Boeing XB-47 Stratojet prototype 46-065 takes off with the help of eighteen RATO bottles fitted to the fuselage

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Ill-Dependent2976@reddit

For those wondering, this has to do with thrust to weight ratios of late war propeller-driven airplanes vs. early jet engines.

Consider a propellor. It has all its power when the air is still. It spins and it can push right off of it. The faster its moving, the faster the air is, the more it has to spin and the hard it is to push. You've got a big fat late war prop engine on a fighter, you just have to push the throttle, and just a few feet later you can left right off a flight deck. You can see on youtube take off clips of airshow stunt planes or Alaskan bush planes just taking right off the ground with practically no run up. Some of the funny ones involve people pushing planes off cliffs and the planes just go flying away.

Now thing of a jet engine. Especially that big fat intake up front. All the power fo a jet engine is when it's already flying at high speed. That air resistance plowing into the airtake just helps the compression. The earlier generations had these incredible long take offs. It also led to a lot of crashes. PIlots who were experienced with propellor planes were expecting to have lots of thrust at low speeds, and when they didn't they had issues with stalling while maneuvering.

This turned out to be a huge issue during the early/mid cold war. During WWII, there were airfields just anywhere that people could carve out of a forest or coral island and they didn't need to be long, because prop planes could just take off from a sheep pasture. Jet planes didn't have that advantage. So there was this huge push for R&D into JATO like this, helicopters, short and vertical take-offs, even things like British Harriers were a result of the problem. One of the Scandinavian countries, I think it was Sweden, had this weird plan to hide their planes in forests next to roads, and just use the roads for runways if war ever broke out.

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lavardera@reddit

I wonder why they made this shift to bubble canopies on bombers early in the jet age, and away from cabin cockpits. The B-52 prototype had a canopy as well, but in production went back to cabin. Was it long flights, and the need to use an onboard head?

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backcountry57@reddit

It was initially thought that bubble canopies were more streamlined. However the cabin design was better for crew communication.

It was decided that it was more important for the pilots to be able to work together.

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jacksmachiningreveng@reddit (OP)

It was initially thought that bubble canopies were more streamlined

Were they though?

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General_Douglas@reddit

Based on how the average airliner looks nowadays, I’d say no

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speedyundeadhittite@reddit

They should switch to bubble canopies, and sell the front seats for big bucks.

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MonsieurCatsby@reddit

The bubble canopy essentially gave better field of view for the pilot, which was deemed a good idea. However having the pilot and copilot in tandem rather than side by side made it more difficult for them to communicate and operate in unison, which on long flights and in a huge aircraft was a priority

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badpuffthaikitty@reddit

Did the B-47 have a yoke or a stick?

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MonsieurCatsby@reddit

Yoke, the early B-52 tandem cockpit also used a yoke

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speedyundeadhittite@reddit

Areonatics engineer: What - I mean - how... (wave hands), why not - ok let's do this thing!

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weird-oh@reddit

There were some staged at PBI during the Cuban Missile Crisis. My buddy and I used to go down there to look at them. When it was over and they departed, it was glorious.

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mizunumagaijin@reddit

The experience of even just watching that thing launch must have been mind-blasting. I can barely even imagine the noise between those old turbojets and 18 bloody rockets.

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jacksmachiningreveng@reddit (OP)

During the late 1940s, the bomber was hailed as the fastest of its class in the world. The first prototypes were fitted with General Electric J35 turbojets, the production version of the TG-180, with 3,970 lbf (17.7 kN) of thrust. Early jet engines did not develop good thrust at low speeds, so to assist take off when heavily loaded, the B-47 had provisions for fitting solid-fuel rocket-assisted takeoff (RATO) rockets, each generating roughly 1,000 lbf (4.4 kN) of static thrust. Early aircraft had mounts for nine RATO units built into each side of the rear fuselage, arranged in three rows of three bottles. Most of the space within the upper fuselage was taken up by self-sealing fuel tanks, the wing having been deemed unsuitable for storing fuel.

extended footage

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erhue@reddit

longer runway ❌

heavy RATO bottles, structural modifications, extra parasitic drag ✅

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jacksmachiningreveng@reddit (OP)

For a certain load you need to reach a certain airspeed to get airborne, with insufficient thrust no length of runway will suffice.

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Deepandabear@reddit

Either way, building a plane so heavy that it has insufficient thrust to take-off on its own sounds like a really bad idea

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mike_jones2813308004@reddit

Big bombs weigh a lot and these things needed to get to Russia somewhat fast.

Jfc wiki says they're made for "loft bombing" nukes. What a time.

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F6Collections@reddit

Well said.

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erhue@reddit

let's say you reach the airspeed and get airborne. Now, you have the problem that those RATO bottles stop producing thrust after a few seconds. How will you remain airborne if you have insufficient thrust to even get airborne with unlimited runway length?

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Decent_Leopard9773@reddit

You can clearly see that the RATO bottles are mounted perfectly level with the plane so if the plane stalls without RATOs, it will stall obviously but if the RATOs are active it will still stall and fall out of the sky anyway because they contribute nothing towards keeping the B-47 airborne.

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jacksmachiningreveng@reddit (OP)

You need less power to stay in the air than to get to the air, especially once the undercarriage is retracted and you've cut down on drag

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wifetiddyenjoyer@reddit

Your submission statement says that it's due to engines being less powerful at lower airspeeds.

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dont_say_Good@reddit

Guess what, you can accelerate after take off while the rockets still help you out

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erhue@reddit

the undercarriage being retracted will reduce drag, but being airborne also means a great increase in induced drag. When on the ground, an aircraft is at roughly 0 degrees of angle of attack, and induced drag is present, but relatively low, since the wings are not fully sustaining the aircraft, and ground effect also helps to reduce this form of drag.

When in the air, the aircraft has to be at a positive angle of attack for level flight. This greatly increases induced drag.

The reason this plane uses RATO is to take off in a shorter distance, not because it'd be "impossible" for it to get airborne with the weight in question.

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jacksmachiningreveng@reddit (OP)

I suppose the question is whether an aircraft with enough power to sustain level flight will also be capable of accelerating to its minimum takeoff speed given sufficient runway. RATO units are certainly used on flying boats that for practical purposes have an infinite runway, although of course they have a lot more drag to deal with compared to a wheeled aircraft.

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erhue@reddit

that makes sense, but it was not the case with the B-47. They didn't use the RATO bottles because the wouldn't be able to get airborne otherwise; they did so to take off where runways wouldn't be long enough.

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vonHindenburg@reddit

You're also burning fuel, so the plane is getting lighter. And, as has been stated elsewhere, jets get more efficient at higher speeds.

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erhue@reddit

yeah, youve burned only a few seconds worth of fuel. You also get lighter as the plane rolls down a long runway. And doesn't need to carry the dead weight of spent RATO bottles.

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wifetiddyenjoyer@reddit

Jesus is the answer.

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gravelpi@reddit

Call me crazy, but having the ability to get airborne faster is a good thing. There might be circumstances, such as your runway has a giant hole in it or the entire area is destroyed, that being able to take off from the partial runway or alternate smaller runways is a benefit.

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erhue@reddit

it is obviously positive. I'm just pointing out the tradeoffs that had to be made to achieve that shorter takeoff - those spent bottles become dead weight less than 20 seconds after their activation. They also require structural modifications that add weight and drag. Must be part of the reason why other aircraft equipped with RATO bottles dropped them after the rocket fuel was spend.

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jacksmachiningreveng@reddit (OP)

I mean technically a rocket is a form of jet engine but yes the term is quite jarring.

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jacksmachiningreveng@reddit (OP)

Going by the numbers the rockets gave an almost instantaneous 75% increase in power compared to the engines at full throttle

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KokoTheTalkingApe@reddit

Holy frijoles. That must be quite a ride.

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erhue@reddit

good post op