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A comment to this question suggested, followed by a smiley, that dropping it [a ramjet engine] from orbit might start it. This got me thinking if this really would be possible.

This Skybrary article suggests that the minimum theoretical starting speed for a ramjet engine is as low as about 100 knots, but the same article, and Wikipedia and many other sources state no significant thrust is produced below mach 0.5 (or even higher).

A streamlined object such as a bullet in freefall will reach a terminal velocity of about 90m/s or 175kts, which would be within the suggested theoretical starting speed.

But this is where my math skills fail me: In order to start, the ramjet will need to be able to force air through itself, this will obviously create resistance. Therefore a ramjet might not be described as a streamlined object, and it might not be able to achieve the speed necessary to start and produce enough thrust to further accelerate to an operational speed range.

Further complicating things migh/will be the altitude from which the ramjet migh be dropped. We know that re-entry from orbit will yield vey high velocities, but these are achieved in very thin atmosphere, and I have no idea how this plays with the operation of a ramjet...

So: is it possible to start a ramjet by dropping it from an altitude, and what might the order of magnitude of this altitude be?

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    $\begingroup$ A bullet is small. Now scale it up to the size of a truck and see it fall at supersonic speed. Scaling laws are your friend here. $\endgroup$ Feb 8 at 18:00
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    $\begingroup$ My math fails me on so many levels here 😃 $\endgroup$
    – Jpe61
    Feb 8 at 18:48
  • $\begingroup$ At operating speed, what is the thrust to weight ratio of the vehicle in question? If it is less than 1, then the vehicle can be started by being dropped, no questions asked. If it is greater than one, then math starts to be involved. $\endgroup$ Feb 9 at 0:01
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    $\begingroup$ This is your captain speaking. Our engines have stalled, so we're going to go into a rapid descent in order to try to restart them. Please return to your seat and fasten your seat belts. $\endgroup$ Feb 9 at 6:09

2 Answers 2

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A well streamlined body with high enough sectional density can easily fall fast enough to reach ignition speed for a suitable ramjet design (some of the heavier gravity bombs of the Second World War had terminal velocity close to Mach 1, and they weren't optimized to minimize transonic drag).

That said, a very heavy object won't do much flying even once you have thrust and a transonic or faster speed. The trick here, then, is to drop from high enough. From sufficient altitude, a falling human in a space suit can exceed the local speed of sound; a streamlined metal object (say, ramjet with fuel tank integrated into duct and diffuser, and some fins for steering, plus batteries and electrics for guidance) should easily be able to carry that kind of speed into thick enough air to support combustion in the ramjet.

As for "how high?" we can reasonably presume that you'll need to be high enough that terminal velocity of a relatively low density object (i.e. one light enough to actually fly once your engine is lit) can get fast enough in thick enough air for combustion. For the latter, you need to be below about 25 km, so I'd guess you need to be above around 30 km ASL to gain enough speed before getting that far down. Getting such a device up to an altitude of multiple tens of kilometers is left as an exercise...

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  • $\begingroup$ There most certainly would not be any sense in this exercise 😂 $\endgroup$
    – Jpe61
    Feb 8 at 17:58
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    $\begingroup$ There most certainly would not be any sense in this exercise Science fiction authors will find some sense in it. :-) $\endgroup$ Feb 9 at 11:41
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    $\begingroup$ In fact, you'd launch something like this the same way you launch a Baumgartner: from a stratoballoon. All you need is higher sectional density and/or lower drag than a human in a pressure suit (both very easy to do), and you can light your ramjet at Mach 1+ and still be around 25 km. $\endgroup$
    – Zeiss Ikon
    Feb 9 at 12:07
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    $\begingroup$ "... is left as an exercise..." often invokes a wry smile. 😉 $\endgroup$ Feb 9 at 21:35
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The inlet of the engine is indeed a source of drag, but it can be covered by some aerodynamic louver until the aircraft picks enough speed. B-36 had the deployable louvers over jet engines to reduce the drag when not running, while these were not ramjets.

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    $\begingroup$ Yes, that helps, but isn't strictly necessary. The ramjet-carrying object must be large and the altitude must be high, then starting the ramjet is straightforward. $\endgroup$ Feb 8 at 18:02

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