> However, two rockets both cost the same to build and one needs less fuel to orbit that directly translates into more payload to orbit. It’s a question of mass

> However, two rockets both cost the same to build and one needs less fuel to orbit that directly translates into more payload to orbit. It’s a question of mass not money. The way you state this is insufficiently constrained as to make the comparison I think you are trying to make. I think what you are trying to suggest that is that if two launch systems have the same per-launch recurring costs and the same gross mass the one that has higher payload mass fraction is better. Well, I can't disagree with the rocket equation. If you are trying to relate this back to air-launch, my point is that for equivalent technology levels (meaning the same propulsion system other than nozzle area ratio, tankage with similar structural efficiency, etc.), the benefits you get in terms of recurring cost per unit mass from air-launch (by reducing rocket propellant consumed and dry mass of expended hardware) pale in comparison to the operating economics of having to additionally operate the air-launch platform. In other words, the recurring cost per unit mass to orbit, for the equivalent mass class, will be higher for air-launch than ground launch. And that doesn't even take into account the non-recurring costs associated with the air platform and need to amortize them. > Alternatively, you can spend that budget on something else like a ‘single stage’ to orbit system which more directly lowers costs. I'm not even sure where "directly lowers costs" applies in the SSTO case. I'm guessing you mean the case of a fully-reusable SSTO vehicle? At the end of the day, your per-fligt costs will reflect the need to amortize the non-recurring costs you've incurred. SSTO systems will tend to very high non-recurring costs, and the extreme sensitivity to dry mass means you will pay more per pound of dry mass for an SSTO vehicle than a multistage vehicle. The only people that think SSTO with today's technology will lower costs are the ones still hoping for the VentureStar to become operational. > As to long term costs, having already built the aircraft that becomes a sunk cost. It might have been a poor choice, but it represents actually value before you can get satilite stock orbit. Which then allows a company to raise more money on better terms. I'm not sure what you're trying to say here. If Strato can find a net cashflow-positive way to use their carrier aircraft, great. I think it's unlikely their launch economics will beat other systems in the marketplace so it won't win on pure cost short of Strato taking a loss on launches. However, in terms of launch, they may get some business if a customer really needs the logistical flexibility mentioned in the root post of this thread.