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Question: Why can solid rockets be both the skinniest and most spherical launch vehicles while liquid fuel rockets have a more limited range of aspect ratios? Are there fundamental engineering principles at work? Simple physics? Or is the devil in the details?

Evidence of solid rocket launch vehicle "noodlicity":

This answer to Most noodle-like (highest aspect ratio) orbital launch vehicle ever?

danger noodle

Scout X-2

Evidence of solid rocket launch vehicle sphericity:

How do spherical SRB's compare to long skinny ones? What do their thrust curves look like?

illustration of one concept for a two-stage, solid-fueled Mars Ascent Vehicle, NASA/MSFC

The first stage of this Mars launch vehicle is pretty squat and is just as spherical as a spherical cow. Source: Spaceflight Now's NASA narrows design for rocket to launch samples off of Mars

This diagram illustrates one concept for a two-stage, solid-fueled Mars Ascent Vehicle. Credit: NASA/MSFC

Counterevidence: squat liquid launch vehicles

  • SERV "VTOVL orbital launch vehicle. Chrysler ballistic single stage to orbit alternate shuttle proposal of June 1971. This was the most detailed design study ever performed on a VTOVL SSTO launch vehicle. The 2,040 metric ton SERV was designed to deliver a 53 metric ton payload to orbit in a capacious 7 m x 18 m payload bay."
  • Apollo Lunar Module
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    $\begingroup$ Liquid-fueled counterexample: astronautix.com/s/serv.html $\endgroup$
    – Organic Marble
    Commented Jun 2, 2021 at 3:24
  • $\begingroup$ @OrganicMarble it's not truly a counterexample until we compare aspect ratios for the launch vehicles, but I'll add a caveat to my question. Thanks! $\endgroup$
    – uhoh
    Commented Jun 2, 2021 at 3:30
  • $\begingroup$ @RussellBorogove ditto $\endgroup$
    – uhoh
    Commented Jun 2, 2021 at 3:30
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    $\begingroup$ Is this related to pressure vessels? Vessels that are designed to hold pressurized contents are almost inevitably curved: sphere, cylinder, or torus. Perhaps solid fuels allow greater freedom? $\endgroup$
    – Tim Randall
    Commented Jun 2, 2021 at 15:11
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    $\begingroup$ I think you need to consider how the fuel in a solid rocket burns. As I understand it (I'm certainly no expert), the fuel is shaped internally to control the rate of burning, and hence the thrust. Probably easier to do this in a long cylinder rather than a sphere. $\endgroup$
    – jamesqf
    Commented Jun 2, 2021 at 17:21

1 Answer 1

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The squat end of the spectrum has little to do with solids versus liquids and everything to do with aerodynamics. Spherical tankage is most weight-efficient, so you'd expect squat stages in cases where aerodynamics don't dominate, such as your Mars Ascent Vehicle (flying where atmospheric drag is on the order of 1% what it is for Earth ascent) or the Apollo LM. Since a solid rocket motor has fuel and oxidizer mixed together in a single container, spherical stages are not unknown, but some liquid stages have side-by-side (Apollo LM, Fregat) or even nested (Briz-M) tankage, for an overall squatter-than-spherical aspect ratio.

At the skinny end of the spectrum, sounding rockets often start with a higher thrust-to-weight ratio than orbital rockets, and solids burn off mass faster than liquids (because of their poorer specific impulse), so solid-motor sounding rockets in particular will hit higher speeds at lower altitudes, meaning the tradeoff between thin-aspect low-drag designs and squat-aspect mass-efficient designs is slightly different.

Solids can probably be built to slightly skinnier extremes than liquids, as SRM casings are generally thicker than liquid fuel tanks, and the fuel grain may provide some structural strength.

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    $\begingroup$ +1 for a concise and thoughtful answer! Skinnier tanks also have more surface/volume and so will either need more insulation (which adds thickness back) for cryo-propellants or accumulate ice/rocket mass faster, or need more aggressive chilling mechanisms for long launch windows. $\endgroup$
    – uhoh
    Commented Jun 2, 2021 at 3:45
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    $\begingroup$ Also, because almost the entire skin of a solid rocket is combustion chamber it needs to be considerably stronger than the tanks of a liquid fueled rocket. While this is a liability in terms of dry mass; it means that the rocket itself is stronger and thus more able to resist differential cross winds between its top and bottom without breaking. $\endgroup$
    – Dan Is Fiddling By Firelight
    Commented Jun 2, 2021 at 18:48
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    $\begingroup$ @DanIsFiddlingByFirelight I think you can break that out as a separate answer, basically the solid rockets are stiff, uncooked pasta while their liquid fueled counterparts are al dente. $\endgroup$
    – uhoh
    Commented Jun 3, 2021 at 0:00
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    $\begingroup$ Air-to-air missiles are also often very skinny relative to their length. Technically still a rocket, just pointed horizontally most of the time. Since the only "cargo" these carry besides their own fuel is an explosive charge, there's no reason for them to be particularly thick, and every reason to keep them skinny so as not to add too much drag on the plane before they're launched. $\endgroup$
    – Darrel Hoffman
    Commented Jun 3, 2021 at 15:05

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