Author Topic: SpaceX F9 : CCiCap In-Flight Abort Test : mid-2019  (Read 41910 times)

Offline woods170

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Re: SpaceX F9 : CCiCap In-Flight Abort Test : Q2-2019
« Reply #140 on: 11/28/2018 11:38 am »
I think this highlighted bit is slightly different from what most have been assuming/picturing, i.e. that aerodynamics would entirely control the flip.  But given the reduced atmosphere at the apogee of the abort trajectory, it's not that surprising that they'll use the small thrusters.  I am surprised that Dragon ends up with so much unused propellant left in her tanks (mentioned elsewhere).

Ascent abort at Max-Q might sound like a nasty environment, but zero-zero (pad abort) is in fact the more demanding abort scenario (in terms of propellant used to reach a safe distance away from the vehicle).
« Last Edit: 11/28/2018 11:39 am by woods170 »

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Re: SpaceX F9 : CCiCap In-Flight Abort Test : Q2-2019
« Reply #141 on: 11/28/2018 11:43 am »
Still, why not burn off the majority of that propellant (save for Draco attitude control) to get as far away from any deflagration and debris of an inflight failure? That would also lower ocean impact speed somewhat.

Offline woods170

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Re: SpaceX F9 : CCiCap In-Flight Abort Test : Q2-2019
« Reply #142 on: 11/28/2018 11:55 am »
Still, why not burn off the majority of that propellant (save for Draco attitude control) to get as far away from any deflagration and debris of an inflight failure? That would also lower ocean impact speed somewhat.

You only burn the SuperDraco's as long as needed. The abort environment is a high-G environment. You only subject the astro's to such an environment as long as necessary. But no longer than necessary, to minimize the risk of physical damage due to the abort.

"As long as necessary" in this case is defined by the amount of time it requires to fly to a safe distance from a failing booster, as well as having enough altitude to dump the trunk, re-orientate for entry and deploy the chutes.

As I explained in my previous post, that requires less propellant at max-Q than it does in a pad abort. That's why Crew Dragon will have a substantial amount of propellant left in the tanks in case of an ascent abort at max-Q.

As to lowering the ocean impact speed: why do you think a fourth parachute was added to Crew Dragon a few years ago?
« Last Edit: 11/28/2018 11:57 am by woods170 »

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Re: SpaceX F9 : CCiCap In-Flight Abort Test : Q2-2019
« Reply #143 on: 11/28/2018 12:07 pm »
You only burn the SuperDraco's as long as needed. The abort environment is a high-G environment.

I am aware of that, but SDs are supposed to be able to throttle down to allow a hover for landing. Are you saying the throttling is also out of the picture now that propulsive landing is scrapped?

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Re: SpaceX F9 : CCiCap In-Flight Abort Test : Q2-2019
« Reply #144 on: 11/28/2018 12:20 pm »
Burning the SDs longer during the abort means increasing the apogee, which is already pretty high for a more of less vertical entry. Maybe they are worried about entry gees and don't want to restart the SDs on the way down if they can avoid it.

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Re: SpaceX F9 : CCiCap In-Flight Abort Test : Q2-2019
« Reply #145 on: 11/28/2018 12:31 pm »
Perhaps and it's a good point for aborts at late stages. In the context of this max-Q abort, I'd think reentry Gs would be of little concern. The vehicle is flying at what, maybe Mach 1.5 - 2 at that point at an altitude of 15-ish km? That's a far cry from for example the recent Soyuz abort, both from a velocity and apogee standpoint.

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Re: SpaceX F9 : CCiCap In-Flight Abort Test : Q2-2019
« Reply #146 on: 11/28/2018 01:24 pm »
Perhaps and it's a good point for aborts at late stages. In the context of this max-Q abort, I'd think reentry Gs would be of little concern. The vehicle is flying at what, maybe Mach 1.5 - 2 at that point at an altitude of 15-ish km? That's a far cry from for example the recent Soyuz abort, both from a velocity and apogee standpoint.

Dragon's apogee (trunk sep) is listed in the table as 32 to 83 km. That's a large range, but the upper end is about what the Soyuz capsule reached during the abort.

Offline Tomness

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Re: SpaceX F9 : CCiCap In-Flight Abort Test : Q2-2019
« Reply #147 on: 11/28/2018 02:01 pm »
Still, why not burn off the majority of that propellant (save for Draco attitude control) to get as far away from any deflagration and debris of an inflight failure? That would also lower ocean impact speed somewhat.

You only burn the SuperDraco's as long as needed. The abort environment is a high-G environment. You only subject the astro's to such an environment as long as necessary. But no longer than necessary, to minimize the risk of physical damage due to the abort.

"As long as necessary" in this case is defined by the amount of time it requires to fly to a safe distance from a failing booster, as well as having enough altitude to dump the trunk, re-orientate for entry and deploy the chutes.

As I explained in my previous post, that requires less propellant at max-Q than it does in a pad abort. That's why Crew Dragon will have a substantial amount of propellant left in the tanks in case of an ascent abort at max-Q.

As to lowering the ocean impact speed: why do you think a fourth parachute was added to Crew Dragon a few years ago?

That is awesome, there is propellant left over to use draco's for orientation, that be cool if they were able to do Soyuz over slam at the end but I guess when go in to the drink, "ground" is ever changing waves.

Offline woods170

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Re: SpaceX F9 : CCiCap In-Flight Abort Test : Q2-2019
« Reply #148 on: 11/28/2018 02:04 pm »
You only burn the SuperDraco's as long as needed. The abort environment is a high-G environment.

I am aware of that, but SDs are supposed to be able to throttle down to allow a hover for landing. Are you saying the throttling is also out of the picture now that propulsive landing is scrapped?

When one is aborting off a failing booster one usually intends to get away from said failing booster as fast as one can go. Which, in the case of SuperDraco, translates into: full throttle (and thus maximum acceleration and G's).
It also means that once one is a safe distance away from the failing booster one cuts the SuperDraco's to get rid of the high-G environment.

Merely throttling down the SuperDracos to reduce G's and burning them longer to burn-off as much propellant as possible just ads unnecessary complexity to an already complex abort scenario.
Adding a fourth parachute, to account for the added weight of a (partial) propellant-load-still-on-board, is deemed safer.
And that is because it has been proven on cargo Dragon.
Cargo Dragon's regularly splash-down with hundreds of pounds of propellant still on-board.

Offline UKobserver

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Re: SpaceX F9 : CCiCap In-Flight Abort Test : Q2-2019
« Reply #149 on: 11/28/2018 08:10 pm »
My first thought on reading that there will be a significant amount of fuel remaining on board, was that this might be due to having separate (i.e. redundant) fuel tanks and systems for the Draco and SuperDraco engines respectively. That way, if you develop a fault/leak in one system in space, you aren't losing all ability to manoever/deorbit, and can improvise by using the other system, as various other spacecraft/satellites have done successfully in the past.

You could choose to fit the two systems with a cross-feed valve, to allow replenishment of one system from the other, and thus utilise propellant that would otherwise be stranded and unusable if one manoevering system developed a problem. However the cross-feed valve would be shut in normal operations, and "manually" opened if needed, to avoid a leak in one system draining both of them. Airliners are built this way, with each wing tank isolated from the other, but with cross-feed valves to allow/stop fuel transfer as desired.

This works very well, but there is a famous example of an aircrew on an A330 (I think) failing to diagnose a major fuel leak from one wing (pylon), electing to open the cross-feed valve to solve what they thought was the problem, and causing the remainder of their fuel to be dumped overboard, resulting in them having to glide their fully laden airliner 200 miles to a deadstick landing in the Azores. Luckily they were close enough. But I digress...

Notwithstanding the above comments about a MaxQ abort requiring less fuel, could part of the remainder be explained by separate Draco/SuperDraco systems?

Offline speedevil

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Re: SpaceX F9 : CCiCap In-Flight Abort Test : Q2-2019
« Reply #150 on: 11/28/2018 08:29 pm »
Notwithstanding the above comments about a MaxQ abort requiring less fuel, could part of the remainder be explained by separate Draco/SuperDraco systems?
A naive calculation assuming 19000 pounds initial mass, and 2000 pounds of propellant gives 250m/s or so of delta-v remaining.
If in fact this is usable, this seems likely to be able to entirely propulsively brake.
(clearly this will not happen in this test).

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Re: SpaceX F9 : CCiCap In-Flight Abort Test : Q2-2019
« Reply #151 on: 11/28/2018 09:14 pm »
My first thought on reading that there will be a significant amount of fuel remaining on board, was that this might be due to having separate (i.e. redundant) fuel tanks and systems for the Draco and SuperDraco engines respectively. That way, if you develop a fault/leak in one system in space, you aren't losing all ability to manoever/deorbit, and can improvise by using the other system, as various other spacecraft/satellites have done successfully in the past.

You could choose to fit the two systems with a cross-feed valve, to allow replenishment of one system from the other, and thus utilise propellant that would otherwise be stranded and unusable if one manoevering system developed a problem. However the cross-feed valve would be shut in normal operations, and "manually" opened if needed, to avoid a leak in one system draining both of them. Airliners are built this way, with each wing tank isolated from the other, but with cross-feed valves to allow/stop fuel transfer as desired.

This works very well, but there is a famous example of an aircrew on an A330 (I think) failing to diagnose a major fuel leak from one wing (pylon), electing to open the cross-feed valve to solve what they thought was the problem, and causing the remainder of their fuel to be dumped overboard, resulting in them having to glide their fully laden airliner 200 miles to a deadstick landing in the Azores. Luckily they were close enough. But I digress...

Notwithstanding the above comments about a MaxQ abort requiring less fuel, could part of the remainder be explained by separate Draco/SuperDraco systems?

The SuperDraco and Dracos use the same fuel tanks. There are multiple separate tanks, though. Several spherical tanks within the "skirt" of the Dragon. See diagram, old picture of Dragon, but red and blue tanks are for the Dracos.
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Offline UKobserver

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Re: SpaceX F9 : CCiCap In-Flight Abort Test : Q2-2019
« Reply #152 on: 11/28/2018 11:32 pm »
The SuperDraco and Dracos use the same fuel tanks. There are multiple separate tanks, though. Several spherical tanks within the "skirt" of the Dragon. See diagram, old picture of Dragon, but red and blue tanks are for the Dracos.

Thanks! Do you know if they are drawing from all of those tanks simultaneously during an abort or are some isolated until needed? What about during in-space operations?

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Re: SpaceX F9 : CCiCap In-Flight Abort Test : Q2-2019
« Reply #153 on: 11/29/2018 03:05 pm »
The SuperDraco and Dracos use the same fuel tanks. There are multiple separate tanks, though. Several spherical tanks within the "skirt" of the Dragon. See diagram, old picture of Dragon, but red and blue tanks are for the Dracos.

Thanks! Do you know if they are drawing from all of those tanks simultaneously during an abort or are some isolated until needed? What about during in-space operations?

From my understanding, there are two separate sets of interconnected tanks, which are hooked up to different sets of Draco thrusters, such that a fair number of them can fail without the Crew Dragon becoming uncontrollable. Each SuperDraco in a pair is running off of one set of tanks, the other, the other. This is all based on my vague recollections from reading things over the years. I may very well be incorrect about some part of that.

But based on how I understand it, in an abort the SDs would draw on all tanks at once. The redundant setup of tanks would reduce the chance that there'd be a failure of the abort. A tank valve can fail and / or an SD flame out and the abort would still succeed.

In-space operations would need to keep the spacecraft's center of mass in mind. Fuel usage would have to be balanced for that.
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