What is the point of landing back on Earth with landing gear?

[Notwal]

I read an article on Dragon V2, and it mentioned that it would be able to land with parachutes or by burning retrograde and landing on legs. What is the point of burning retrograde when you can just use parachutes and have a lighter craft?

[Dres]

Ironically, because parachutes are heavy. It is much more cost effective to forego adding parachutes and instead just land it using rockets. You really don't want heavy anything, because it cuts down on available payload fraction. Legs are generally just metal sticks, so they weigh less than those complicated bundles of canvas and wires.

Edit: Another thing I remembered, using engines allowed for more controlled descent. If you want something to land gently and be recoverable (which SpaceX wants), you need finer control than falling with style.

Edited March 15, 2016 by Dres

[GoSlash27]

Notwal,

 If you land with parachutes, whatever takes the brunt of the impact with the surface is destroyed. If it lands in salt water, the corrosion will render it unusable.

 The idea of retroburning and landing on gear is that you can refurbish and reuse everything.

Best,
-Slashy

[sevenperforce]

Splashdown requires recovery by ship, which is costly and requires a ton of infrastructure and support. The Soyuz capsule parachutes to land...but it actually requires its own set of solid-fueled retrorockets mounted on the parachute cables to slow down at the very end. A parachute which actually slows down a 7-tonne capsule to a gentle landing speed would be absolutely massive and extremely heavy. If the retros on the Soyuz fail, you live, but you break most of your teeth and possibly your spine.

The Dragon V2 uses the landing rockets as an emergency launch abort system as well, saving weight at launch. Most capsules have had an external abort tower that is discarded every mission it isn't used. 

The landing legs soften touchdown and will probably collapse/crush in the event of a parachute landing on land, destroying themselves in the process but saving the occupants. 

[wumpus]

The original plan was to recover Falcon (1) with parachutes.  I doubt that space-x has made it completely clear if the problem was control, regulation, or getting too wet.  If you launch at Florida, prevailing winds are going to be from land to ocean.  You have to bring your vessel back into the atmosphere well over land (good luck getting permission from the FAA), drift over Florida, and hopefully bring the thing down on land.  If you want a barge landing, you need finer control than most parachutes have (see falling with style comments above).

I'm still shocked that they don't carry *any* parachutes, I would think that drogue chutes would weigh less than the fuel burned, and vastly cut into just how tricky the burns are.  My guess is that it has more to do with the throttling: anything that provides lift to the rocket makes the suicide burn that much harder.  They really do want to support the full weight to reduce the strength of the rocket (and maximize the margin of error for the suicide burn).  On the flip side, since Blue Origin *can* hover, they have no interest in parachutes.  Remember: throttling is one of those things where KSP vastly differs from real life.  Very few rockets throttle at all, and Merlin engines impressively throttle to 60%.  This is still too much to hover and they are forced to do suicide burns during landing.

[sevenperforce]

Putting chutes on the Falcon first stage would not reduce its speed enough to let it touch down gently.

[fredinno]

2 hours ago, wumpus said:

The original plan was to recover Falcon (1) with parachutes.  I doubt that space-x has made it completely clear if the problem was control, regulation, or getting too wet.  If you launch at Florida, prevailing winds are going to be from land to ocean.  You have to bring your vessel back into the atmosphere well over land (good luck getting permission from the FAA), drift over Florida, and hopefully bring the thing down on land.  If you want a barge landing, you need finer control than most parachutes have (see falling with style comments above).

I'm still shocked that they don't carry *any* parachutes, I would think that drogue chutes would weigh less than the fuel burned, and vastly cut into just how tricky the burns are.  My guess is that it has more to do with the throttling: anything that provides lift to the rocket makes the suicide burn that much harder.  They really do want to support the full weight to reduce the strength of the rocket (and maximize the margin of error for the suicide burn).  On the flip side, since Blue Origin *can* hover, they have no interest in parachutes.  Remember: throttling is one of those things where KSP vastly differs from real life.  Very few rockets throttle at all, and Merlin engines impressively throttle to 60%.  This is still too much to hover and they are forced to do suicide burns during landing.

Parachutes won't allow for the needed precision.

[wumpus]

2 hours ago, sevenperforce said:

Putting chutes on the Falcon first stage would not reduce its speed enough to let it touch down gently.

 

4 minutes ago, fredinno said:

Parachutes won't allow for the needed precision.

Is anybody here a skydiver?  Do you know the conditions of Florida?  Considering just how flat the place is, I'd expect that wind is a huge consideration for using a parachute in Florida.  Reducing the vertical speed is straightforward engineering.  The problem is that as you reduce your vertical speed, you increase your horizontal speed (pretty much up to wind speed).  I'm guessing nothing will remain vertical with a parachute (hints for KSP play: landing big rockets is easiest if you land them horizontally in the ocean.  Kerbin's oceans are salt free).

Of course, while it seems easier to get FAA approval for a parachute landing over land, it pretty much means re-entry and deploying chutes over populated areas.  Don't think that *ever* will happen, which means the needed precision has to be done over water (I'm sure you could hit the right sized parking lot, assuming you were willing to go asymmetrical and land horizontal.  You just will never get the permit).

[sevenperforce]

1 hour ago, wumpus said:

Is anybody here a skydiver?  Do you know the conditions of Florida?  Considering just how flat the place is, I'd expect that wind is a huge consideration for using a parachute in Florida.  Reducing the vertical speed is straightforward engineering.  The problem is that as you reduce your vertical speed, you increase your horizontal speed (pretty much up to wind speed).  I'm guessing nothing will remain vertical with a parachute (hints for KSP play: landing big rockets is easiest if you land them horizontally in the ocean.  Kerbin's oceans are salt free).

While your considerations regarding horizontal speed are entirely valid, I'd also point out that reducing vertical speed becomes prohibitively difficult below a certain point, because terminal velocity is proportional to the square root of your parachute cross-sectional area.

[Nibb31]

The Shuttle SRBs were the largest items ever parachuted down, and carried the biggest parachutes ever made. Each chute system, with covers, pyros, etc, weighed about 2 tons for an SRB that weighed 90 tons empty. Even with those massive chutes, the SRBs splashed down at a pretty high speed and were often damaged.

Contrary to what people think, parachutes are heavy and expensive. They need a lot of manual processing, and the bigger they are, the more complex it is to pack and handle those chutes.

[sevenperforce]

1 hour ago, Nibb31 said:

The Shuttle SRBs were the largest items ever parachuted down, and carried the biggest parachutes ever made. Each chute system, with covers, pyros, etc, weighed about 2 tons for an SRB that weighed 90 tons empty. Even with those massive chutes, the SRBs splashed down at a pretty high speed and were often damaged.

Contrary to what people think, parachutes are heavy and expensive. They need a lot of manual processing, and the bigger they are, the more complex it is to pack and handle those chutes.

Yeah, this is what it looked like when the SRBs hit the water.

Impact speed: 52 mph. The chute system weighs three tons, actually. If you want to cut the speed to 26 mph (which, I'll point out, still won't allow a touchdown on land), you'd have to increase the chute area by a factor of 4, increasing their mass to well over 12 tonnes.

[wumpus]

42 minutes ago, sevenperforce said:

Yeah, this is what it looked like when the SRBs hit the water.

Impact speed: 52 mph. The chute system weighs three tons, actually. If you want to cut the speed to 26 mph (which, I'll point out, still won't allow a touchdown on land), you'd have to increase the chute area by a factor of 4, increasing their mass to well over 12 tonnes.

Of course, Space-x needs about 10% of the fuel for the return trip, which corresponds to shockingly close to 10% of the launch weight (of just the booster).  This might be a bit less than the mass of the parachute needed, but I suspect the "final design" before space-x gave up the parachutes involved a slightly lower ratio of parachute weight, and a higher speed.  The parachute would be then cut (similar to a Mars rover) somewhat before landing.  It would then do an extremely short burn to land.

Problems:

No land option in Florida (you would have to point the rocket directly over inhabited land, and let the parachute drift toward Cape Canaveral.  Might be possible to drift toward the Mexican coast from Vandenburg).

No precision: While the retrorocket option might give a fairly precise control in a couple of dimensions (vertical and the horizontal component of prograde), it won't really give any sort of correction you need to hit a barge.

Even worse timing: One of the reasons I'm less concerned about the size of the parachute is that it will likely be dumped before the landing burn.  Doing the landing burn while on a parachute would be rather similar to the three engine burn (way to high TWR) that poked a hole in "Of Course I Still Love You".  Dropping the parachute early enough to counteract this (and keep the thing from falling over) might spoil all the benefits of the parachute.

 

As far as I know, for all the obvious problems, space-x went ahead and tried to recover the Falcon (1) via parachute.  I can only imagine what the "even worse timing" problems were: imagine a falcon 9 booster trying to land with all 9 engines firing (roughly the TWR of a falcon 1 with its single Merlin firing).  I suspect that while doing the detailed design, it became obvious that they weren't going to recover until the Falcon 9 came along and could dial down the TWR of the landing boost to a dull roar.   I still think they kept trying to recover the Falcon 1 with parachutes anyway.

[problemecium]

This thread has taught me many new things o_o

[Atlas2342]

Plus, retrorockets do not require careful packing and they do not suffer from tangled parachutes. They also allow for a more gradual descent without adding more weight (drogues)  and points of failure...

[kiwi1960]

Agree with all that has been said BUT....

Landing gear is way better for being able to actually walk away from a landing.... Just ask Jeb!

 

 

[RainDreamer]

It also gives more control in landing than just let it drop and pray it will be in the vicinity of where you want it to be.

Although I wonder what else they have in the event that the retro rocket fail.

[Atlas2342]

20 minutes ago, RainDreamer said:

It also gives more control in landing than just let it drop and pray it will be in the vicinity of where you want it to be.

Although I wonder what else they have in the event that the retro rocket fail.

Ironically, i think parachutes...

[RainDreamer]

14 minutes ago, Atlas2342 said:

Ironically, i think parachutes...

You are not kerbal enough. The answer is always more rockets.

[sevenperforce]

2 hours ago, RainDreamer said:

It also gives more control in landing than just let it drop and pray it will be in the vicinity of where you want it to be.

Although I wonder what else they have in the event that the retro rocket fail.

Those hypergolic engines are extremely reliable and deeply throttleable; they can lose any two and so have enough thrust to make a safe landing. So in a sense the answer is more rockets. 

But the backup parachutes (four in all) have enough area to slow the capsule down considerably. Splashdown with parachutes will be gentle enough to preserve the capsule for reuse as long as the saltwater corrosion isn't too bad. Parachuting to a landing on terra firma will crush the landing legs to splinters, but they will absorb enough of the shock to save the occupants from discomfort. 

[Atlas2342]

15 minutes ago, sevenperforce said:

Those hypergolic engines are extremely reliable and deeply throttleable; they can lose any two and so have enough thrust to make a safe landing. So in a sense the answer is more rockets. 

But the backup parachutes (four in all) have enough area to slow the capsule down considerably. Splashdown with parachutes will be gentle enough to preserve the capsule for reuse as long as the saltwater corrosion isn't too bad. Parachuting to a landing on terra firma will crush the landing legs to splinters, but they will absorb enough of the shock to save the occupants from discomfort. 

Meh, Kerbal or not, still half right....That's close enough for me...

PS: DEFFO KERBAL WAY!!!

[Motokid600]

Its a lot more then just slowing down. The gridfins alone are not enough to control the rocket especially at slower speeds. They need to fire the engines so the gimbals can put it on target. That's part of the reason why the last landing attempt failed. Three engines meant for a shorter hover slam therefore less time gimbaling which equals less control. ( though granted the speeds were higher so the burn may have been a similar length as with a single engine )

[sojourner]

Some of you guys are talking about the Dragon V2 and some of you are talking about Falcon 9 first stage.  You really should clarify for the OP.

Edited March 16, 2016 by sojourner

[Nibb31]

Well, first of all, although it's designed for the purpose, it's unlikely that we ever see a manned Dragon V2 land propulsively. The Commercial Crew missions will all be splashing down on parachutes. Maybe some cargo return missions will test the propulsive Dragon V2 landings and reusability, but in only 6 to 10 planned flights, they won't build up enough confidence to risk manned flights on it.

Since NASA is the only customer for Dragon, it's unlikely that it flies again after 2024, unless a new customer pops up.

Edited March 16, 2016 by Nibb31

[fredinno]

20 minutes ago, Nibb31 said:

Well, first of all, although it's designed for the purpose, it's unlikely that we ever see a manned Dragon V2 land propulsively. The Commercial Crew missions will all be splashing down on parachutes. Maybe some cargo return missions will test the propulsive Dragon V2 landings and reusability, but in only 6 to 10 planned flights, they won't build up enough confidence to risk manned flights on it.

Since NASA is the only customer for Dragon, it's unlikely that it flies again after 2024, unless a new customer pops up.

V2 will never be used for cargo return, it doesn't have the correct docking port.

[Nibb31]

SpaceX's CRS proposal involved a mix of V1 and V2 Dragons for cargo depending on requirements. I suspect that they will use V1 for the early flights and try to transition to V2 for missions that don't require CBM berthing. That way they can put the V2 propulsive landing through it's paces on unmanned flights.