# 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?

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?

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?

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
• Liquid-fueled counterexample: astronautix.com/s/serv.html – Organic Marble Jun 2 at 3:24
• @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! – uhoh Jun 2 at 3:30
• @RussellBorogove ditto – uhoh Jun 2 at 3:30
• 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? – Tim Randall Jun 2 at 15:11
• 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. – jamesqf Jun 2 at 17:21

• +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. – uhoh Jun 2 at 3:45