Calling Hohmann from a public URL
On 13 August 2026 I pointed a client at https://sidus.tools/api/mcp and called hohmann with r1_m = 6578137 and r2_m = 42164137. Those two radii describe a circular transfer from about 200 km LEO to GEO, computed with the educational Earth μ and kept in SI the whole way. The tool answered with a first burn of dv1 2454.59 m/s and a second burn of dv2 1477.27 m/s, for a total of 3931.86 m/s. The time of flight came back as 18931.92 s (about 5.3 hours), with a transfer semi-major axis of 24371137 m. The same response carried v1 7784.26 m/s and v2 3074.66 m/s.
sidus.tools is a page where you enter the inputs and get a space-engineering number in SI, with the formula next to it. If you type the same two radii into the browser, the page also returns 3931.86 m/s, because the page and the public URL import the same function. The physics lives in one place.
One function behind the page and the URL
The number above is an educational two-body Hohmann transfer. The same function produces it whether a person types the radii into the page or a client sends them to the URL. If Hohmann is wrong in one place, it is wrong in both, so you fix it once. I published sidus.tools so these checks would live in SI, both on a page I could read and on a URL a client could hit without a second copy of the physics quietly drifting away from the first.
What SIDUS is
SIDUS (from the Latin sidus, a constellation) is a free catalog of educational space-engineering checks in SI. It is an independent project, with no agency behind it. As of 4 September 2026 the live catalog holds 201 tools across orbital mechanics, propulsion, RF link budgets, launch, crew life support, and the smaller geometry around them. You do not need an account to use any of them. The source is at github.com/massimodeluisa/sidus-tools. The card at the top of this article is the live Open Graph image from the site.
I write software, and I am not a space engineer. The formulas and any code a page exports should be reviewed and tested before you rely on them, because every tool in the catalog is an educational check. Each page shows its formula next to the number, which means that review can start on the same screen as the result.
Models small enough to hold in your head
The defaults describe an educational WGS-84-class Earth, with μ⊕ = 3.986004418×10¹⁴ m³/s² and an equatorial radius of 6 378 137 m. The models built on those defaults stay small on purpose: two-body motion, impulsive burns, circular Clohessy-Wiltshire, the ideal rocket, and ISA aero. A model of that size lets you hold the assumption in your head while you read the result.
Two-body pages cite Vallado’s Fundamentals of Astrodynamics and Applications and Curtis’s Orbital Mechanics for Engineering Students. A tool does not ship until its citation is a real HTTPS URL, so each formula leads back to a source a reader can open. Units stay in SI everywhere. If you want kilometres, you convert on purpose, because that conversion is a decision and I want it visible. A page can also export the same number into code you compile. If the two disagree, I have a bug.
Hohmann is one page among many
The transfer at the top of this article is one page in the catalog. Next to it sit Lambert, SGP4, a rocket equation, an RF link budget, cabin atmosphere, and an Earth-Mars sketch, each with its own inputs and its own formula on the page. On any of them you can disagree with the model, see the formula it uses, and call it again with different numbers.
Calling it from an agent
The same catalog is available at https://sidus.tools/api/mcp over Streamable HTTP, with no API key. A GET request returns discovery. A POST request is the tool call, which is how the hohmann result at the top of this article came back. You do not need to clone the repository to use it.
If you want the catalog as an agent skill, install it with this command:
npx skills add massimodeluisa/sidus-tools --skill sidus
The skill is listed at skills.sh/massimodeluisa/sidus-tools/sidus. If a default is wrong, the page has an edit link on GitHub.
References
- Vallado, Fundamentals of Astrodynamics and Applications, Microcosm Press. microcosmpress.com/vallado
- Curtis, Orbital Mechanics for Engineering Students, Elsevier. shop.elsevier.com