If someone were strapped down underneath a Harrierto keep from blowing away, and it then did its VTO, would the thrust from the ‘vectored jets’ (is that right?) be enough to crush the person flat? If you think about it, the amount of thrust would, considering it has to push down enough to lift that piece of hardware upwards.
The thrust would not crush you. The full weight of the airplane would not be exerted as pressure on your body. The pressure is distributed over a pretty wide area but I couldn’t do much more than make a WAG but I doubt if it would be more than a few PSI.
You would however be burned very severely in short order. I don’t know if this has happened with a Harrier but I personally know of at least one incident with an A7 on the USS Constellation. A guy new to the flight deck panicked and tried to hide near the mast at the island. A plane was turned so the exhaust was pointed at him and he was quickly burned to death.
Would the pressure be enough to even make my ribcage compress a little bit? I’m not sure what kind of damage a couple psi can do . . . .if any.
I think you may be confusing thrust and pressure. The Harrier lifts off because the thrust of the engines overcomes the force of gravity, not because of any sort of downward “pressure”.
Think of it this way, how hard would the wind have to blow to crush you?
Well, people have survive hurricane-force winds, so I’d say being crushed is unlikely. Being suffocated though, is a serious threat, since a steady firm pressure on your ribs can keep your lungs from expanding.
The harrier, though, can only hover for a short time (at most ten minutes under ideal conditions), so even you stopped breathing while it was over you, prompt application of mouth-to-mouth or a ventilator might bring you back, albeit short a few brain cells.
If the marines wanted to kill you, there are easier ways.
With regard to landing remaining ordnance might be a factor in being squished, but WRT takeoff, I’d be worried about being cooked.
And the Harrier’s native environment is underwater. 
I’m gonna say, “maybe”. The Harrier engine generates somewhere around 21000 – 23,800 lbs of thrust, depending on which engine/model you’re talking about. That thrust is split between four thrust vents. I don’t know if the Harrier is at full throttle when it’s hovering, but probably close to it, given that its VTOL take-off weight is 18,000 lbs. So you’re looking at an in-your-face 4000 – 6000 lbs of thrust. Now even if that thrust were fairly sharply focused so it just covered 1 square foot (144 sq inches), that’s only a pressure of 27 lbs sq inch. Which doesn’t sound like a lot, but you can’t really get around the fact that if someone dropped 6000 lbs on your torso, you wouldn’t survive the event.
So we need to know how sharply focused the thrust is and how far from the thrust nozzle you are.
But the short answer is, the same thing would happen to you if you stuck your head in a Harrier thrust jet as would happen if you stuck your head behind just about any jet engine.
Thinking about it just a bit more – I haven’t found any information on how fast the exhaust is moving. But to generate that kind of thrust, we’re talking a lot of air, moving very fast. Probably much more than hurricane strength – maybe 400-500 mph? My guess is that after being flayed and burned alive, you’d be grateful for the eventual crushing.
You’d be killed quickly.
I used to fly both fighters & airliners. At takeoff power, the exhaust from a (non-afterburning) jet engine of that size is traveling 400+ mph at a temp of 400+ degrees F.
The cross-section of a Harrier’s exhaust nozzle is roughly 2 feet in diameter.
So here we have a source of F5+ tornado windspeed at temps high enough to sear meat instantly, and of a cross section about the size of your torso & you’re going to position yourself within 3 or 4 feet of the outlet.
Don’t worry, it won’t hurt for long.
I once saw an 737 blow a full-sized pickup truck end over end across the ramp. A single 737 engine has about the same thrust as a Harrier, and the 737 wasn’t at full power when he did it.
When taxiing a big jet, concern for what you’re blasting behind you is actually a bigger issue than avoiding hitting anything. With a flick of the wrist you can kill people and spend a LOT of your employer’s money in just a few seconds.
Said another way, a Harrier at takeoff power is burning roughly 1/2 gallon of kerosene per second. That’s a pretty bogacious fire. And you want to be just a couple of feet away from that fire, on the output side to boot? Not a good plan.
p.s.
Afterburning engines raise all the numbers by roughly an order of magnitude.
Did you see this in person? I only ask because I vaugly remember seeing something on TV about this, on mythbusters or something. Unfortunately I cannot remember what the conclusion was. I do remember that it was a truck driving on a road near the 747, and drove up behind it just as the jet was preparing the engines for takeoff.
Ahh, found it, I think I did see it on mythbusters. It was on episode 13, they got jet engine to blow a taxicab 75 feet into the air. I remember enjoying that episode. 
Quoth daffyduck:
Huh? How do you propose having a force spread over an area without having a pressure?
Hm, I just watched that episode on TV the other month and they were unable to get the taxi to move much at all (certainly not 75 feet), although they were using smallish jet engines. They certainly made a mess out of the cab though!
They did, however, track down an actual case of this happening (although there is no video of the event) - a taxi in Brazil (IIRC) drove behind a jet at an airport and the cab was blown some distance. There were photos of the taxi being recovered and it was totalled. I think a lot of the structural damage came from the resulting crash, not the jet exhaust.
Yeah, now that you mention that, I remember it, and cannot remember the outcome of their tests. I might have missed it. Something on another channel must have caught my eye. I just got the description off the mythbuster web site. Sorry.