Delivering broadband connectivity to unconnected areas is extremely challenging. The emergence of Low Earth Orbit (LEO) satellite systems has been seen as a potential solution for connecting remote areas where engineering terrestrial infrastructure is prohibitively expensive to deploy. Despite the hype around these new technologies, we still lack an open-source modeling framework for assessing the techno-economics of satellite broadband connectivity which is therefore the purpose of this paper. Firstly, a generalizable techno-economic model is presented to assess the engineering-economics of satellite constellations. Secondly, the approach is applied to assess the three main competing LEO constellations which include Starlink, OneWeb and Kuiper. This involves simulating the impact on coverage, capacity and cost as both the number of satellites and quantity of subscribers increases. Finally, a global assessment is undertaken visualizing the potential capacity and cost per user via different subscriber scenarios. The results demonstrate how limited the capacity will be once resources are spread across users in each satellite coverage area. For example, if there is 1 user per 10 km 2 we estimate a mean per user capacity of 24.94 Mbps, 1.01 Mbps and 10.30 Mbps for Starlink, OneWeb and Kuiper respectively in the busiest hour of the day. But if the subscriber density increases to 1 user per km 2 , then the mean per user capacity drops significantly to 2.49 Mbps, 0.10 Mbps and 1.02 Mbps for Starlink, OneWeb and Kuiper respectively. LEO broadband will be an essential part of the connectivity toolkit, but the results reveal that these megaconstellations will most likely have to operate below 0.1 users per km 2 to provide a service that outcompetes other broadband connectivity options. The open-source codebase which the paper contributes is provided with the hope that other engineers will access, use, and further develop the satellite assessment capability.