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Our Heroes are a small group of people around the level of the heroes of The Skylark Of Space. Operating on a budget of maybe ten million dollars, they've built a spacecraft with a mass somewhere around 75 metric tons, which can take off from an ordinary runway using jet engines and then, once out of the atmosphere throttle, the specific impulse up to 8000 seconds or beyond.

  • Mass: 75 tons

  • Mass Ratio: 3

  • Propellant: Water or Liquid Methane

  • Crew: 2, plus up to 2 passengers, plus 250 kg cargo

  • Powerplant: Proton-Proton Fusion, 5 GW output electron beam

  • Engines: 2x Thermal Turbojet/Rocket, 1x high-Isp-low-thrust plasma

The problem? They want to be able to fly this wonder around, land at various airstrips, not get chased by the Air Force in the air or the FAA on the ground or the news media in general, have access to spare parts and facilities, and not be forced to give up the secret of tiny lightweight high-power proton-proton fusion before they've built up a substantial space infrastructure to flee to.

Their cunning plan? Claim that they just legitimately developed a high-performance jet aircraft with a vastly less dramatic description (but fitting its appearance and atmospheric flight performance), then register their spaceship as an instance of that aircraft.

The question: What barriers are they going to face to this? "This is air traffic control, did you just accelerate to FIFTEEN TIMES your Vne?!" is obvious, unless they can drop off the radar somehow. And I imagine that somebody might want to do too close of an inspection at some point. How long are they going to be able to keep up the game, and how is it going to fall apart? What about third-world countries?

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    $\begingroup$ Standardization and inspection groups exist for the approval of jets...this isn't opinion based. $\endgroup$ – James Mar 16 '17 at 13:50
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    $\begingroup$ What threat model are we up against? Are we trying to avoid accidental discovery, or are we trying to avoid a focused military effort to find our plane? $\endgroup$ – Cort Ammon Mar 17 '17 at 1:50
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    $\begingroup$ Why to disguise at all? First time you go into space, everyone knows that. No surprise after that. Wouldn't it be cheaper to have a plane and spacecraft as separate items? And just keep spacecraft hidden, ready to launch? $\endgroup$ – Mołot Mar 17 '17 at 7:24
  • $\begingroup$ Cort: Nobody knows what they are doing in the beginning, nd then they are trying to keep from being discovered, even if people become suspicious (which they surely will eventually.). Molot: is it necessarily impossible to disguise orbital ascent and descent, at least to the point that nobody connects it with the plane? They want to be able to go back to Earth. $\endgroup$ – ikrase Mar 17 '17 at 19:21
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    $\begingroup$ Is there a reason they don't simply use obscurity as a cover, instead of secrecy? If you are sticking to not-busy airfields (vs. busy ones) and work out all the airworthiness & spaceworthiness issues correctly (shouldn't be hard for a well-designed SSTO), I don't think the FAA would pester them too hard about giving up their actual secrets... $\endgroup$ – Shalvenay Mar 18 '17 at 2:33
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Your main problem would be during an FAA Airworthiness Inspection that must be issued before your plane is approved for a flight certificate.

I'm guessing that most of your space-flight modifications can't easily be disguised as your normal Boeing equipment.

Without FAA approval, it's very unlikely you'll get approval to fly or land anywhere. You'll get a visit from the local air force pretty quickly after you take off, and the military police as soon as you land.

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  • $\begingroup$ How much deception on the part of Our Heroes does that account for, and also how much Experimental Aircraft exception rules? They never "hide" the "fact" that they developed their own high-performance jet aircraft, possibly with peculiar but permissible design features. $\endgroup$ – ikrase Mar 16 '17 at 12:09
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    $\begingroup$ @ikrase deception at the level of engines and fuel. If you have second set of engines, ones performing better in vacuum, and tanks of oxidizer, it'll be known. For experimental, these generally don't happen over populated areas. So it's kinda worse. $\endgroup$ – Mołot Mar 17 '17 at 7:22
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Plane spotters

Their biggest problem — that will hit them instantly — are all the plane spotters, that will instantly realize that this is a very exotic plane, and post out requests on the Internet asking "What is this aircraft type?!".

And even if they make their plane a copy of an existing model, these annoying nerds cunning enthusiasts will take note of the registration number. If your heroes use an unknown registration number, this will be noticed. If they use a known number, they are likely to run into the case where the "same" plane is spotted in two different locations at the same time, which will be noticed.

Solution

Your heroes need to buy an existing plane, hide that, and then make their space plane look like and identical copy of that plane.

As for operations, there are vast areas that are not controlled by Air Traffic Control, like out over the big oceans. So over land they would need to mimic a normal aircraft and fly it like the aircraft of that type. One obstacle here is that they need to be as knowledgeable as licensed pilots. But with enough hours on public flight simulator servers and listening to enough ATC chatter, they can probably fake it, especially if they keep to small regional airports.

Then they just need to make it out over open water, turn off their transponders, and go to space.

...stealthily. They do not want to trigger a nuclear attack alert.

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Radar invisibility that can be turned on and off

You've raised a good point: our airspace is currently monitored permanently by radar.

Any unplanned disappearance below or above radar level, is very likely to be detected and will raise serious questions.

The only solution to that is to be invisible to radar, but that also means you can never officially land on any airstrip that is managed by air-traffic control, because that would draw attention and questions.

Your question says they want to be able to land at various airstrips, so they must be able to be visible on radar when they want, and invisible when they don't want.

You must of course be extremely careful when going from one mode to another. From visible to invisible, you need to give air-traffic control a good reason why you're going below (landing somewhere?). The other way, again, you need to be below the radar when switching.

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There are actually two separate issues here (which were noted by other posters as well), detection in flight and detection on the ground.

Detection in flight is not only by radar, although the vast majority of the worlds ATC systems use conventional radars. Various military systems use radars in different bands, and some may actually use multi spectral radars which can "chirp" at several different frequencies at once (postulated as a means to defeat "Stealth" aircraft.

This vehicle also uses a nuclear fusion reaction which will emit a massive amount of heat energy, and you have stated the output is a relativistic electron beam (used to heat the reaction mass), so there will be some pretty impressive electromagnetic signatures from the engine as well. Modern fighter aircraft have targeting pods which passively seek out emissions, so this is another way the craft will be spotted and identified as being very much out of the ordinary.

Once on the ground, the issue is not only nosy "plane spotters", but various customs agencies, not to mention aviation officials and quite possibly intelligence agencies (both national forces and industrial spies) will be looking for reasons to get close to the aircraft.

Given the parameters, perhaps the only way to really disguise the fact you have such a vehicle is to go the route of the classic 1960's TV show "Thunderbirds". The high performance vehicles were hidden in a secret base and launched only when necessary. The heroes lived in a much less intrusive society (no cell phones and cell phone cameras), so the issue of landing and doing things was less difficult. Today, the craft might have to only be launched at night, do "point to point" flights from secret base to secret base, and otherwise avoid well travelled air routes or obvious military choke points (like the Greenland-Iceland-UK gap which is patrolled by the RAF).

enter image description here

Nobody's going to notice....

Covering the craft with metamaterials might limit the visibility on particular radar frequencies, but the craft has other signatures (including sonic booms if you are very careless) which metamaterials don't guard against.

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  • $\begingroup$ Lots of ATC radar is "secondary radar", so basically a transponder interrogator. For those, just turn off the transponder, and the aircraft disappears from ATC radar screens; they might get a "blip" on the screen, but it will be unidentified. Military radar is much more likely to be "primary radar", relying primarily or only on the signal reflection by the aircraft. $\endgroup$ – a CVn Oct 13 '17 at 19:29
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Biggest problem might be building more planes.

So, if they scrounge up some funding, and start building one or two actual planes of that description - maybe with some of the tech they used to make the spaceship for genuinely better designs, or maybe just regular technology gussied up - they can alter them as much as possible to resemble the spaceship-as-a-plane. The more they can make the interior resemble their spaceship, the better.

And once they have a regular-ish plane or two, they can file themselves as a small business flying cargo or passengers or whatever, with a plane that can, for example, pass FAA inspection, won't need shenanigans around radar detection, doesn't matter if people see it or ask questions about the model, and basically have nothing to hide. And that has flight plans and cargo runs and passengers that do, in fact, exactly what they say they're going to do.

The spaceship, therefore, is a hidden "extra" plane. Depending on how closely it matches the real planes, it can be occasionally swapped out for one of the real planes (swapping all attendant paperwork), set up as a real plane by certifying one of the real planes a second time with a new registration and re-purposing the old papers, kept in storage or on display to be touted as a prototype or a backup (that sometimes goes missing when actually needed), possibly modified to pass FAA inspection once they have firsthand knowledge of what they're looking for, or simply hidden away until it's ready to be used... and if used when a real plane is in the area, any sightings will be attributed to that plane, see?

And, as a bonus, they could perhaps pick up enough real work with the planes they build to make the business pay for itself, or even give them a bit of profit to plow into other parts of the greater plan.

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So you have a spaceship. I'm pretty sure you built it to actually go to space, eventually.

Assuming you came up with a lot of clever tricks to avoid anyone noticing (the other answers already cover how difficult that will be), you have one obstacle you won't be able to surpass.

Even if you managed to reach space unnoticed (which you won't. Assuming your launch was in the US, then the russians will be all over the "red telephone" before you left the atmosphere), how are you going to hide your re-entry?

Someone will deinitely notice that. While it might first be mistaken for a comet or something, the fact that it doen't go splash where it should will raise questions. Long before you are near enough to ground level to attempt a landing the place will be swarming with fighter jets curious to see what kind of object is invading their airspace.

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  • $\begingroup$ Yeah, that sounds problematic. This ship can, of course, easily outrun present day fighter jets, but maybe only at altitude: do you think it's going to be impossible to get far enough away from the predicted impact point (including radar!) to hide? $\endgroup$ – ikrase Mar 17 '17 at 19:40
  • $\begingroup$ You cannot hide from radar. It is possible to "fly under the radar" with ground-based radar. But AWACS and the radar in fighter jets looks from above. you would need to be covered from above, like in a tunnel. So, no, you can't. At least not twice. If you manage to evade the fighters once, you're narrowing down the region where they need to search. And i assume you want your ship to be re-usable. $\endgroup$ – Burki Mar 20 '17 at 8:00
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1) Your craft as configured will not reach space in the first place. Your good engine can only be used once you are pretty close to orbit.

2) You are going at it the wrong way anyway. A quick perusal of the rules shows that you can fly an aircraft you designed and built but the certification process will require a lot of disclosure that your people do not want to make. Besides, that's a needless cost and safety issue.

Given the specs you provide they shouldn't build an aircraft at all. Rather, they should buy an ordinary airplane that can be modified to do the job. Provide some means to fold the space engines into the airframe so it looks normal. My understanding of the rules is that so long as the plane still flies by the book (a type-rated pilot unaware of your mods could fly it normally) it doesn't matter what cargo you carry--even if said cargo is a space drive.

Now, actually using that space drive will bring down the wrath of the FAA and a few other agencies because there are treaties about space launches. However, you can hide from them in this case. Fly out over the ocean under VFR rules until you're beyond radar range. Now climb to space. I would advise a pure black airplane with the best in stealth you can devise as you don't want to be detected by the orbit-watching systems. Go into deep space as soon as possible, don't linger near the Earth where you'll be detected in time. (One detection isn't going to be a big deal. Repeated detections will eventually lead to the realization that there's someone operating spacecraft with technology far beyond what we have as you won't be leaving behind the launch plumes that mark every other rocket.)

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  • $\begingroup$ I explicitly said that it has turbojet-rocket combo engines. So it can reach orbit, and a good deal besides. The bigger question is whether any aircraft available to civillians is capable of hypersonic flight and atmospheric reentry, even with modifications like "magical heat-resistant paint coating" and that has cargo doors mechanically/thermally capable of surviving being opened at hypersonic speed. I know that there are some 1970s and later jet fighters in private hands but even that is pushing it. $\endgroup$ – ikrase Mar 18 '17 at 1:28
  • $\begingroup$ @ikrase Missed the rocket combo, it started out saying it flew out of the atmosphere on jets--something that's not going to happen. It doesn't fix the problem, though--your rockets won't have enough fuel. Besides, it almost certainly makes more sense to have separate rockets and turbojets, the design is too different. $\endgroup$ – Loren Pechtel Mar 18 '17 at 3:59
  • $\begingroup$ @ikrase With the ISP you give the lack of a supersonic airframe doesn't matter--you simply fly subsonic until you run out of lift then you make up for it with your rockets. Likewise, you use your engines to slow down rather than going through the fire. The problem is your high ISP engine doesn't work until you're in orbit and your design can't possibly reach orbit. $\endgroup$ – Loren Pechtel Mar 18 '17 at 4:01
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So the world's first space plane, the X-15 "Achieved" sub-orbital flight in 1963. I say achieved because it flew at an altitude that would merit USAF Astronaut wings (80 km) but FAI metrics for space flight records are higher (100 km).

The X-15 is also the uncontested speed demon of the skies, with a top speed of Mach 6. This is 3 times the max speed of the F-22 and about twice the speed of the SR-71 (officially Mach 3.2 but we'll expand upon this).

The trouble you will face with this design is that it could never become space worthy without assistance. Space missions were achieved by a "mother-ship" launch, or strapping it to another plane to achieve altitude and then release the X-15 from there to continue the ascent (Superman Returns has an intended idea with the falling plane sequence). It simply couldn't carry the fuel needed to put itself into suborbital flight.

The SR-71 is an unusual plane that might meet your specs (except size... it came in 1 and 2 man variants). Optimal performance of it was achieved at Mach 3.2, though just how fast it could go was never really discussed. Officially, Wikipedia lists its top speed at Mach 3.3 but during test flights it was observed at speeds of Mach 3.4 and in one combat mission it was reported at speeds of Mach 3.5 (notably, higher speeds seemed to reduce fuel consumption as the pilot that achieved this speed returned with way more fuel than he should have had). While not stealth as we think of it, it had a low radar cross section which was enough to shake radar lock from SAMs by slightly changes in speed, altitude, or heading. It could fly at just shy of a 26 km ceiling (F-22 has a 20 km ceiling by comparison), and with the combo of speed and altitude, it was impossible for intercept craft to catch up with it. The only two lost in active service were bot a result of mechanical issues, not enemy actions. This plane was famously used as the model of the famed X-Jet from X-Men, and although it's fictional cousin is much bigger, the boast "If they have anything that can pick up our jet, they deserve to catch us" is pretty accurate of the SR-71, even after it's retirement two decades ago.

As a final point, the speed of the Shuttle at Launch in Atmoshpere is about Mach 2.6, though orbital speeds are about Mach 23-25 (though most note this as if it was in atmosphere... not being a physicist, I wouldn't know why the distinction is such that it is.)

As noted elsewhere, since all of these craft are designed to be the fastest thing in the sky, they have little practicality as much else... SR-71 could be sized up to include an additional crew and cargo with only minimal strain on the fabric of reality (again, "Physics works" is the most I know on that subject). You'd have a lot of fuel issues and the real SR-71 had a ton of maintenance quirks that amounted to a week of downtime following a single flight to get it back up to flight standards for next mission. It also never got to any height remotely space worthy, but as far as the best military craft are concerned, it might as well be in outer space. It could go from NYC to LA in under 2 hours. With those numbers, outer space isn't required.

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