Quote from: the_other_Doug on 12/27/2015 01:35 amAlso, I believe the entry burn is designed to create a plasma sheath between the atmospheric heating region and the octoweb. In other words, it acts to provide protection against entry heating. And trust me, at the altitude from, and speed at which the first stage comes in, there is significant entry heating. The entry burn keeps that from affecting the stage itself, without needing to cover the base of the stage with some additional TPS rated for entry heating.That's likely, plus I believe it also provides stability in a region where the fins are still not effective.This is the biggest piece of secret sauce they developed. When and how much. They were lucky and got it to work on the first try (CASSIOPE IIRC) but then made it work on the subsequent launch that had much lower margins... And it has worked on each and every subsequent launch.
Also, I believe the entry burn is designed to create a plasma sheath between the atmospheric heating region and the octoweb. In other words, it acts to provide protection against entry heating. And trust me, at the altitude from, and speed at which the first stage comes in, there is significant entry heating. The entry burn keeps that from affecting the stage itself, without needing to cover the base of the stage with some additional TPS rated for entry heating.
Quote from: meekGee on 12/27/2015 04:12 amQuote from: the_other_Doug on 12/27/2015 01:35 amAlso, I believe the entry burn is designed to create a plasma sheath between the atmospheric heating region and the octoweb. In other words, it acts to provide protection against entry heating. And trust me, at the altitude from, and speed at which the first stage comes in, there is significant entry heating. The entry burn keeps that from affecting the stage itself, without needing to cover the base of the stage with some additional TPS rated for entry heating.That's likely, plus I believe it also provides stability in a region where the fins are still not effective.This is the biggest piece of secret sauce they developed. When and how much. They were lucky and got it to work on the first try (CASSIOPE IIRC) but then made it work on the subsequent launch that had much lower margins... And it has worked on each and every subsequent launch.I don't believe they were lucky.I believe they have good engineers, who know what they are doing. As shown by the fact it has worked on every launch. Including the first.
Quote from: JamesH on 12/27/2015 10:28 amQuote from: meekGee on 12/27/2015 04:12 amQuote from: the_other_Doug on 12/27/2015 01:35 amAlso, I believe the entry burn is designed to create a plasma sheath between the atmospheric heating region and the octoweb. In other words, it acts to provide protection against entry heating. And trust me, at the altitude from, and speed at which the first stage comes in, there is significant entry heating. The entry burn keeps that from affecting the stage itself, without needing to cover the base of the stage with some additional TPS rated for entry heating.That's likely, plus I believe it also provides stability in a region where the fins are still not effective.This is the biggest piece of secret sauce they developed. When and how much. They were lucky and got it to work on the first try (CASSIOPE IIRC) but then made it work on the subsequent launch that had much lower margins... And it has worked on each and every subsequent launch.I don't believe they were lucky.I believe they have good engineers, who know what they are doing. As shown by the fact it has worked on every launch. Including the first.Talent, no doubt... but an environment that thrives on innovation and empirical testing (with its associated kabooms) is as vital. They don't study problems to death -- they try things that will probably be close to the answer, watch the response, and adjust before trying again. Brilliant.
Here's a question to which I don't have the answer, though -- is the ballistic trajectory worse in terms of heating regime than the looped-back trajectory achieved with the boostback burn? The boostback does cancel out the rather significant downrange velocity, after all.I recall that ballistic trajectories have high deceleration rates; do they also have hotter entries than a loop-back? If so, would that limit the speed and angle at which a falcon can enter on a ballistic trajectory, even with an engine's exhaust pushing back the entry heating?
By environmental conditions do you mean air temp, pressure and wind speed? I'm guessing wind speed and sea state may be significant, but not temperature and barometric pressure. Apart from that, it is also a matter of probabilities as far as actual engine performance that may leave more or less fuel once the velocity target is reached for MECO.
Quote from: Jcc on 12/27/2015 01:51 pmBy environmental conditions do you mean air temp, pressure and wind speed? I'm guessing wind speed and sea state may be significant, but not temperature and barometric pressure. Apart from that, it is also a matter of probabilities as far as actual engine performance that may leave more or less fuel once the velocity target is reached for MECO.No -- I'm talking about the entry conditions -- speed, angle of entry, tie of peak heating, and deceleration stresses. For example, let's say that a boostback to an RTLS trajectory results in deceleration stresses of about 5G to 6G, whereas a ballistic entry results in 15G to 20G stresses. Or there is a difference between 30 seconds of entry heating and, say, two minutes, or a significant increase in entry heating, from a ballistic to an RTLS trajectory? Are there outlier conditions on ballistic entries where the deceleration and thermal stresses result in breakup of the stage?Again, not talking about the wind conditions at the desired landing point, or the sea state. I'm talking about surviving the entry back into the atmosphere, and whether or not there is a point at which a Falcon stage will not survive a ballistic entry, even if it has enough delta-V on paper to perform entry and landing burns. Or is the question of whether or not a given trajectory allows stage recovery based solely on the remaining delta-V in the stage, and the entry conditions are not ever going to be a limiting factor?
No -- I'm talking about the entry conditions -- speed, angle of entry, tie of peak heating, and deceleration stresses. For example, let's say that a boostback to an RTLS trajectory results in deceleration stresses of about 5G to 6G, whereas a ballistic entry results in 15G to 20G stresses. Or there is a difference between 30 seconds of entry heating and, say, two minutes, or a significant increase in entry heating, from a ballistic to an RTLS trajectory?
Is there an update thread for the SES-9 SpaceX Mission... I'd like to know more details about where this all stands in terms of Launch Readiness ie is the LV ready, is the PL ready, what is the hold up...
Quote from: the_other_Doug on 12/27/2015 02:09 pmNo -- I'm talking about the entry conditions -- speed, angle of entry, tie of peak heating, and deceleration stresses. For example, let's say that a boostback to an RTLS trajectory results in deceleration stresses of about 5G to 6G, whereas a ballistic entry results in 15G to 20G stresses. Or there is a difference between 30 seconds of entry heating and, say, two minutes, or a significant increase in entry heating, from a ballistic to an RTLS trajectory? Maybe I'm missing something and perhaps this is just semantics, but the only entry type a F9 stage can perform is effectively a ballistic entry. It has no significant lift, the grid fins serve only to fine-tune the landing point, I don't think they have enough control authority to significantly shallow-up the trajectory. The only difference between trajectories it can do is the entry angle you set up.
Quote from: ugordan on 12/27/2015 02:32 pmQuote from: the_other_Doug on 12/27/2015 02:09 pmNo -- I'm talking about the entry conditions -- speed, angle of entry, tie of peak heating, and deceleration stresses. For example, let's say that a boostback to an RTLS trajectory results in deceleration stresses of about 5G to 6G, whereas a ballistic entry results in 15G to 20G stresses. Or there is a difference between 30 seconds of entry heating and, say, two minutes, or a significant increase in entry heating, from a ballistic to an RTLS trajectory? Maybe I'm missing something and perhaps this is just semantics, but the only entry type a F9 stage can perform is effectively a ballistic entry. It has no significant lift, the grid fins serve only to fine-tune the landing point, I don't think they have enough control authority to significantly shallow-up the trajectory. The only difference between trajectories it can do is the entry angle you set up....Which is exactly what the poster is asking about.
Yeah, but he was talking about RTLS and ballistic trajectories as if they're somehow different. My point was that they're both really ballistic.
Quote from: cro-magnon gramps on 12/27/2015 02:18 pmIs there an update thread for the SES-9 SpaceX Mission... I'd like to know more details about where this all stands in terms of Launch Readiness ie is the LV ready, is the PL ready, what is the hold up...Not yet, it's too early. The Jason-3 update thread was only created yesterday.
OK --to try and wrap up this little side-thread, here, let me try and put the question a little better:Will a ballistic landing downrange, with little to no boostback burn (as will be required for some heavier payloads), create a more challenging entry environment, in terms of both entry heating and deceleration stresses, than entry after a boostback to an RTLS, or near-RTLS, profile? And does the Falcon have limits beyond which it can't survive a ballistic entry without a boostback (or at least a lofting/shaping) burn? Or is the entry burn capable of handle all possible conditions, from pure unmodified ballistic entries to full boostback RTLS trajectories?I guess I'm trying to visualize the boundary between the case of being able to recover a first stage, and the case where the stage must be expended. I know there's a line out there, defined by the remaining delta-V in the stage after BECO, below which there is not enough energy left to achieve both an entry burn and a landing burn. But are there ballistic trajectories that are non-survivable, even if there is theoretically enough energy left in the stage to accomplish a downrange recovery? In other words, is the definition of "must be expended" entirely defined by the remaining delta-V in the stage, or does the entry environment also play a role in defining a mission where the first stage cannot be recovered?
Quote from: the_other_Doug on 12/27/2015 01:36 pmOK --to try and wrap up this little side-thread, here, let me try and put the question a little better:Will a ballistic landing downrange, with little to no boostback burn (as will be required for some heavier payloads), create a more challenging entry environment, in terms of both entry heating and deceleration stresses, than entry after a boostback to an RTLS, or near-RTLS, profile? And does the Falcon have limits beyond which it can't survive a ballistic entry without a boostback (or at least a lofting/shaping) burn? Or is the entry burn capable of handle all possible conditions, from pure unmodified ballistic entries to full boostback RTLS trajectories?I guess I'm trying to visualize the boundary between the case of being able to recover a first stage, and the case where the stage must be expended. I know there's a line out there, defined by the remaining delta-V in the stage after BECO, below which there is not enough energy left to achieve both an entry burn and a landing burn. But are there ballistic trajectories that are non-survivable, even if there is theoretically enough energy left in the stage to accomplish a downrange recovery? In other words, is the definition of "must be expended" entirely defined by the remaining delta-V in the stage, or does the entry environment also play a role in defining a mission where the first stage cannot be recovered?Well, your first comment about wrapping up the side-thread, now a dozen or so posts later looks like the side thread is still going on. Including this-I seem to have some knowledge relevant to the necessity of boostback but my memory is foggy and a quick search didn't come up with anything useful. Posting here in hopes that it triggers someone else to remember and flush out the details. [IIRC] All ocean recovery experiments including the ones prior to ASDS platforms have had what SpaceX has called a boostback burn even though that burn was only to slow horizontal velocity not reverse it (as was done with the CCAFS landed Orbcomm-2) - with one exception. The one exception was relatively recently, approximately Eutelsat, in March. That heavy payload took too much energy to do a boostback burn. It seems as if there was a tweet or some other source that stated the kinetic energy being carried into the re-entry burn was to be 8x what had succeeded on previous recovery attempts. And it made it through.[/IIRC]I can't emphasize enough that this is only foggy memory stuff, not necessarily fact.
DSCOVR was the fast and hot one way downrange...