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This is a new subject for Space Industry – heavy spacecraft (1200-ton) in LEO that have to burn hundreds of tons of propellant to get going to their destination.

It looks inefficient to launch 7+ fuel tankers just to fill up SpaceX Starship in LEO and then burn 40% of Starship’s fuel in a first orbit change.

Could a light space tug with expendable solid fuel boosters assist Starship in LEO?

Solid rocket fuel has all kinds of disadvantages compared to a liquid fuel.

But would solid fuel have an advantage in the case of a light space tug (less than 5 tons dry mass) with a small liquid engine and attachments for a large solid rocket boosters (expendable)?

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The rocket equation tells us that solid rocket boosters are at a great disadvantage in this sort of comparison.

$$ \Delta v = I_{sp} \ g_0 \ \ln\left(\frac{m_0}{m_f}\right) $$

Let's make up some numbers and see how the SRBs stack up*.

Given:

  • dry mass of 150,000 kg
  • desired delta-v of 1000 m/s
  • liquid engine Isp of 350 s - rounding down
  • solid motor Isp of 250 s - rounding up

For the liquid engine, you need ~51,000 kg of propellant

For the solid motor, you need ~75,000 kg of propellant

There would have to be some massive* advantage to make lifting 50% more propellant into orbit worth it.

* pun intended

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  • $\begingroup$ That is a good answer. It is also consistent with Elon Musk' vision. $\endgroup$ Mar 3, 2023 at 17:50
  • $\begingroup$ @TheMatrixEquation-balance noting in the last paragraph, the one exception here is if you are not lifting the stuff into orbit from earth, see ALICE en.wikipedia.org/wiki/ALICE_(propellant). Still really only applicable for first stages on the body you start from, not in orbit tugging where you need to get your tug BACK after use. $\endgroup$ Mar 3, 2023 at 23:10
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    $\begingroup$ This reference researchgate.net/figure/… lists ISP for ALICE as 210sec, making utility of ALICE solid fuel boosters even more dismal than @Organic Marble's estimate. $\endgroup$
    – Woody
    Mar 4, 2023 at 15:43
  • $\begingroup$ @Woody thanks! the question was edited to add the Al-H2O thing after my answer was posted. $\endgroup$ Mar 4, 2023 at 15:56
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The answer here by Organic Marble explains the ISP math. This is a supporting answer on why the ISPs are different. High ISP comes from high exhaust velocity, and getting that is easiest with lighter elements. Hence nuclear thermal rockets normally assume pure hydrogen and liquid fuel tables starting with Hydrogen-Oxygen, then Methane-Oxygen (methane being mostly Hydrogen with some Carbon).

So making a high performance solid rocket involves trying to stuff as many light atoms in there as possible, with the difficulty that light atoms tend to make molecules that are not solids at room temperature, meaning otherwise valid solid rocket chemistry becomes a liquid mono-propellant instead (see proposal mentioned in Ignition by Clark for an Oxygen/Methane mono propellant).

The next challenge is mechanical, the solid fuel grain needs to stay put during handling and use, so most compounds need binders of sub optimal rocket performance to keep them in place. For space assembled solid rockets this might be less relevant.

Solid rockets are a subclass of mono propellants which means the single fuel grain needs to be an optimal fuel/oxidizer mix, which may be chemically complicated. Where a liquid engine can just tweak the pump pressures to tune things you cannot remove/add fractional hydrogen atoms from your compound. Composite propellants try to solve that by cocktailing compounds, but at the cost of adding lower performance compounds to the mix.

Finally the solid rocket needs to be stable enough to not burn or explode early. Hydrogen and Methane will happily explode with Oxygen, but can be used in rockets by careful injector and ignition design. Solid fuel needs to prevent detonation with chemistry rather than engineering.

All of this constrains solid rocket performance unless someone finds some really oddball chemistry, restricting solid rockets to applications where high thrust, shelf life or simplicity matter more than raw performance numbers.

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  • $\begingroup$ "GremlinWranger" - misgivings about solid rocket fuel are old and plentiful. But how do you see the alternative with hundreds of tones of cryogenic LO and Methane stored in LEO for long periods of time? Is it more reliable than solid fuels? $\endgroup$ Mar 4, 2023 at 1:16
  • $\begingroup$ Or, should SpaceX pay $200,000,000 for 1000-ton of Hydrazine (if they cannot guarantee cryogenic storage of LO and Methane in space? $\endgroup$ Mar 4, 2023 at 1:28
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    $\begingroup$ @TheMatrixEquation-balance the issues with solid rockets are not misgivings, they are physics. The problem is we live in a boring reality where cool stuff turns out to be hard. Writing this answer I did delete a section musing on what happens if you re-define 'solids' away from the military driven long shelf life/wide temp range to lower temperatures achievable just by using a sunshade in orbit and on the spot mixing, like ALICE. Is an area with little money spent so far (no military application) and might produce something relevant to your interests. $\endgroup$ Mar 4, 2023 at 1:56
  • $\begingroup$ "GremlinWranger" - Thank you. I did not know. "ALICE provides thrust through a chemical reaction between water and aluminum. As the aluminum ignites, water molecules provide oxygen and hydrogen to fuel the combustion until all of the powder is burned. "ALICE might one day replace some liquid or solid propellants, and, when perfected, might have a higher performance than conventional propellants," Pourpoint said. "It's also extremely safe while frozen because it is difficult to accidentally ignite. Aluminum particles 80 nanometers combust more rapidly... " $\endgroup$ Mar 4, 2023 at 2:54
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    $\begingroup$ "just by using a sunshade in orbit and on-the-spot mixing" and a machine for producing the aluminum and water, one for forming aluminum nanoparticles (under atmosphere, presumably, or they'll try to cold-weld together), purifying, mixing, packaging the mixture into a rocket, testing... I mean, it'll probably be a cool fuel someday, but it's got a lot of boring, hard stuff to get past first. $\endgroup$
    – Cadence
    Mar 4, 2023 at 8:52
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Others have pointed out the problems with performance of the solid propellant itself, but there are also logistical issues with actually applying that propellant to the problem of accelerating your payload.

Firstly, you can't adjust the propellant load in a solid motor, at least not without non-trivial changes to its performance and significant manufacturing work to assemble a variable-size booster. Once cast, a propellant grain has to be burned in one piece. Also, you have to launch that propellant in the form of boosters packed into the launch vehicle, with lots of overhead and some wasted capacity. Liquid propellants can just be launched and transferred in the quantity needed, filling the launch vehicle to capacity. With the Starship architecture, you may not even need additional tanks to hold them, just stretch the main propellant tanks of the tanker variant.

Second, solid rockets do have a limited shelf life. Yes, ICBMs can sit around for decades, but they have much higher acceptable failure rates. You won't be able to just stockpile generic solid booster segments in orbit without some eventually expiring and having to be discarded...hardly an efficient use of mass launched to orbit.

Then there's the actual problem of using them. Liquid propellants can just be transferred through a docking connection. A liquid fuel depot can be some variation of a tanker, a solid booster depot needs to be some kind of specialized orbital dock/spacecraft construction facility.

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