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?

  • 2
  • @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. – karthikeyan Nov 20 at 20:32
  • 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). – tl8 Nov 21 at 1:36
  • 2
    slightly related, w/ photos of struts: Shear forces between Shuttle, tank, and boosters - what pushes what? – uhoh Nov 21 at 1:57
  • 1
    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. – Antzi Nov 21 at 5:56
up vote 32 down vote accepted

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.

  • 8
    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 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 at 0:49
  • 21
    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 at 7:53
  • 3
    @user71659: Now I feel like a gorilla for snapping one. – Joshua Nov 21 at 20:18
  • 1
    @ChrisStratton [citation needed] There are companies selling quarter inch bolts with tension failure loads of 4.3 tons – TemporalWolf Nov 21 at 21:51

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.

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

This is evidently wrong, but I always assumed that the most important thing being transferred from the side boosters is not thrust, but fuel.

The thrusters on the side boosters just need to keep up with the main booster until they run low on fuel. At that point, they detach, and the main booster still has some fuel to spare, since it has been fed fuel continuously.

Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

  • 5
    Unfortunately no one has ever flown a cross-fed booster system like you describe: no rocket has ever simultaneously run an engine from one tank and pumped fuel to another tank. Falcon Heavy was originally planned to do this, but it proved too expensive to design for its usefulness. – Nathan Tuggy Nov 21 at 9:24
  • 5
    @Osthekake Welcome to Space Exploration SE! Try to always include sources in your answer. In this case, trying to find a source will probably lead you to the conclusion that boosters are not there to provide fuel to the main booster. Cross-feeding can also be called "asparagus staging". You can find a discussion of "asparagus staging" here for example: forum.kerbalspaceprogram.com/index.php?/topic/… – BlueCoder Nov 21 at 9:32
  • (In any case, even when cross-feeding/asparagus staging, the boosters do not simply provide fuel, the thrust they provide is useful too) – BlueCoder Nov 21 at 9:37
  • 3
    In the case of the American Space Shuttle (which I think was mentioned before), the boosters are named SRB from Solid fuel Rocket Boosters. The shuttle engines use the fuel from the tank - the russian "shuttle" (the Energia/Buran complex) had rocket engines on the central tank itself – Calin Ceteras Nov 21 at 14:24
  • 2
    @CalinCeteras Yes, the Shuttle looks like a case of cross-feeding...although the central tank wasn't a booster, but just a tank :) – BlueCoder Nov 21 at 15:07

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

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