an hour. Had the X15 been able to sustain Mach 6, it would have encountered very serious heat problems, instead of the somewhat serious heat problems it experienced in its short dashes to high speed.

So, with current tech, Mach 3 is about the practical limit for sustained flight. Beyond that, the problems increase exponentially. And that's not even getting into the expense... remember that the SR71 required a special, high flash point JP7, plus special oil, plus a lot more. It was retired largely due to the expense of operation.

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  • $\begingroup$ What stops a rocket powered aircraft/spacecraft from simply approaching suborbital (or higher) altitudes to avoid the problem with heat? $\endgroup$ – AlphaCentauri Feb 22 '18 at 14:47
  • $\begingroup$ As you decrease the air density, then the type of engine needed to power the aircraft becomes different. A jet turbine engine depends on being able to ingest air (and therefore not requiring an oxidizer tank) to function. This same air that a jet engine uses to generate thrust is what causes the problems with heat. For the air to combust properly in a standard turbine, it needs to be slowed down sub-mach 1 speeds. As you get up at about Mach 2.5+, ram air engines start becoming more efficient. $\endgroup$ – Tom K Jun 8 '18 at 1:44

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