Hi,I would like to understand better which trajectory the Falcon 9 follows during ascent. During the launch it is evident that the trajectory bend quite soon and become almost tangential. This saves a lot of fuel from gravity drag. The problem is that if you want to RTLS, the additional horizontal component of the velocity must be compensated for, inverted etc. This would cost a lot, really a lot, in terms of fuel. You really need to do an almost vertical launch (at least in the first 100 km) to RTLS. Alternatively, you need much higher exhaust velocity to reach orbital speed at the orbital altitude. So, I was wondering if someone has a reference or something for the trajectory actually used by Falcon 9 v1.1 before first stage separation in particular before first stage separation. Something relating the altitude with the angle respect to the vertical. By mean of the other published parameters (thrust, weight, etc.) it would then be possible to calculate the complete flight parameters.Thanks
You really need to do an almost vertical launch (at least in the first 100 km) to RTLS.
ISTM that the more vertical the first-stage trajectory, the easier the first-stage recovery.However, ISTM that the intention should be to give the best chance of delivering the payload to orbit, and this ends up with the opposite constraint. For a very light payload, the first stage can afford a *very* flat trajectory, and still have prop left to achieve an RTLS, despite needing a huge boost-back. In the event of a first-stage issue pre-MECO, this will maximise the chance that second stage can achieve the contracted orbit, at expense of the first stage abandoning hope of recovery.As payload size increases, the overall vehicle has much less margin, and at the limit of reusability ISTM it must fly a more lofted trajectory which both delivers the needed dV from 1st & 2nd stages, while leaving just enough prop to RTLS the first stage.So, ISTM the requirement to maximise chance of payload-to-orbit implies the same un-lofted trajectory as disposable for very small payloads, with lofting increasing where payloads are prepared to trade a higher mass for a greater chance of failure during second-stage burn.cheers, Martin
that's exactly the kind of information I would like to find to have a better estimate through the Tsiokolsky equation. Can you please tell me where to find them?
I think people underestimate the propellant needed for a RTLS. The real challenge is the RTLS, not the powered descent, because the V_hor component.
However, when you add a not negligible horizontal component of the velocity, whatever you do you have to zero it after staging, and then go back to return to site. This is a 2 x DV_hor, where DV_hor is the horizontal component of the velocity at staging.
Frankly speaking I do not see that. Whatever you do you have to revert the horizontal speed compoent because the speed at lift-off is 0, and the speed at landing must be again 0 (in the rest reference frame). It's a matter of kinetic energy conservation. Can you explain better your point please?thanks for your time, guys.
You are right guckyfan: I wrote "DV_hor", as delta-v horizontal component, but actually wrote several times "horizontal component of the velocity". What I meant was delta. Thanks for noticing.
I think the error he was pointing out was that you implied that you need to build up the same horizontal velocity for the RTLS as you have going down range. That is not so, is the F9 stages at Mach 6, you may only need to go Mach 1 or 2 on the return and take longer to do it.
What really matters is the delta-v in rocket propulsion. Specially when the gravitational drag must not be taken in account (and we are talking about the horizontal velocity). So, in terms of propellant to be used for such a maneuver, it doesn't matter if you build up speed slowly or not or if you have more or less time.