21
$\begingroup$

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?

Improve this question
$\endgroup$
6
  • 2
    $\begingroup$ Related: Are any strap-on boosters held in place by actual straps? $\endgroup$
    – Russell Borogove
    Nov 20, 2018 at 20:22
  • $\begingroup$ @RussellBorogove in your answer linked, the case is Soyuz which has the strap on in flush with the core. But the case I'm looking at specifically are the FH, Delta, etc where there is just 2 thin rods that connect on the top and bottom. $\endgroup$
    – karthikeyan
    Nov 20, 2018 at 20:32
  • $\begingroup$ Something worth noting, the struts on top would also be being pushed into the core because of the torque action of the side boosters (depending on any vectoring). $\endgroup$
    – tl8
    Nov 21, 2018 at 1:36
  • 2
    $\begingroup$ slightly related, w/ photos of struts: Shear forces between Shuttle, tank, and boosters - what pushes what? $\endgroup$
    – uhoh
    Nov 21, 2018 at 1:57
  • 1
    $\begingroup$ Remember that the strap on has to lift itself first before transmitting force to the core stage. So not all the thrust is directly imparted to the struts. $\endgroup$
    – Antzi
    Nov 21, 2018 at 5:56

2 Answers 2

Reset to default
33
$\begingroup$

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.

Improve this answer
$\endgroup$
5
  • 8
    $\begingroup$ 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. $\endgroup$
    – Russell Borogove
    Nov 20, 2018 at 23:24
  • $\begingroup$ 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”. $\endgroup$
    – Quentin Clarkson
    Nov 21, 2018 at 0:49
  • 21
    $\begingroup$ 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. $\endgroup$
    – user71659
    Nov 21, 2018 at 7:53
  • 3
    $\begingroup$ @user71659: Now I feel like a gorilla for snapping one. $\endgroup$
    – Joshua
    Nov 21, 2018 at 20:18
  • 1
    $\begingroup$ @ChrisStratton [citation needed] There are companies selling quarter inch bolts with tension failure loads of 4.3 tons $\endgroup$
    – TemporalWolf
    Nov 21, 2018 at 21:51
22
$\begingroup$

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.

Improve this answer
$\endgroup$
14
  • 3
    $\begingroup$ Does anyone know how much thrust is transferred through the top (resp. bottom) attachments in each case? $\endgroup$
    – Steve Linton
    Nov 20, 2018 at 20:46
  • 9
    $\begingroup$ @karthikeyan remember that a significant portion of a side booster's thrust is used to accelerate its own mass $\endgroup$
    – Jack
    Nov 20, 2018 at 21:23
  • 2
    $\begingroup$ 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. $\endgroup$
    – Russell Borogove
    Nov 20, 2018 at 21:29
  • 6
    $\begingroup$ @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. $\endgroup$
    – Luaan
    Nov 21, 2018 at 11:59
  • 4
    $\begingroup$ @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. ;) $\endgroup$
    – Russell Borogove
    Nov 21, 2018 at 15:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.