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Consider a rocket using nuclear fuel, where the energy density is far higher. Let's stipulate that in an SSTO configuration, it's got a payload capacity like the Falcon Heavy. So, at that point, you don't need the boosters...but it seems like with the boosters, your payload capacity is higher still.

Is there a tipping point where boosters no longer make sense or are you always better off with boosters using the same high-density fuel as the main vehicle?

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There's no fundamental difference between a rocket with boosters and a bigger rocket. For a given set of propulsion technologies, the payload capability of a launcher is generally proportional to the launch mass of the rocket -- some features don't scale that way, but the fuel load does, and makes up over 90% of the mass.

So if you want more payload capacity, adding boosters is a good thing.

In practice, we often see boosters using a cheaper and less efficient type of propulsion than the core stage -- solid rocket motors, usually. These are less mass efficient than liquids, but much more cost efficient, so they're advantageous to use at the very beginning of flight.

You mentioned nuclear fuel; assuming you're talking about a nuclear thermal system like NERVA or Timberwind, those have relatively low thrust, so you'd almost certainly pair them with solid rocket boosters for a practical launcher.

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  • $\begingroup$ I'm not sure the boosters being cheaper really applies anymore. Sure 5 or 10 years ago it wasn't a bad way to get a bit more performance out of an existing rocket but SRBs will never be cost competitive with thew new standard of reusable rockets and there is really no realistic option for cost effective SRB reuse. $\endgroup$ – Evan Steinbrenner Mar 2 '18 at 23:07
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    $\begingroup$ Nuclear fuel can negate the need for boosters, if you have an outrageous design such as the "Liberty Ship". It uses closed cycle gas core nuclear thermal rockets, the good old "nuclear lightbulb" projectrho.com/public_html/rocket/… $\endgroup$ – Winchell Chung Mar 3 '18 at 13:13
  • $\begingroup$ Yeah the NLB is partly what I had in mind with this question. But are you better with two nlb boosters strapped to the sides! $\endgroup$ – Chris B. Behrens Mar 4 '18 at 4:43
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There will come a point where the bracing needed to deal with the extra boosters costs as much as the gain from the booster.

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