Author Topic: Could the CRS-7 first stage have saved itself?  (Read 18082 times)

Offline Dave G

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Re: Could the CRS-7 first stage have saved itself?
« Reply #40 on: 08/11/2015 12:39 pm »
Much has been made of the fact that, for want of software, Dragon could have saved itself and its payload.

It just occurred to me that as of soon (now?) the first stage is also a valuable piece of hardware that could potentially save itself in the event of a second stage failure.

A lot of discussions on this site are like this, asking if something can be done, then going through the details of how to do it. But good engineers should also look at the cost/benefit analysis.  What's the bang-for-buck here?

For Dragon, its obvious.  They're already working on the abort software for Dragon 2.  Applying that to Dragon 1 won't require much additional development.  And since the CRS-7 Dragon was carrying some expensive hardware, the bang-for-buck to software upgrade Dragon 1 is a no-brainier.

But for the first stage, bang-for-buck is much less obvious.  First, think about how many times something like CRS-7 will reoccur.  Assuming SpaceX fixes the underlying problem with the seconds stage, it may not reoccur at all.  But for the sake of discussion, let's say CRS-7 type events do reoccur, perhaps once every 5 years.  For me, this seems highly improbable, but let's run with it.  If the first stage costs around $30 million, that would be a cost of 6 million a year.  So that would be the benefit: maybe $6M/year max. 

But that sort of assumes that first stages can be reused indefinitely.  If I remember correctly, Gwynne Shotwell said they intend to re-fly F9 first stages around a dozen times before they're decommissioned, at least initially.  So that would lower the benefit.

Now, what would be the additional development cost to achieve that benefit?  How much does an engineer cost?  When you total the costs of leasing office space, paying utilities, secretaries, managers, etc., plus health care and other benefits, the total cost of an engineer is around $500K/year.

We need to remember that reusability is not mission critical.  In the end, reusability boils down to a numbers game - dollars and cents.  If they lose the first stage once in a blue moon, no big deal. 

As another example, let's say a first stage engine fails, like it did on CRS-1.  Yes, the remaining 8 engines will complete the mission, but that requires more fuel, so the first stage won't have enough fuel to return to the launch site.  Some have suggested that SpaceX always have the landing barge (ASDS) in place for such scenarios, but how much would that cost?  If engine failures are very rare, operating the landing barge may be more expensive than simply losing a stage once every hundred missions.

In fact, I suspect the whole landing barge idea will be a short-term thing.  In 5-10 years, SpaceX won't use them anymore.  Falcon Heavy will lift 7 tons to GTO, with all 3 first stages returning to the launch site.  7 tons to GTO covers the entire current launch market.  The only missions that require more payload are beyond Earth orbit, and using FH BEO would probably require an expendable configuration anyway.  When they start launching MCT, the BFR first stage will be too big for a landing barge.  So I predict ASDS will be short lived.  Again, its a numbers game.
« Last Edit: 08/11/2015 01:00 pm by Dave G »

Offline mme

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Re: Could the CRS-7 first stage have saved itself?
« Reply #41 on: 08/11/2015 09:04 pm »
Much has been made of the fact that, for want of software, Dragon could have saved itself and its payload.

It just occurred to me that as of soon (now?) the first stage is also a valuable piece of hardware that could potentially save itself in the event of a second stage failure.

A lot of discussions on this site are like this, asking if something can be done, then going through the details of how to do it. But good engineers should also look at the cost/benefit analysis.  What's the bang-for-buck here?

For Dragon, its obvious.  They're already working on the abort software for Dragon 2.  Applying that to Dragon 1 won't require much additional development.  And since the CRS-7 Dragon was carrying some expensive hardware, the bang-for-buck to software upgrade Dragon 1 is a no-brainier.

But for the first stage, bang-for-buck is much less obvious.  First, think about how many times something like CRS-7 will reoccur.  Assuming SpaceX fixes the underlying problem with the seconds stage, it may not reoccur at all.  But for the sake of discussion, let's say CRS-7 type events do reoccur, perhaps once every 5 years.  For me, this seems highly improbable, but let's run with it.  If the first stage costs around $30 million, that would be a cost of 6 million a year.  So that would be the benefit: maybe $6M/year max. 

...
I don't think that the Dragon 2 abort software has anything to do with the Dragon 1 deploying parachutes.  Dragon 2 abort software is way more complicated then what Dragon 1 "needs".  Dragon 1 just needs to be able to:
1. Arm deploying the drogue (that should never be possible by default on ascent.)
2. Deploy the drogue.

Maybe it needs to disconnect from the trunk, maybe it can "lawn dart" or tumble until the drogue deploys.  That's it.  It may be commanded rather then automated because the simpler the better.

As for stage one saving itself, I agree it's not worth it.  Frankly I think spending any effort on making the first stage "save itself" would be outright insane.  What's valuable is a reputation for reliably delivering payloads and humans safely.  It's not worth diverting any time, money, or engineering resources on creating a bunch of new failure modes that do not improve the reliability for the customer.
Space is not Highlander.  There can, and will, be more than one.

Offline Dave G

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Re: Could the CRS-7 first stage have saved itself?
« Reply #42 on: 08/12/2015 06:21 am »
I don't think that the Dragon 2 abort software has anything to do with the Dragon 1 deploying parachutes.  Dragon 2 abort software is way more complicated then what Dragon 1 "needs".  Dragon 1 just needs to be able to:
1. Arm deploying the drogue (that should never be possible by default on ascent.)
2. Deploy the drogue.

Maybe it needs to disconnect from the trunk, maybe it can "lawn dart" or tumble until the drogue deploys.  That's it.  It may be commanded rather then automated because the simpler the better.

Yeah, I could have phrased that better. 

Here's how Elon says it:

Quote
The software to initiate the parachute deployment was not in Dragon 1. It certainly was planned for Dragon version 2, and one of the things we've decided to do is to advance the software that was meant for Dragon version 2 into Dragon version 1 so that if something like this were to happen in the future, the Dragon spacecraft would save itself.
http://shitelonsays.com/transcript/elon-musk-talks-failed-crs-7-dragon-mission-2015-07-20

Obviously Dragon v1 already knows how to land using parachutes.  That's how all Dragons have landed so far, from COTS-1 through CRS-6.  But until now, Dragon v1 didn't know how to use parachutes on an abort, which is why CRS-7 Dragon was lost.

If Dragon v2 aborts, it will use parachutes to land.  Yes, a Dragon v2 abort involves much more than this, but the bit of software that deploys parachutes on a Dragon v2 abort can be applied to Dragon v1. 



« Last Edit: 08/12/2015 07:55 am by Dave G »

Offline MattMason

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Re: Could the CRS-7 first stage have saved itself?
« Reply #43 on: 08/12/2015 01:30 pm »
I don't think that the Dragon 2 abort software has anything to do with the Dragon 1 deploying parachutes.  Dragon 2 abort software is way more complicated then what Dragon 1 "needs".  Dragon 1 just needs to be able to:
1. Arm deploying the drogue (that should never be possible by default on ascent.)
2. Deploy the drogue.

Maybe it needs to disconnect from the trunk, maybe it can "lawn dart" or tumble until the drogue deploys.  That's it.  It may be commanded rather then automated because the simpler the better.

Yeah, I could have phrased that better. 

Here's how Elon says it:

Quote
The software to initiate the parachute deployment was not in Dragon 1. It certainly was planned for Dragon version 2, and one of the things we've decided to do is to advance the software that was meant for Dragon version 2 into Dragon version 1 so that if something like this were to happen in the future, the Dragon spacecraft would save itself.
http://shitelonsays.com/transcript/elon-musk-talks-failed-crs-7-dragon-mission-2015-07-20

Obviously Dragon v1 already knows how to land using parachutes.  That's how all Dragons have landed so far, from COTS-1 through CRS-6.  But until now, Dragon v1 didn't know how to use parachutes on an abort, which is why CRS-7 Dragon was lost.

If Dragon v2 aborts, it will use parachutes to land.  Yes, a Dragon v2 abort involves much more than this, but the bit of software that deploys parachutes on a Dragon v2 abort can be applied to Dragon v1.

And that revised software presumes that a D1 is in any condition after a mishap to accept commands. I think someone already noted that D1 hasn't any practical abort separation actions, so a rescue of a D1 would occur with luck, as CRS-7's demise just happened to blow the Dragon off the stack before consuming itself. It got lucky. A more critical failure (like STS-51L) would cause greater aerodynamic forces that might consume the Dragon as well. But in that scenario, it's better to have a salvage option than not.

That brings up a question: Are D1 spacecraft also equipped with flight termination charges? In a similar mishap where the D1 separates and the parachutes do not deploy, I'd hate to see this kind of Dragon eat people or property from an uncontrolled fall.
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Offline Jim

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Re: Could the CRS-7 first stage have saved itself?
« Reply #44 on: 08/12/2015 02:30 pm »

That brings up a question: Are D1 spacecraft also equipped with flight termination charges?

No.

Offline Paul451

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Re: Could the CRS-7 first stage have saved itself?
« Reply #45 on: 08/12/2015 05:24 pm »
That brings up a question: Are D1 spacecraft also equipped with flight termination charges? In a similar mishap where the D1 separates and the parachutes do not deploy, I'd hate to see this kind of Dragon eat people or property from an uncontrolled fall.

The purpose of the charges is not to break up the vehicle into pieces too small to cause damage to property. Hence there's no reason to destroy Dragon. Instead, it's to prevent the out-of-control vehicle from deviating away from the exclusion zone and becoming a risk to people/property. (A secondary benefit is that the fuel/oxidiser is released/burned at altitude, reducing the risk of energetic detonations at ground level. A tertiary benefit is that it makes debris safer to approach, sooner.)

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