# Is a Falcon Semi-Heavy plausible?

Some rockets use strap-on boosters to match the payload requirements, even sometimes launching with only one strap on.

Is this a plausible scenario for Falcon to be strapped together with only one side booster?

• Not sure I understand your question. A Falcon Heavy is 3 first stage cores and a single upper stage. Do you mean using only one first stage? That's a Falcon 9. Or do you mean two first stages - the "central" core and a one additional "strap-on"? If the latter, the problem is assymetric thrust. I have no idea if the thrust vectoring can correct for that, but I suppose it's theoretically possible to make it work. I doubt it's worthwhile though - with full reuse, it seems cheaper to just use an FG than to design for this approach. Dec 11, 2017 at 15:44

The main concern with this configuration would probably be asymmetric thrust. If the thrust is aimed straight down the rocket would start to rotate clockwise because the center core is heavier than the booster while having the same amount of thrust. This turns into even more of a problem later into the flight because they throttle down the center core.

This asymmetric thrust needs to be compensated by changing the direction of the thrust vector. Luckily the Falcon rockets are capable of vectoring their engines, but the exact range is unknown. I made the following drawing to do some guesstimating on the required vectoring range for a Falcon Semi-Heavy: Some assumptions I made:

• Payload capacity would be 40,000 kg (FH = 63.8 F9 = 22.8)
• Core throttles down to 50%
• Booster remaining fuel at separation is 30%
• Core remaining fuel at separation is 50%

For the rocket to stay stable in flight the thrust vector needs to be in line with the center of mass and center of thrust. As long as the thrust vector lines up with these two points the rocket will be able to fly in a straight line.

So if you draw a line between the center of thrust and center of mass in these drawings you will see that this results in a required vectoring range of around 3 degrees, which seems quite reasonable.

(Some sources for the numbers I used: (1)(2)(3)(4))

• Could you include the calculations for the thrust vectoring angle ? Mar 8, 2018 at 1:28
• @Antzi I added an explanation to how I got this number. It's still very much a guess though. But because the Falcon is such a slim rocket, the required vectoring range is fairly low and certainly way lower than what I had initially expected. Mar 8, 2018 at 10:32

With first stage reuse, there is a simpler configuration: Keep both strap-on boosters, but only fill them part way with fuel. Since each of the two boosters fires for about half as long as if there was one, they only accrue about half the wear. You could also fire only some of the engines on each booster as another way of limiting wear. This seems easier to manage than asymmetric thrust. Also of note is how small the boosters are on the Atlas V compared to a second complete first stage.

• -1. "Since each of the two boosters fires for about half as long as if there was one, they only accrue about half the wear." Citation needed. The majority of wear could occur when the engine turbupumps spin up, or when the fuel begins flowing, or when the engine shuts down. Wear may not be linear. In both cases, full and half-full, the engines have to ignite once. Presuming that the wear is perfectly matching with time is flawed. Dec 12, 2017 at 1:13
• Fair enough, and this answer is speculation, I have no citation. You could still limit the engines that you start up. There will also be wear on all nozzles during reentry.
– Lex
Dec 12, 2017 at 17:26
• More likely they'd fill them all the way up and fly the center core back to the pad as well. Feb 14, 2018 at 2:56

This is very unlikely to happen for a number of reasons. The biggest reason is that the Falcon Heavy in particular is designed to be reused. If you would only need 2 cores instead of 3, then just land the 2 cores back at your main base, which would save a lot of fuel. It is thought that it will be cheaper to fly a Falcon Heavy with 3 core reuse then it would be to fly a Falcon 9 with the same payload that requires the Falcon 9 to use up it's landing fuel reserve. I can't find performance on the expendable Falcon 9 configuration, but here is the performance differences between the various Falcon configurations for C3, per NASA's Launch Vehicle Performance website. Note that there isn't a huge difference between a Falcon 9 expendable and a Falcon Heavy Recoverable.

• Are you saying that the payload range for a F9 with no reuse overlaps that of a FH with 3 core reuse (and maybe even all three cores landing at the takeoff point). If so, that is actually the main answer to the question -- "there is no need for a Falcon Semiheavy because you either use a FH with maximum reuse or a Falcon 9" Mar 8, 2018 at 10:48
• That is my understanding. Will see if I can dig up some more information on it later. Mar 8, 2018 at 11:31
• silverbirdastronautics.com/cgi-bin/LVPcalc.pl suggests about 17t to LEO for a fully expendable F9 vs about 20t for a FH with all three boosters landing back at Canaveral. Which does indeed suggest that a semi-heavy is simply not needed. Just find a booster close to end-of-life and finish it off as a F9. Mar 8, 2018 at 13:32

As @jgalak notes, the problem would be asymmetric thrust, but I believe the vectored thrust on the Falcon would be sufficient to keep things balanced. Since the rocket wouldn't be going straight up, there would be a net sideways aerodynamic force on the stack, but Atlas V seems able to deal with that in its asymmetric single-booster configuration. At first glance it seems plausible to me.

• But Atlas V is not a Flying Noodle!
– uhoh
Dec 28, 2017 at 0:58