Question about Tallboy/Grand Slam bombs
 

Leo McKinstry in his book Lancaster claims the Tallboy bomb designed by Barnes Wallis could reach a terminal velocity of 3700 feet per second. He also says the Grand Slam bomb exceeded the speed of sound when dropped from 12,000 feet. These incredible speeds seem to defy the laws of physics and I have trouble believing this was possible. Can someone please clarify this.

In 'The Dam Busters' by Paul Brickhall he comments at Barnes Wallis was trying to develop big bombs to create earthquake effects & that he calculated the bombs needed to be dropped from 40,000ft to reach the necessary velocity. This was very early in the war and he realized he'd have to develop a bomber (the 'Victory Bomber') to get a bomb that high. In the end he settled on bigger bombs dropped from a lower height by the Lancaster. The Tallboy first due to the difficulty of making the Grand Slam. I know in that book it mentions that the Grand Slam bombs had to withstand impact speeds of about 1000mph.
I'll try to dig it out later today to see if / how it mentions how these velocities were arrived at. I thought that falling bodies would reach a 'terminal velocity' and wouldn't be that great a speed though?

I think that Tallboy, dropped from as high as a Lancaster could carry it would just go supersonic. Grand Slam maybe a bit faster. Certainly not 3700 fps.

After an excellent landing you can use the airplane again.

I seem to recall a statement by Eric Brown in one of his books that when they were doing high speed research with Spitfires i.e. vertical dives, they would reach a point where throttling back the engine had no effect because all the speed was being provided by the aircraft mass - and they were hitting 500 mph with a lot more drag and a lot less mass than a Tallboy!

Also I think it was Sir Stanley Hooker who once did a back of a fag packet calculation that a Spitfire flat out in level flight was producing about 1000 lb of thrust. Sabres got close to the sound barrier in level flight with about 5000 lb thrust.

So a well streamlined bomb with 12000 lb (or 22 000 lb) of gravitational thrust? Through the sound barrier easy.

:ok:

It's going to be pure physics on terminal velocity innit? Drag, mass, gravity...

Slight thread drift
 

Was it really true that a Grand Slam/Tallboy parked on its nose outside an RAF airfield guard room was moved for road widening and found to be full of explosive? Or is it just another urban myth?

Anyway looking at the damage caused by BW's bombs [TV Documentary] supersonic or not, they made very big bangs/shifted shedloads of soil! :)

Inspired by Load Toad, I did a bit of back of the fag packet (OK - I used a calculator) calculation using very old O Level physics - having heard the same story myself in my youth.

I know this is very flawed, but I used 1100 fps as the "speed of sound" (OK, that's ICAN Sea Level) and 32 fps/s as g.

Using basic formulae (OK, I know that's in a vacuum!) of v=gt and d=1/2 x g x t x t, it gives an absolute minimum of about 19 000 feet, which makes claims of 3700 fps from 12 000ft seem very unlikely.

Clearly effects of air density (which would vary) and coefficient of drag (which wouldn't) would probably increase this figure - the CD certainly would.

After this my brain started to hurt ......

This doesn't answer your question but may be of interest - see the second post down.

Make a bit of head room, get a prisoner on death row and the morning papers read...

I don't know about the exactly velocity reached by either the Tallboy or the Grand Slam, but both had offset fins to give them gyroscopic stability to counteract the effect of transonic shock waves forming and disturbing the path of the bomb.

The Grand Slam had offset fins from the first one, but the Tallboy was not fitted with them until after the first drop trials because it was affected in this way. This information came from Paul Brickhill's 617 Squadron book.

Very high accuracy was being demanded, the SABS bomb sight needed all sorts of corrections applied resulting in very high crew workload and having an errant bomb would have negated all that work put in to ensure accuracy.

Hipper,
Thanks for that - fascinating! By the way, I was in the Rathaus (townhall) in Leipzig after they restored it - I didn't realise it was damaged in the war as it was in perfect condition.

Tallboy Bomb
 

The Tallboy had to withstand an impact velocity, if dropped from its optimum height of 18000 feet, of 750 mph after falling for 37 seconds .

Reference Dambusters by Wood, Lee and Watchel.

It seems McKinstry is a little bit high for the terminal velocity of the Tallboy but 750 mph is pretty fast.
Lancaster is not a bad book - over 500 pages of it. It's well written but there are a number of errors. However, it's still worth reading particularly because of the first-hand descriptions form just about everyone involved with the plane including those on the other side.

In his book "No 5 Bomber Group", W.J. Lawrence wrote of the Tall Boy:
"it was ballistically perfect, and in consequence had a very high terminal velocity estimated at 3,600 and 3,700 ft per second".

Don't know what bombing height that was at.

Bob C

Tallboy Terminal Velocity
 

The rule of thumb formula we used at NASA for quick calculations of terminal velocity was:

Vt = sqrt((2.m.g)/Cd.p.a))

Cd is drag coefficient, p is air density, a is cross sectional area.

If you plug in some “best guestimate” figures for the tallboy you get terminal velocity figures close to the quoted 3,700 ft/sec.

Bob C

A few quick sums:
 

If you neglect air resistance you have to start from 18,906 feet according to Newton.

(I remember this from a maths lesson in 1967 "If a plane flying at 19,000 at drops a bomb what speed does it hit the ground at? ignore the effect of air resisitance." I got to 1100 feet per second but the teacher wanted it in miles per hour and I got that bit wrong, which is why I remember it! Very irascible Scotsman was Laurie Andrew)

VnV

Bouncing-Bomb Man by Iain Murray (Haynes 2009) p129:
"were dropped from 18,000'.... average striking velocity 1100fps"

A Hell of a Bomb by Stephen Flower (Tempus 2002) p98:
"from 18,000'......had taken 37 seconds to fall, impacting at just over 1,000 ft per second."

It would be nice to see the original test reports, I wonder if they are accessible anywhere; IWM or perhaps the Brooklands museum.

Its 1100 feet per second in fact.

Which is about 2,500 miles per hour or a tad over Mach 3 as it gets near the ground...

I don't see that as impossible for a streamlined and spin stabilised vehicle with 12,000 pounds of gravitational "thrust" but it would have to be dropped from a height that would exceed the ability of any optical bombsight to aim it. So, its the age old engineering problem of optimising the design - in this case, impact speed and impact resistance versus accuracy, that ends with an optimum dropping height of 18,000 feet and an impact velocity of 1,100 feet per second to achieve the desired penetration of, lets say, a submarine pen.

I remember a grand slam on display at RAF Lindholme in the 60s. Does anyone know what happened to it?

Hooey!

Gravity is not thrust, it is a fixed rate of acceleration and is totally independant of the mass of the object as every schookid knows.

A ton of lead falls as fast as an ounce of feathers in a vacuum which is what most of the above calculations are based on.

3700ft/sec is probably a good number for terminal velocity. The problem would be getting a Lanc to 220000 ft to drop it.

After an excellent landing you can use the airplane again!

Tallboy Bomb
 

Further to my previous post 12 , Dambusters also explains that the depth of penetration into the earth , dependant on fuse setting ,was estimated to be capable of displacing one million cubic feet of earth whilst creating a crater requiring five thousand tons of earth to backfill.

Some muckspreader ,could spoil ones day if dropped in ones backyard !!

Grand Slam
 

As a schoolboy in Wilhelmshaven after ww2 It was possible to go to many places that Health & Safety would have had a fit about these days. I was shown a hole in the top of the U-boat pens supposedly made by a Grand Slam bomb which subsequently exploded deep underground. Also remember the Army engineers detonating some dangerous explosive
which caused most of the remaining windows in that part of town to shatter. Apparently they forgot to inform the locals what they were up to.

50°55'32.80"N 1°43'3.00"W. (New Forest)

Pop that into Google Earth and see how big that hole is...the pond inside it is 50 x 60 ft! What else could have made that but a Grandslam, and they were tested there apparently.

I remember reading somewhere a long time back, that the New Forest was the only place where the grand slam was tested.

Aviate1138. It was Scampton and I've been assured that it was an urban myth.

I thought they were tested at New Forest only and with Grand Slam they were so valuable they only tested one before going operational.

"Tested" may be a bit of an exaggeration but there was a Grand Slam outside the museum at Fort Lewis, Washington State in the 80s. It was found on the ranges. My guess is that it was from B-17 trials & was jettisoned.

I seem to remember that it was not the blast itself but the shock wave produced that caused the main damage such that a near miss could do more than a direct hit. Barnes Wallis thought it up after he heard that shock waves from driving pilings was causing collateral damage at a site in London.

My guess is that there is more chance of a Grand Slam getting airborne with a B-17 slung underneath, than a B-17 getting airborne with a Grand Slam. the largest bomb carried by the B-17 was 2000lb.

Probably a typo mate, he meant B-29....

http://img831.imageshack.us/img831/6...lam4895776.jpg

True, which is the reason for the inverted commas (somebody else used the expression), but the inertia isn't independent and thats where the penetration comes from - 12,000 pounds travelling at 1100 feet per second when dropped from 18,000 feet using a modified optical bombsight.

Ayn idea of the name of the air base just north of the above site

50°55'32.80"N 1°43'3.00"W - Google Maps

Just curious

That would be RAF Stoney Cross I believe. (Would have been a good one for the 'Which Aerodrome' thread)

Apart from its very unusual layout which is a dead giveaway.

TCraft41, Welcome to PPRuNe.

As StbdD said, it's Stoney Cross - a disused wartime airfield, which is used as a Visual Reporting Point for VFR traffic skulking round the edge of the "busy" Bournemouth zone.

IF you are interested in UK airfields, download this Google Earth 'kmz' file:

http://www.rickdavis.co.uk/home/UKAirfields.kmz

Here's the whole of the UK:

http://i30.photobucket.com/albums/c3...Kairfields.jpg


Another interesting 'kmz' file that I found recently is http://is.gd/hAMrD
which contains old aerial photos of airfields.

Here's the one for Stoney Cross (with Ab's Grand Slam crater in the top left corner):
http://i30.photobucket.com/albums/c3...toneyCross.jpg

I42

I tried to "load" the second kmz, bur without visible success. Any ideas? (I run a Mac, could that be a glitch?)

The Rick Davis stuff I have and it works perfectly.

Reg

Gentlemen go to Brooklands where all 3 types of bomb 22,000, 12,000and "Bouncing" bombs are displayed and an informative text is displayed as well .

There's confidence for you! At school I was taught that gravity is the force of attraction between two masses and was proportional to the masses of the two objects and their distance apart.

On the earth the gravitational force is sufficient to give an object an acceleration toward earth's center of 32ft/sec/sec (roughly)

Dick

Sorry, I'm a page out of date. I was replying to Ag. bis.

RegDep,

I have a Mac too, so that is not the problem. ;)

I clicked on the link I posted and downloaded the file directly to GE, but I noticed that it wasn't automatically turned on. Look at the bottom of the Places list, under Temporary Places. You should see 'airfield overlays28-1.kmz'. Click the box to turn it on.

PM me if you cannot get it to work.

India Four Two

Thanks.

Besides just a historical interest of England I am friends with an Eighth Air Force vet of WW II. B-17 waist gunner with 50 plus missions.

I am trying to get him to record his war memories before it is too late. It may already be as he is having problems remembering exactly where he was based in England.

The Google location should help.