Pros and Cons, Dragon V2, CST-100 and Dream Chaser

[ChrisWilson68]

A pusher system, with all the weight of the spacecraft above it instead of below, can put more pressure on the computers controlling the abort during the critical first second or so when the spacecraft is getting away from the rocket.

I think there is no nonsense in either opinion.

Well, there's still a bit of nonsense.  This quote shows that even some at NASA are not immune to the error of the rocket pendulum fallacy.

A pusher system, with all the weight of the spacecraft above it instead of below, can put more pressure on the computers controlling the abort during the critical first second or so when the spacecraft is getting away from the rocket.

Think of balancing a baseball bat on the palm and how many adjustments it takes to keep it balanced.

[TrevorMonty]

And exactly what is clumsiness in the Orion LAS? It only has three motors.

According to a 2011 NASA fact sheet, the entire Orion stack is 69,181 lbs, of which the LAS is 16,125 lbs.  That's about 25% of the whole system.

Anyone have a guess what the equivalents would be for Dragon V2, CST-100 and Dream Chaser?  My guess would be less.

That disposable LAS is 16,125lbs less mass the SLS needs to deliver to BLEO and 16,125lbs less mass on re entry. The Orion is optimised for performance which means reducing mass where possible. This is why they choose parachutes and water landings.

[Proponent]

While watching the traditional tractor abort system tests of the Orion Spacecraft  I'm struck by the clumsiness and inelegance of that system. So many events, so many parts. It appears, to my untrained eye, to be extremely limited, expensive, heavy, inefficient and fraught with failure modes. DV2's approach seems simple, elegant, relatively cheap and as others have said nothing is wasted.

From the view point of flight control, a tractor system is inherently simpler and more stable than a pusher system. When the abort motor is working the CoG of the stack is far below the center of thrust so the capsule effectively act as a stabilizer. It’s bad for a vehicle that requires agility, but all you want of a LAS is to pull the capsule up and high really fast with blunt force. 

And exactly what is clumsiness in the Orion LAS? It only has three motors. The abort motor is a big dumb booster, the jettison motor is also a no-brainer. Only the attitude control motor looks a bit clumsy, but it is really just one solid motor with many controllable exhausts (so you don’t have to gimbal the nozzle). I think it looks inelegant primarily because of the dirty solid engine exhaust.

As ChrisWillson68 points out, having the "center of thrust" (never heard that term before) ahead of the center of mass does not provide stability.  However, having the center of mass ahead of the center of pressure does.  For that reason, the tractor system probably does offer a bit more stability than a pusher system.  Apollo's LES had, IIRC, half a ton of depleted uranium in it to shift the center of mass forward in order to provide stability.

Orion's escape motor fires upward and then is ducted aft.  This design allows for a shorter tower.  I suspect that means it is shorter than Apollo's tower and doesn't shift the center of mass as much.

More to the point, though, with modern control systems, why do you need natural stability?  In Apollo's day, it was probably essential, but now it's no more than a nice-to-have.

[clongton]

This is why they choose parachutes and water landings.

Not quite.

In the beginning of the CxP program, Orion was to return under parachute, but to a land landing using bags, just like CST-100. This capability was scrapped under the ZBV program and never reinstated for mass reasons. The ZBV program was attempting to trim the spacecraft of all non-essential mass in an effort to make Ares-I workable as Orion's launch vehicle. The bags never made it back in because a splashdown was still available as a "free" mass option.

As to the tractor LAS, there was a time when NASA was considering a pusher abort system for Orion. But they were never able to make it work from a mass perspective because they didn't consider the dual use possibility. Even with the pusher abort motors NASA intended to use parachutes as the main return mode. I don't believe they ever considered powered return if there was no abort so even under parachute Orion would have been carrying back a lot of unused propellant, adding significant mass to the spacecraft that the parachutes would need to handle, increasing their size (and mass). While true that Dragon is similar except in terms of the powered landing capability, Orion was dealing with a launch vehicle that was increasingly unable to perform the launch requirements  that its spacecraft imposed on it. Dragon's launch vehicle has no such crippling handicap. Had Ares-I been more capable, it is possible that bags on the ground would have been reinstated and even Griffin's pusher abort motors may have become baseline. Had that happened, some smart design engineer at NASA may have seen the very real potential for the powered return, with parachute backup, just like Dragon. But remember, Orion is a much larger and much heavier spacecraft than Dragon. For Orion to do what Dragon is going to do would be a major redesign effort and have made the spacecraft even heavier.

[SWGlassPit]

WRT Dragon's "powered landing with parachute backup" approach --

Isn't there a period in which a failure of the landing motors would not leave the backup chutes enough time to open?  Say, for example, complete loss of thrust at ~100 ft AGL.  I'm curious how that possibility, however remote, is addressed.

[rpapo]

WRT Dragon's "powered landing with parachute backup" approach --

Isn't there a period in which a failure of the landing motors would not leave the backup chutes enough time to open?  Say, for example, complete loss of thrust at ~100 ft AGL.  I'm curious how that possibility, however remote, is addressed.

Yes, and a parachute can fail too.

[Elmar Moelzer]

WRT Dragon's "powered landing with parachute backup" approach --

Isn't there a period in which a failure of the landing motors would not leave the backup chutes enough time to open?  Say, for example, complete loss of thrust at ~100 ft AGL.  I'm curious how that possibility, however remote, is addressed.

Yes, and a parachute can fail too.

I think that if not all engines fail, Dragon can use the remaining ones together with the parachutes to make the landing at least survivable without injuries.
The good thing is that Dragon has multiple redundancies and having all the engines fail after they were tested at a high enough altitude seems like an unlikely problem.

[rcoppola]

As fascinating as this is and it is, the fact remains that not one of the 3 have puller LAS. So why is this relevant to the pros & cons?

[rcoppola]

WRT Dragon's "powered landing with parachute backup" approach --

Isn't there a period in which a failure of the landing motors would not leave the backup chutes enough time to open?  Say, for example, complete loss of thrust at ~100 ft AGL.  I'm curious how that possibility, however remote, is addressed.

Testing, testing and more testing. That's the primary role of the DragonFly testing program starting over at McGregor this year. The new parachute system has also been modified for a quicker deploy. What the ceiling is for that in case of an SD failure closer to ground? I'm not sure we know yet, At some point things need to work as designed. We never have and never will mitigate every potential failure mode.

[bilbo]

I'm thinking the biggest issue for DC is that it has control surfaces, Space-planes are inherently harder to make then a capsule. DC does have some advantages though, it has a lower g force on re entry, and it can land on any runway.

I think the toughest choice for NASA will be to chose between Boeing and Spacex.
I'm guessing the decisions already been made by now,all we have to do is wait for them to release it. all I do know is there will be a bunch of butt hurt Spacex amazing peoples if they don't get the contract and Boeing does.

 I like Boeing's design because its probably the safest, capsule with parachutes, no fancy powered landings, just made to get the job done. Dragons design however is good to, because it has a heatshield capable of interplanetary reentry, and it also has parachutes.

Only time shall tell )

[pagheca]

to be honest I can't believe they will actually choose one of the three and throw away the other twos. They are all so different and developed at this point!

Maybe there will be appeals, the unlucky two will accuse the committee of unfair conduct and a "salomonic" process will follow...

The problem (but also the good) with this is that all the 3 candidates have done an amazing job. I think the CCiCap etc. have been a wonderful scheme and should become the paradigm of how to manage this kind of ultra-high-tech development with technological benefits for the future. What has been done in the last few years can really be considered as a sort of Renaissance of the space sector.

[tobi453]

@TheLurioReport  ·  2 Std.

Conflicting reports tonight - CCtCap announcement on 22 August or slip to September for Congress notification requirements./I ask:or other?

 @TheLurioReport  ·  3 Std.

Likelier to put _cautious_ bet on SNC in CCtCap than whether SpaceX or Boeing wins other contract. Dream Chaser a 'fan favorite' at NASA/ULA

DC winner? anouncement tomorrow or September.

[Lars_J]

My guess/prediction is that the two winners will be DC and Dragon.
- DC a sentimental favorite (wings!), plus lots of progress recently
- Dragon provides the biggest contrasting capabilities (capsule AND different LV, selecting both covers the most bases)
- Boeing has been very quiet about CST-100 lately

[pagheca]

My guess/prediction is that the two winners will be DC and Dragon.
- DC a sentimental favorite (wings!), plus lots of progress recently
- Dragon provides the biggest contrasting capabilities (capsule AND different LV, selecting both covers the most bases)
- Boeing has been very quiet about CST-100 lately

On one side, Boeing is the only one of the three that could survive a withdraw with only lateral damage. The other two would risk to die or at least to be dwarfed.

On the other side is clearly the most politically powerful of the three. I do not know if any (or both) of these two elements may impact on the selection process.

[kevinof]

I hope you're right. My heart says Dragon and DC but my head says Dragon and Boeing.

For some reason I have in my head that CST is "old tech". It's a capsule that lands using parachutes (ok on land instead of water) and we've been there and done that. Dragon and F9 represent  a different way to thinking and one that would/could deliver some great flexibility in the future. Can't wait to see a Dragon land, no chutes and just the Super Dracos blasting away. What a sight that would be.

DC? Well yes it has wings and that is sentimental but also because by choosing DC you cover the bases. You have a capsule design (above) but you also have "mini shuttle" that will evolve and provide a different technology path down the road.

But my head says Dragon and CST simply because of the politics.

My guess/prediction is that the two winners will be DC and Dragon.
- DC a sentimental favorite (wings!), plus lots of progress recently
- Dragon provides the biggest contrasting capabilities (capsule AND different LV, selecting both covers the most bases)
- Boeing has been very quiet about CST-100 lately

[A_M_Swallow]

My guess/prediction is that the two winners will be DC and Dragon.
- DC a sentimental favorite (wings!), plus lots of progress recently
- Dragon provides the biggest contrasting capabilities (capsule AND different LV, selecting both covers the most bases)
- Boeing has been very quiet about CST-100 lately

That would be:

Dragon
Falcon 9 (man rated)
DreamChaser
Atlas V (man rated)

So all 3 companies would get money, Boeing as a subcontractor.

[jsgirald]

That would be:

Dragon
Falcon 9 (man rated)
DreamChaser
Atlas V (man rated)

So all 3 companies would get money, Boeing as a subcontractor.

Exactly my view. That is a very political way of making everyone happy.
After all even Boeing didn't look too enthusiastic about CST-100, and they get their piece of the cake too.

[TrevorMonty]

One big plus Dragon has over DC and CST100 is its trunk. NASA has made excellent use of this for delivering bulky external cargo to ISS. 

[Zed_Noir]

My guess/prediction is that the two winners will be DC and Dragon.
- DC a sentimental favorite (wings!), plus lots of progress recently
- Dragon provides the biggest contrasting capabilities (capsule AND different LV, selecting both covers the most bases)
- Boeing has been very quiet about CST-100 lately

That would be:

Dragon
Falcon 9 (man rated)
DreamChaser
Atlas V (man rated)

So all 3 companies would get money, Boeing as a subcontractor.

The Atlas V will only be man-rated after a dual engine Centaur upper stage has successfully flown. It's been a long time since a DEC upper stage has flown.

[oiorionsbelt]

The Atlas V will only be man-rated after a dual engine Centaur upper stage has successfully flown. It's been a long time since a DEC upper stage has flown.

Why is DEC required for man rating and is that that pacing item?