sevenperforce

Crap. I was looking at the videos to see the moment of BECO to try and figure out whether it was a flame out or a planned cut off. Didn't think about looking earlier.

Do we know this for sure? As far as I know, we have never filmed the reentry of a craft not meant to survive reentry from inside the craft. There is actually quite a bit of good information we could glean. Which modules survive the longest? Do lightweight craft self-stabilize, or do they tumble wildly? How far can the hull breach before it triggers catastrophic unplanned disassembly? Is there just one failure mode, or are there several, possibly complementary ones? Plus, if several modules were broken off and separated by a mile or twenty for staggered re-entry, we could set them up to film each other. It would be quite a ride....

Wow, I never would have imagined such an innovative and powerful use of sonic frequencies to achieve their goal. I should have known they were never gonna give up. The MCT is certainly not a let down! More logos running around the body never hurts. I thought realistic plans were deserted, but...say goodbye to doubt.

ALL HAIL APOLLO Seriously, there's a reason they call it a moon shot and not a moon dunk.

...good lord.

Wasn't really intending this particular dV map for use in the game; that's why I said it was for planning lunar missions. I could make the table for Kerbin easily enough.

You solve the vis-viva equation. Or, you just measure it. The table is the set of solutions to the vis-visa equations for the orbits provided. How would you measure it? Like, use a simulator or something?

Talk about guts. 'So you want me to climb into a metal cone on top of the biggest stack of explosives ever built, let it push me off the world and onto a gravity-defying loop around another world, then break my ship in two, turn upside in mid-air, ram my half back into the other half, pull off a piece of it, then flip back around and use a separate stack of explosives...these lethally toxic...to push me onto a collision course with the other world? Then you want me to turn off the explosives and wait until I'm really close, then turn the explosives back on and get moving fast enough to fall around the other world without hitting it, then get out of the metal cone and into the tin can I pulled out of the other half of the ship, break it off of my ship again, and then use a third set of explosives to slow me down until I'm back on a collision course with the other world? Then you want me to lay down and let a massive, clunky, slow computer control the flow of explosives to lower me down to the surface, then sit back up and use the explosives to manually steer this tin can to a landing on its tail while depending on some guys a hundred thousand miles away for almost all the information I need? And that's just the first half? Sure, when do we start?" And the idiot at my bar last night assured me that we never could have landed on the moon and it was all propaganda.

v = sqrt(mu/r) gives you circular velocity, sure. How is that going to help you find apsis speed for an elliptical orbit with one apsis at r? It doesn't. Finding the periapse and apoapse velocities of an elliptical orbit based only on the periapse and apoapse distances requires you to solve for specific orbital energy using the semimajor axis, add the negative gravitational potential for the point you're interested in, and then convert specific kinetic energy into speed. Only then will you know how much delta v a given maneuver will consume. These tables provide the velocity for you, and the dV map takes it a step further by subtracting the circular velocities appropriately to show transfer dV at each point. If you think there's a simpler way to do it, I'm all ears. Tell me how you would calculate dV to get from a 185x185km circular equatorial parking orbit onto GTO, and how much additional dV it would take to get from GTO to GEO. And sure, there will be variation in launch dV. That's why people would typically use quoted launch system payload capacity and start from LEO. But quoting estimated launch dV allows for comparative adjustment, so that's still useful while in no way detracting from the rest of the dV map.

Beautiful. Just beautiful. We will never replicate that. Landing manually on another world for the first time in a tin can using the crudest possible computers with virtually all mission control being processed through a three-second delay one hundred thousand miles away...unbelievable. No wonder they launched with such high fuel margins.

That's not necessarily the case...it can happen, but often it would end up in a non-atmosphere-crossing orbit. Unless the mass driver can eject pieces at greater than Earth escape velocity...or break it up into small enough pieces to not cause damage. But even laser ablation should be enough to use that 400 tonnes for quite a while.

You're missing the point: you don't know the velocity at the first apsis. If you need to do a Hohmann transfer from one circular orbit to another circular orbit, all you have is the distances. You don't have velocities at all.

Oh, I have no doubt. But for executing Hohmann transfers, you need to know periapse and apoapse velocities so that you know what delta-v to apply.

Packing that thing full of cameras would be a great idea. Each one going to a black box with a shield and an inflatable float. Heck, set it up with a few of Jeb's explosive decouplers set to go off right before the station crosses the Karman line, and then let the two space station pieces film each other on the way down. Or here's another idea. This might be crazy, but...that's 400 tonnes of potential reaction mass, already sitting there in orbit. I wonder how hard it would be to build a space tug with a thruster that can use space junk as reaction mass....

Two points. First of all, the numbers for transfers between various orbits are still entirely valid even if you don't use the numbers for launch. For example, if you're staging for a cislunar mission at the space station, then you can simply use your launch vehicle's stated ISS payload and start from there. Moreover, because the periapse and apoapse burns are shown separately, you can start or end halfway through a Hohmann transfer. If your launch vehicle can deliver 10 tonnes to GTO then you can still calculate the required GEO circularization burn without necessarily knowing the exact dV burned to get onto the GTO; if you're coming back from a cislunar mission and passing through EML-1 then you'll be able to estimate the apogee burn for aerobraking without worrying about the launch dV. Second, and more to the point, the surface periapse velocities given in the tables are decidedly not the same as the ones provided in the dV map. Rather than going through the excessive pain of adjusting for T/W ratio and atmospheric drag and calculating the ideal gravity turn and everything else, I've simply used an average figure for total gravity drag and atmospheric drag in launches -- 1.5 km/s -- and added that on to the putative Hohmann transfer perigee velocity when determining the values for the dV map. I also subtracted the 465 m/s rotational speed of the Earth at the equator for the best-case simplification. Thus, even though a launch vehicle obviously does not make an instantaneous horizontal burn equal to the Hohmann transfer perigee velocity at the launch pad, it will be as if it burned that amount of dV plus an additional 1.5 km/s in order to get onto a trajectory that can be circularized. That's what is shown on the dV map. Naturally, the launch dV requirements will be highly dependent on the launch vehicle; a narrow rocket with a high T/W ratio will be able to get away with lower launch dV than a wide rocket or a VTHL spaceplane with poorer thrust. But the launch dV values provided give a reasonably good estimate for the dV you need. In most cases, serious mission planning will involve the use of actual quoted figures for a given launch system, so you know what can be delivered to your initial parking orbit and you can plan from that starting point. The launch dV values are also useful for comparative reasons. If your launch vehicle can deliver a given payload a 100x100km LEO with 1200 m/s of remaining dV, then you can predict the remaining dV for a launch to the ISS by looking at the difference between the circularization dVs and the launch dVs. This works with both the table and the map. Launching from the surface directly to the ISS will cost you 88 m/s more in your initial burn and 86 m/s more in your circular burn than launching to 100 km would. This may or may not be more efficient than first launching to 100x100 km and then doing a Hohmann transfer up, depending on inclination changes. However, if it's something like a launch to EML-1, then you will have significant savings by launching directly from the surface compared to launching to 100x100 km and then doing a transfer. This will limit your launch window, though. All of these factors need to be considered. This just presents a common starting point.

Yeah, the graphic is very large. You can download the pdf version here. The dV numbers shown in red on the map are insertion burns which don't exactly match a specific Hohmann maneuver. For the ascent burns from Earth, I have factored in the Earth's equatorial velocity as well as gravity drag and atmospheric drag; these are all simplified best-case-scenario coplanar ascents. If you have a higher-latitude launch site or an inclination change then you'll need to increase your launch dV accordingly, but this can be done by simply adding the additional dV to the ascent dV shown. Same for on-orbit inclination changes; these can be calculated separately and tacked on. You should always do inclination changes at the highest apogee possible for maximum efficiency. For cislunar transfers, inclination changes at EML-1 are pretty miniscule. So if you want to launch from Cape Canaveral to a 100x100km parking orbit and then do a GTO transfer, you'll want to increase your dV to make up for your latitude change and inclination change appropriately. But someone could create a separate reference table showing the additional dV required for each launch site and each destination.

Apparently they were adjusting the mixture ratio mid-ascent -- I'm not sure why -- and ran out of oxidizer 5.4 seconds earlier than planned. Not sure whether it flamed out or if the computer automatically triggered BECO due to a low LOX volume message. I'll see whether the launch videos are available online...if so, comparing this launch to the prior launch should show a noticeable difference at BECO if it was a flameout...probably a hot fuel-rich flare. Does SpaceX use tighter margins? Yes. But with reusable first stages (or at least planned-reuse first stages), this would never happen; they have a comparably high volume of extra propellant left over for their landing attempt, so a launch anomaly can be compensated for with relative ease at the expense of narrower landing margins.

The colors are pink, purple, and pale blue; depending on your computer settings the pale blue may appear to have a greenish tinge. The italized axis labels represent varying apsides. When the apsis on the horizontal axis matches the apsis on the vertical axis, the orbit is circular and has only one speed; when the axes don't match, there is a high apsis (the apoapse/apogee) and a low apsis (the periapse/perigee). Because the axes have the same labels, each possible combination appears twice, once in the pink region and once in the blue region. But since this is now an elliptical orbit without constant speed, the minimum speed (at apoapse) is shown in blue while the maximum speed (at periapse) is shown in pink. For most coplanar orbits, the most efficient way to transfer between circular orbits at different altitudes (for example, to go from a 100 km circular parking orbit to the 400 km circular orbit of the ISS) is to enter an elliptical orbit with a perigee at 100 km and an apogee at 400 km, ride it up to its highest point, then transfer into a circular 400 km orbit with a circularization burn. Virtually all orbital maneuvers involve some sort of elliptic transfer. This table allows you to figure out how much speed you need to enter or to leave any of the given orbits. That's the thing; you don't necessarily have the velocity at a given apsis. If I'm in a 185x185 km circular parking orbit and want to send a satellite to geostationary orbit, I won't know how large of a burn I need. But glancing at the table, I see that the perigee speed for a 185km x 36,786km elliptical orbit is 10,268 m/s. My parking orbit speed is 7,793 m/s (shown in purple). So I need 3,275 m/s of dV to enter the geostationary transfer orbit. Once the satellite completes the transfer, I can look at the apogee speed for that same 185km x 36,786km orbit to figure out how much additional velocity the satellite will need in order to circularize to a 36,786x36,786 km geostationary orbit. So it's not just about finding perigee speed for a given apogee (or vice versa); it's about calculating the energy requirements for every different transfer or maneuver. The dV map I linked to shows all this even more clearly; it was just too large an image to embed here.

I got tired of doing a complex set of calculations from scratch every time I wanted to find the dV of a given Hohmann transfer, so eventually I sat down and made an excel calculator to do it for me. Which led to this: And this: These are reference tables for periapse and apoapse velocities for Hohmann transfers between numerous orbits of interest around the earth and the moon. They should be pretty self-explanatory. These won't give you dV directly; instead, you have to subtract your current velocity from your target velocity. So if you're at a low orbit and want to go up to a higher one, subtract your circular-orbit velocity (in purple) from the periapse velocity (in pink) matching your orbital altitude to the target altitude and execute that burn. Once you reach the apoapse of the Hohmann transfer at your target altitude, subtract your new velocity (in blue, matching the new altitude to your starting altitude) from your target orbit's circular velocity (in purple) and execute that burn to circularize. To drop to a lower orbit, do the same thing in reverse. The EML-1 and EML-2 points are reference for an orbit at that distance; the perigee burn is the same, but the apogee burn needs to match the lunar-circular velocity instead. Actually that's not perfectly correct (since it matches period not speed), but I'm using patched-conic anyway so it's close enough. I've made a correction to the original so that the EML-1 and EML-2 circular velocities are the period-matching velocities rather than the reference velocities for an orbit at that distance. On the lunar side those points are stationary so you don't have to match velocity at all, In the lunar reference table, the circular velocities for EML-1 and EML-2 are for orbits with that distance but at other points; if you are actually reaching one of these points, you just kill your elliptic-orbit apolune velocity. Then I decided to go ahead and create a complete dV map for all major cislunar transfers. I can't attach it here but I posted it at the following link: http://forum.nasaspaceflight.com/index.php?topic=39942.0 Here's a reduced-size version of the dV map; if you want the full-size version, you'll have to go to the link above or click here.

I'm pretty sure an airbreathing LANTR pushing liquid hydrogen at launch would have enough T/W to take off vertically. Then you could slowly dial down the propellant flow as airflow took over, use that to get up as high as it will allow, then switch back to pure hydrogen.

Well, it could give us a rough idea of how fast the stage must have been going. Though obviously the stage will splat at lower speeds than the deck will boom, so...

I was thinking more of something like a negative lift-to-drag ratio -- having an aeroshell that would passively re-orient after dropping below a certain speed, but do so at an angle that converted a portion of the drag into negative lift and thus circularize. It wouldn't get you entirely out of the atmosphere, but it might be circular enough that a dedicated space tug could match trajectory near apogee, couple with the payload, and then raise perigee. That would avoid needing any sort of correction on the payload itself. You could use spin-stabilization to keep the payload in a low-drag orientation through the lower atmosphere, designed in such a way that the angular momentum would decay at around 70 km and allow it to tumble into the higher-drag negative-lift orientation.

Hmm. Strictly speaking, the last point where the orbital velocity changes would be approximately where it crosses the Karman line. Until then, it is highly subject to drag. Is there a way to design a projectile which will passively re-orient such that a significant portion of its radial velocity is converted to tangential velocity?

Yeah, the weight of the stage doesn't really matter. This is one of the time when the difference between weight and mass actually makes...a difference. Instead of static weight, we are looking at two things: momentum and kinetic energy. If you want to know (roughly) whether the deck was going to give way, you would take the momentum (mass times velocity) and divide by the "crunch time" (i.e. the time it took the stage to crumple to a stop) to get the impulse force. Then you can divide by average contact area to get the pressure. This depends on some assumptions but can provide a first-order estimate of whether the forces exceeded the load bearing capacity of the deck. Now, to determine how far the stage would penetrate, you need to consider kinetic energy. In cases where there is little cohesion in the target material and the impactor is massively supersonic (e.g. a meteor impact or an antitank missile hitting a ceramic plate), you can use Newton's approximation, but that's not the case here. Instead, divide the kinetic energy of the stage by the compressive strength of the deck, then divide by the impact area. This gives you the distance across which a given amount of force produces work equal to the kinetic energy. Of course some kinetic energy is absorbed by the crumpling stage but this is still just first order.

Well, if he has a forcefield capable of containing quantum neutron degeneracy pressure, it ought to be trivial for him to open up one end of said forcefield and create a rocket with a specific impulse of three million seconds. Simulations suggest that there is a small set of proton+neutron combinations a little larger then unbiseptium which have half-lives of minutes-to-days. The simulations are consistent with what we have currently created, but we haven't gotten any further than unbiseptium (118 IIRC) so far.