Frequently Asked Questions

Is Delta-V Desk free to use?

Yes. Every calculator and guide is free, with no account or download. The site is funded by advertising, and all calculations run in your browser.

Which constants do the calculators use?

Earth uses a gravitational parameter of 398,600.4418 km³/s² and the WGS-84 equatorial radius of 6,378.137 km. Other bodies use published gravitational parameters and mean radii, standard gravity is 9.80665 m/s², and planetary orbits use mean semi-major axes and sidereal periods. Each result shows the values it used.

Why does my textbook give a slightly different answer?

Usually because of constants. Many books use Earth’s mean radius of 6,371 km, a rounded μ or g = 9.81 m/s². Escape velocity at the surface is 11.180 km/s with the equatorial radius and 11.186 km/s with the mean radius. Differences in the third or fourth significant figure almost always come from this.

Do I enter altitude or distance from the centre?

Altitude above the surface, in kilometres. The calculators add the radius of the body for you. Entering a radius instead counts the planet twice and is the most common cause of a wrong answer.

Why are higher orbits slower?

Gravity weakens with distance, so less speed is needed to keep falling around the planet. Circular speed is about 7.67 km/s at 400 km and 3.07 km/s at geostationary altitude. Getting to the higher orbit still costs more energy, because the climb works against gravity.

Do the results include air drag and gravity losses?

No. The tools give ideal two-body values with instant burns. Reaching low Earth orbit needs about 9.4 km/s of Δv in practice, roughly 1.5 to 2 km/s more than the orbital speed, because of gravity losses, drag and steering.

Are plane changes included in the Hohmann transfer?

No. The Hohmann calculator assumes both orbits lie in the same plane. A plane change costs 2v sin(Δi/2) on its own and is cheapest when combined with a burn at the slowest point of the orbit. Our geostationary transfer guide works one through.

How accurate are the Mars launch window figures?

They are idealised values for circular, coplanar orbits: a synodic period of 779.9 days, a transfer of 258.9 days and a phase angle of 44.3°. Real windows vary by weeks because Mars’s orbit is noticeably elliptical. Mission planners use porkchop plots built from ephemerides.

Can I use the rocket equation calculator for a model rocket?

Only to understand mass ratios. It gives ideal Δv in space and does not predict altitude, which for a model rocket is dominated by drag and the motor’s thrust curve. For flight safety, follow your national rocketry association’s safety code.

Can I use these tools for a real satellite or mission?

No. They are educational and use simplified models. Operating real spacecraft requires precise orbit determination, numerical propagation and review by qualified engineers.

Are my inputs stored or sent anywhere?

No. Values you type stay in your browser and disappear when you close the page. See the Privacy Policy for the routine server logs and advertising cookies that apply to page visits.

Can teachers use the site in class?

Yes. Students need no accounts, the default inputs reproduce the worked examples in each article, and you may share results and quote short passages with a link.

How do I report an error or suggest a calculator?

Email logan.luo@adsnova.cn with the calculator, your inputs and the result you expected, or with the tool you would like us to add.