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Strap on boosters of a lot of heavy launch vehicles seems to be attached with just a few "flimsy" struts.

Here are few images for reference : Falcon Heavy image, on LC39A Delta 4 launch

Some of the possibilities:

  1. These support structures from the strap-ons are extremely strong and well designed to transfer the thrust.

  2. There are other hidden support structures to bear/transfer the thrust to the central core.

  3. Somewhat a mix of the above two possibilities or something other than the listed.

SO, how do the thrust actually get transmitted to the central core?

Dragongeek
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karthikeyan
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2 Answers2

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It’s a sense of scale issue. As much as the struts might look like flimsy bits of drainpipe, those rockets are around 15 meters wide, and the struts are more like the heavy steel beams used to hold up entire buildings.

So yes, they’re just really strong.

Quentin Clarkson
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    Falcon Heavy core diameter is 3.66m; from the picture I estimate the upper struts are about 24cm or 9" in height. Delta Heavy core is 5.1m; the picture is too fuzzy for me to accurately estimate the size of the struts. – Russell Borogove Nov 20 '18 at 23:24
  • Using a similar approach I got 36cm (14”) for each of the two lower attachment points on the Delta Heavy. But yeah, it’s tricky finding something much more accurate than “way bigger than they look”. – Quentin Clarkson Nov 21 '18 at 00:49
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    Also, people underestimate the strength of solid metal. A scientist was amazed when I told him a single high-grade 1/4 inch (6 mm) steel bolt is good for 3-4 tons. – user71659 Nov 21 '18 at 07:53
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    @user71659: Now I feel like a gorilla for snapping one. – Joshua Nov 21 '18 at 20:18
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    @ChrisStratton [citation needed] There are companies selling quarter inch bolts with tension failure loads of 4.3 tons – TemporalWolf Nov 21 '18 at 21:51
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For Delta IV Heavy, according to Spaceflight101:

The CBCs functioning as boosters are attached to the central core using thrust struts that interface with the interstage section of the launcher to transfer loads from the boosters to the rest of the vehicle. Additional attachment points reside in the base of the vehicle right above the engine heat shields.

The "thrust struts" described are the thin horizontal pieces near the top of the boosters. The "additional attachment points" are at the bottom of the cylindrical portion of the cores, just above the tapered boat-tail heat shields around the engines. This diagram gives a slightly better view of the latter:

enter image description here

All the force differential between the boosters and center core goes through those attachment points.

For Falcon Heavy, the setup is similar:

The boosters are attached to the central core stage via structural interfaces in the aft section and interfaces that connect the upper portion of the boosters to the interstage area of the Falcon Heavy via thrust struts to transfer loads to the vehicle. Separation of the boosters is accomplished using collets in the structural interfaces, avoiding the use of pyrotechnics since SpaceX prefers to use systems that can be tested and re-used. The reaction control system of the boosters ensures a clean separation from the core stage.

The base connection points appear to be singular, rather than dual as on the D4H:

enter image description here

The struts are pretty substantial; from the first picture in the question, I estimate the height of the upper struts to be about 24cm.

For Soyuz, the force seems to all be transmitted to sockets near the top of the core stage which are engaged by the nose of the boosters; I believe the straps at the base hold the boosters in position without transmitting significant load.

Russell Borogove
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    Does anyone know how much thrust is transferred through the top (resp. bottom) attachments in each case? – Steve Linton Nov 20 '18 at 20:46
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    I am really surprised at how such tiny pieces can transfer such huge loads.. I am also curious to know the share of load as pointed out by steve. @russell are there any references/calc/explanation as to why the load on top strut of a strap on will be the largest? – karthikeyan Nov 20 '18 at 21:12
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    @karthikeyan remember that a significant portion of a side booster's thrust is used to accelerate its own mass – Jack Nov 20 '18 at 21:23
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    No idea how the load is distributed. Jack's point is good, although in both the D4H and Falcon Heavy case, the center core is throttled down to ~55% while the side boosters are burning, and the center stick supports the mass of the upper stage and payload. – Russell Borogove Nov 20 '18 at 21:29
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    @Jack I was thinking along the same lines, but wouldn't the argument lead to a point questioning the added advantage of boosters?! I have not worked out the math, but qualitatively discussing! – karthikeyan Nov 20 '18 at 21:29
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    The shuttle external tank and solid rocket boosters were held together by only a few bolts and fittings. Large bolts compared to everyday experience, but still. – Organic Marble Nov 20 '18 at 23:18
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    @karthikeyan No, that's just the fundamental problem with rockets. Most of the mass of the spaceships is there to move the vehicle in the first place. Mass ratios of about 25 are around the limit of our technology (with staging!) - Saturn V's payload was 4% of its launch mass; the Shuttle only had 1%. Rockets are a ridiculous way to get things into orbit, we wouldn't use them if we had anything better :D The SRBs on the Shuttle had launch thrust only twice their own weight - so indeed, at launch, half of their thrust was to lift themselves. Then, 24-35mm solid struts are massive. – Luaan Nov 21 '18 at 11:59
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    @Luaan Shuttle put about 4.5% of its launch mass into orbit; it was only for sentimental reasons that most of it was returned to Earth. ;) – Russell Borogove Nov 21 '18 at 15:48
  • @RussellBorogove: Part sentimental reasons and part wanting to avoid lawsuits from the families of crewmembers. – Vikki Nov 21 '18 at 23:35
  • The difference in vertical force -- which is the only force occuring between each booster and the core -- is just the force needed to accelerate the mass difference between booster and middle section, i.e. roughly the second stage, faring and payload. The second stage appears to have a mass including fuel of about 100t, which would need 4MN (equiv. 400t) for an acceleration of 4g (or ~40m/s^2). The 400t would be provided by all 3 first stages, makes ~133t (1.3MN) force per stage, which should be the differential force. A far cry from the 7.6MN of thrust per booster/core. Add payload. – Peter - Reinstate Monica Nov 22 '18 at 08:25
  • @PeterA.Schneider Core is throttled to ~55%. – Russell Borogove Nov 22 '18 at 09:39
  • @RussellBorogove To save fuel for continuing after booster separation? – Peter - Reinstate Monica Nov 22 '18 at 09:51
  • @PeterA.Schneider And to avoid accelerating too quickly. – Russell Borogove Nov 22 '18 at 09:54
  • @RussellBorogove So what's your best guess what max force the struts must withstand? What's max g with boosters attached? What's the differental force then if the boosters also must accelerate the heavy core? (Of course it's dependent on the core's fuel level since that's the bulk mass, so for a given acceleration the forces -- overall and hence differential -- would be largest when full.) – Peter - Reinstate Monica Nov 22 '18 at 11:28