tag:blogger.com,1999:blog-3784553273521533552.post3953399382145569286..comments2023-05-05T01:47:38.139+10:00Comments on Casey ex Australia: SpaceX Mars plan analysisCasey Handmerhttp://www.blogger.com/profile/10894614796135316267noreply@blogger.comBlogger12125tag:blogger.com,1999:blog-3784553273521533552.post-82845779023690737772016-10-02T09:37:10.794+11:002016-10-02T09:37:10.794+11:00I think this code started life as an orange cannon...I think this code started life as an orange cannon trajectory modelling system! <br /><br />That said, there's more to the physics than that. Both drag (and hence lift) and centrifugal force scale with v^2. So the most important handle is ambient density. But in the regimes we're talking about, the entering vehicle is pulling 4-8gs, Mars contributes 0.38gs, and the centrifugal term may be as high as 1.5gs, so I think there's still a good deal of headroom when it comes to the heating constraint.<br /><br />The heating problem was the one part of the SpaceX presentation I thought looked a bit funny, as a 2000K max heat shield temp seems a bit low.<br /><br />The aerodynamic numbers are somewhat worse than shuttle. Closer to AMaRV, and bracketed with 100% variation in the parameter. I also turned the mass up as high as it would go.<br /><br />If you're creative enough with the lift function, you can get my code to do a skip entry. Depending on the heat shield material, you may want to avoid more than one swing in temperature. Aerobraking sometimes confers a delayed-action heat pulse that starts making trouble. I think the ablators, like PICA-X, partially side step this issue.Casey Handmerhttps://www.blogger.com/profile/10894614796135316267noreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-39440220870722360722016-10-02T07:35:31.880+11:002016-10-02T07:35:31.880+11:00I thought so. Don't worry, forgetting centrifu...I thought so. Don't worry, forgetting centrifugal terms when extrapolating from atmospheric to orbital flight is a common mistake. And by common, I mean I've made it 3 times in a professional setting. A good rule of thumb is that if your velocities are being expressed in km/s, then you should include centrifugal terms.<br /><br />If you got those aerodynamic numbers from shuttle, then I would say they are optimistic. For a wingless blunted vehicle of this size, I would recommend using values closer to the DRM lifting body:<br /><br />C_d = 2.5<br />C_l = 1.4<br />L/D = .56<br /><br />I think you will find that this also changes your integrated thermal load to unrealistically high levels. It is for this (and other) reasons that I strongly suspect the vehicle will be making at least one braking orbit before entering.Max Faginnoreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-71646390128963812912016-10-02T07:06:31.406+11:002016-10-02T07:06:31.406+11:00I will update the code shortly. Centrifugal terms ...I will update the code shortly. Centrifugal terms added. Regarding inverted flight at 7000m/s, given the IMS numbers, a Cd of 1.5 and a Ld of 0.88, which is reasonable for a lifting body like this, the atmosphere is thick enough to maintain horizontal flight below about 40km. Casey Handmerhttps://www.blogger.com/profile/10894614796135316267noreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-77029347310608379692016-10-02T06:47:59.305+11:002016-10-02T06:47:59.305+11:00Yep, the world is flat. I'll throw in centrifu...Yep, the world is flat. I'll throw in centrifugal terms and see what happens.Casey Handmerhttps://www.blogger.com/profile/10894614796135316267noreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-48475977235463265322016-10-02T06:44:29.759+11:002016-10-02T06:44:29.759+11:00Sustained level flight at higher than circular orb...Sustained level flight at higher than circular orbital velocity doesn't just require bank modulation, it requires bank modulation while flying inverted. And you show the vehicle capable of doing so at altitudes as high as 30 km. We may be assuming different maximum lift values, but as far as I can tell, that is impossible. At that altitude and speed, the vehicle can't generate enough lift to avoid skipping, even by flying with the lift vector fully down.<br /><br />As such, I was wondering if you might be using simplified EOMs that omitted centrifugal terms.Max Faginnoreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-50096599666128620532016-10-02T04:50:40.917+11:002016-10-02T04:50:40.917+11:00There's no aero going on under the hood, and n...There's no aero going on under the hood, and no aerodynamic inertia either. What do you mean by impossible?<br />The EOMs are on line 5ish, called "eqn" or something like that.Casey Handmerhttps://www.blogger.com/profile/10894614796135316267noreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-37624479114532542652016-10-01T19:28:53.856+10:002016-10-01T19:28:53.856+10:00I don't see your EOMs explicitly listed anywhe...I don't see your EOMs explicitly listed anywhere in your code, so I can't be sure, but it looks to me like you are running a 2D planar simulation well outside the range of its validity. For example, you are showing ITS engaging in level flight at 20 km and 6000 m/s. That's impossible for this vehicle, even by flying inverted. Can you confirm whether or not these charts were generated from planar or spherical EOMs?Max Faginnoreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-63779698623578347622016-10-01T10:00:54.755+10:002016-10-01T10:00:54.755+10:00I think if you have a fission engine, or warp driv...I think if you have a fission engine, or warp drive, there's not much you can't do in space. But the specific thrust of a fission engine probably isn't good enough for launch, and there's also the niggling issue that they have a very low TRL.Casey Handmerhttps://www.blogger.com/profile/10894614796135316267noreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-18976900225349116392016-10-01T08:13:05.418+10:002016-10-01T08:13:05.418+10:00I find myself stunned, even now, that this venture...I find myself stunned, even now, that this venture is being forecast using chemical rockets. It's utter madness, and the description of the Raptor and aerobraking in this article only reinforced my predudice. A fission thruster is required for the transfer stage, that can also detach as a lander, leaving the habitat in LMO. It can then collect reaction mass on the surface quickly and easily.Such a machine could make multiple journeys between the surface and orbiting habitat.adearthichttps://www.blogger.com/profile/13063618488208442643noreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-194300674104862572016-10-01T00:52:54.991+10:002016-10-01T00:52:54.991+10:00Dr. Handmer.
Hello. I would be interested in confe...Dr. Handmer.<br />Hello. I would be interested in conferring with you more on EDL. Please find me at your leisure FB/Linkedin: Donald C Barker (Houston).<br />Cheers.Donald Barkerhttps://www.blogger.com/profile/17265502147916360466noreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-84965507880857055882016-09-29T14:32:10.223+10:002016-09-29T14:32:10.223+10:00Heat shield all around! I would hope it has some d...Heat shield all around! I would hope it has some dihedral for stability, but anything is possible.Casey Handmerhttps://www.blogger.com/profile/10894614796135316267noreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-38930012245935941102016-09-29T14:08:20.750+10:002016-09-29T14:08:20.750+10:00I wonder if they plan to rotate during aerobrake t...I wonder if they plan to rotate during aerobrake to spread the energy around the entire rocket.Darrin Taylorhttps://www.blogger.com/profile/03179252192582355933noreply@blogger.com