Case Study: Why the World's Comsats Fly from the Equator

"Real estate is about location, location, location. So, it turns out, is spaceflight."

Executive Summary

For decades, the operators of the world's most valuable satellites — geostationary communications platforms — paid a premium to launch from a mosquito-ridden coastal town in South America. The reason is pure geometry: the Guiana Space Centre at Kourou sits at just $5.2^\circ$ north latitude, and that low latitude buys a geostationary satellite hundreds of kilograms of propellant it would otherwise have to carry. In this case study we audit that advantage. We take one realistic $3{,}500\ \text{kg}$ GEO communications satellite and ask what it costs to put it on station from three real launch sites at very different latitudes — Kourou ($5.2^\circ$), Cape Canaveral ($28.5^\circ$), and Baikonur ($45.9^\circ$) — using nothing but the launch-site geometry of §30.3 and the rocket equation. We will find a difference of well over a tonne of launch mass between the best and worst sites, quantify exactly where it comes from, and then see how reusability is beginning to change the calculus.

Skills applied

• Computing the eastward rotation credit and minimum inclination from launch-site latitude (§30.3).
• Combining a plane change with circularization at GTO apogee (§30.3; Ch. 10).
• Converting a delta-v penalty into a propellant mass via the rocket equation (Ch. 3).
• Reading a real industry pattern as an engineering consequence, and knowing the Tier of every number.

Background

The satellite and the maneuver

A geostationary communications satellite must end in a circular, equatorial ($i = 0^\circ$) orbit at $35{,}786\ \text{km}$ altitude. Launch vehicles do not deliver it there directly; they drop it into a geostationary transfer orbit (GTO) — an ellipse with perigee in LEO and apogee at GEO altitude — and the satellite's own apogee engine finishes the job. That final burn does two things at once: it raises the perigee to circularize at GEO, and it removes whatever orbital inclination the launch site imposed. Both come out of the satellite's own propellant tanks, so both are the satellite operator's problem, not the launch provider's.

The key numbers, from Chapter 9 and vis-viva (Chapter 6): GTO apogee speed $v_a \approx 1.60\ \text{km/s}$, GEO circular speed $v_{\text{GEO}} \approx 3.07\ \text{km/s}$. The combined circularization-plus-plane-change burn at apogee costs

$$ \Delta v = \sqrt{v_a^2 + v_{\text{GEO}}^2 - 2\,v_a\,v_{\text{GEO}}\cos i}, $$

where $i$ is the launch (GTO) inclination — which, at best, equals the launch-site latitude $\phi$.

All satellite masses and the apogee-engine $I_{sp}$ below are illustrative (Tier 3), chosen to be realistic for a mid-size comsat; the site latitudes and the $0.465\ \text{km/s}$ rotation figure are Tier 2. The point is the pattern and its magnitude, not any one satellite.

Phase 1: The launch-site scorecard

For each site, compute the eastward rotation credit $0.465\cos\phi$ and the minimum GTO inclination (= latitude):

Two separate advantages already point the same way: the equatorial site gives slightly more free launch velocity and a far smaller inclination to clean up later. For a GEO mission the second effect dominates, so let us price it.

Phase 2: The plane-change-plus-circularization cost

Plug each site's inclination into the combined-burn formula (with $v_a = 1.60$, $v_{\text{GEO}} = 3.07\ \text{km/s}$):

• Kourou ($i = 5.2^\circ$): $\Delta v = \sqrt{1.60^2 + 3.07^2 - 2(1.60)(3.07)\cos 5.2^\circ} = \sqrt{2.56 + 9.42 - 9.78} = \sqrt{2.20} = 1.48\ \text{km/s}.$
• Cape Canaveral ($i = 28.5^\circ$): $\Delta v = \sqrt{2.56 + 9.42 - 9.82\cos 28.5^\circ} = \sqrt{11.98 - 8.63} = \sqrt{3.35} = 1.83\ \text{km/s}.$
• Baikonur ($i = 45.9^\circ$): $\Delta v = \sqrt{2.56 + 9.42 - 9.82\cos 45.9^\circ} = \sqrt{11.98 - 6.84} = \sqrt{5.14} = 2.27\ \text{km/s}.$

Sanity check: with $i = 0$ the formula gives $3.07 - 1.60 = 1.47\ \text{km/s}$ — the pure circularization cost. Kourou sits just above it; the Cape and Baikonur climb steadily as the plane change grows. The ordering (equator cheapest, high latitude dearest) is exactly what §30.3 predicts.

Phase 3: Turning delta-v into propellant

Now the payoff. The satellite arrives on station with a dry-plus-payload mass of $m_f = 3{,}500\ \text{kg}$ and burns an apogee engine of $I_{sp} = 320\ \text{s}$, so $v_e = 320 \times 9.81 = 3{,}138\ \text{m/s} \approx 3.14\ \text{km/s}$. The propellant it must carry is $m_p = m_f\,(e^{\Delta v/v_e} - 1)$:

Read the last column. To place the same $3{,}500\ \text{kg}$ satellite on station, a launcher must throw about $5{,}610\ \text{kg}$ to GTO from Kourou, $6{,}270\ \text{kg}$ from the Cape, and $7{,}210\ \text{kg}$ from Baikonur. The high-latitude site demands roughly 1.6 tonnes more mass to GTO for an identical delivered satellite — mass that is either a bigger, costlier rocket or a heavier satellite full of propellant it will burn just to fix a problem the launch site created.

Phase 4: The audit's verdict, and its limits

The geometry is decisive and it explains history. For decades Ariane, flying from equatorial Kourou, dominated the commercial GEO launch market precisely because it handed customers this latitude discount; Russia's Proton, flying from high-latitude Baikonur, fought the penalty audited above and even inspired sea-based, on-the-equator launch ventures to escape it.

But an audit must state what it leaves out, and reusability is rewriting the bottom line:

• Supersynchronous GTO. A launcher with margin to spare can drop the satellite at an apogee above GEO, where it is moving even slower and the plane change is cheaper still; the satellite then descends to GEO. SpaceX uses this to blunt Cape Canaveral's latitude penalty — trading launch-vehicle performance for satellite propellant.
• Price, not just physics. A reusable Falcon 9 launching from the "disadvantaged" Cape can still underprice an expendable rocket from the "advantaged" equator. The latitude penalty is real and worth hundreds of kilograms — but if reuse cuts the launch price by more than that penalty costs, the customer flies from Florida anyway. Physics sets the penalty; economics decides whether it is the binding constraint.

🔧 Engineering Reality: Our clean model gave each site its best-case inclination (= latitude). Real sites are often worse: range-safety corridors can forbid a due-east launch, forcing a higher inclination than latitude alone would imply (Baikonur's actual GTO inclinations run higher than $45.9^\circ$ for exactly this reason). The direction of every correction is the same — more inclination, more penalty — so our audit understates the high-latitude disadvantage rather than overstating it.

Discussion Questions

1. The eastward rotation credit differs by only ~0.14 km/s between Kourou and the Cape, but the GEO propellant difference is far larger than that gap would suggest. Which of the two latitude effects is doing the real work for a GEO mission, and why?
2. Why is the plane change performed at GTO apogee rather than at perigee or during launch? (Recall Ch. 10.)
3. Baikonur's real GTO missions are penalized even more than our $45.9^\circ$ model shows. Give the physical reason, and say whether it makes the audit's conclusion stronger or weaker.
4. If a reusable rocket from the Cape is $30\%$ cheaper per launch than an expendable one from Kourou, how would you decide which to choose for a specific comsat? What would tip the decision each way?

Your Turn: Extensions

• Option A (analysis). Redo Phase 3 for a heavier $6{,}000\ \text{kg}$ on-station satellite. Does the absolute propellant penalty between Kourou and the Cape grow, shrink, or stay the same? Explain via the mass ratio.
• Option B (computation). Write a function gto_to_geo_dv(i_deg, va=1.60, vgeo=3.07) and a companion apogee_propellant(mf, dv, ve=3.138), and reproduce the Phase-2 and Phase-3 tables. Do not run it — hand-trace and add # Expected output:.
• Option C (design). Your Track-A comsat masses $2{,}200\ \text{kg}$ on station. Compute its GTO→GEO propellant from both Kourou and the Cape, add the results to your MDR, and state which launch site you would prefer and why.

Key Takeaways

1. Launch-site latitude is worth real propellant for a GEO mission, mostly through the inclination it forces the satellite to remove, not the rotation credit.
2. The penalty compounds through the rocket equation: ~0.35 km/s more from the Cape than Kourou becomes hundreds of kilograms of satellite propellant; ~0.79 km/s more from Baikonur becomes well over a tonne.
3. Equatorial launch historically dominated the GEO market for exactly this reason — geometry, audited, explains the industry map.
4. Reusability now competes with geometry: supersynchronous transfers and cheap reused boosters can outweigh a latitude disadvantage, so the binding constraint is increasingly price, not place.