XpertKerbalKSP

Ideas for a fully re-usable launch vehicle?

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Hello, fellow kerbonauts!

   After seeing SpaceX make history by launching the world's first re-used rocket and landing it back on the droneship, I thought: What if you could re-use, not only the first stage, but the second stage, the fairing, and - well - everything else? Is this even possible? Would it be worth it?

   I have come up with a few designs for a possible spacecraft, but I also want to know what every one else thinks of this idea. Do you think this would be possible? And do you think it would be worth it? If you have any design ideas, feel free to draw them up (either by hand or using computer software) and share them with everyone else.

Looking forward to hearing your thoughts!

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Remember to not only design, but also engineer. A nice spacecraft isn't going to do you much good if all it does is stylishly fail to achieve orbit. :P

You need to be able to field 9,500 m/s, better 10,000 m/s worth of dV with significant (5-6 tons) payload attached... after deducting all the fuel you need to reserve for making recovery possible (if your scheme calls for reserving fuel, anyway). And after adding all the extra dry mass for systems that enable recovery. For this, keep in mind certain physical limitations, such as tank dry mass ratios. In launch-grade tanks, you might get near 30 for most liquid fuels, but only near 10 for hydrogen; for long-term storage tanks, hydrogen drops down to below 4 (and getting even worse as tank size gets smaller).

Most reusable vehicle ideas run into some of these physical limitations. The tank dry masses make SSTO designs uncompetitive, the poor storability and high handling cost of hydrogen nullifies its Isp advantages, and fuel reserves or added dry mass for recovery reduces payload capacity dramatically.

SpaceX is going after full reusability, too. And they are struggling with the requirements, too. They recover only the first stage right now because it's an order of magnitude easier to pull off... and it still took them 15 years from forming the company to reflying a recovered first stage. If we're lucky, we might see an experimental second stage return attempt with the Falcon Heavy demo flight this fall, but considering how much other important stuff SpaceX has on its plate, I wouldn't hold my breath.

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Oh, tremendous. See, I've been meaning to make precisely this same thread for a while now. I've got a pretty neat design I've been kicking around, so I'll run all the numbers and do a lineart render later today...still excited to see what everyone else comes up with, though!

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I'd say go with something like SpaceX has but scaled up. 9 Raptor engines on a single core would be a bit more liftoff thrust than the Falcon Heavy but with higher efficiency. Second stage would be an ITS mini powered by a single Raptor Vacuum.

 

First stage lands like a normal Falcon 9 second stage (might need a bigger boat).

Second stage has the heat shield on its nose and reenters that way to protect the delicate engine nozzle. A set of superdraco engines also pointing forward allows it to land on its nose.

 

Could probably put like 30 tons into orbit fully reusable.

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1 hour ago, Frozen_Heart said:

I'd say go with something like SpaceX has but scaled up. 9 Raptor engines on a single core would be a bit more liftoff thrust than the Falcon Heavy but with higher efficiency. Second stage would be an ITS mini powered by a single Raptor Vacuum.

First stage lands like a normal Falcon 9 second stage (might need a bigger boat).

Second stage has the heat shield on its nose and reenters that way to protect the delicate engine nozzle. A set of superdraco engines also pointing forward allows it to land on its nose.

Could probably put like 30 tons into orbit fully reusable.

The OP specified total reuse, which means the payload fairing either needs to be incorporated into the second stage or independently recoverable. One consideration is that for a man-rated RLV, you have a lot of additional constraints. If you want commonality between your manned and unmanned launches, you'll need to factor at least some of your man-rating designs into your unmanned design.

Note that SuperDracos are not necessary; if you have Raptor engines, you can use the pressurized intertanks to run a few of the hot-gas oxy+methane thrusters intended for the ITS Spaceship RCS/OMS system. They come in at around 86 kN SL thrust and 93 kN vacuum thrust; we can ballpark their TWR at around 190:1 for a dry mass of 45 kg each. Specific impulse is probably a little lower than Raptor.

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5 hours ago, Streetwind said:

Remember to not only design, but also engineer. A nice spacecraft isn't going to do you much good if all it does is stylishly fail to achieve orbit. :P

You need to be able to field 9,500 m/s, better 10,000 m/s worth of dV with significant (5-6 tons) payload attached... after deducting all the fuel you need to reserve for making recovery possible (if your scheme calls for reserving fuel, anyway). And after adding all the extra dry mass for systems that enable recovery. For this, keep in mind certain physical limitations, such as tank dry mass ratios. In launch-grade tanks, you might get near 30 for most liquid fuels, but only near 10 for hydrogen; for long-term storage tanks, hydrogen drops down to below 4 (and getting even worse as tank size gets smaller).

Most reusable vehicle ideas run into some of these physical limitations. The tank dry masses make SSTO designs uncompetitive, the poor storability and high handling cost of hydrogen nullifies its Isp advantages, and fuel reserves or added dry mass for recovery reduces payload capacity dramatically.

SpaceX is going after full reusability, too. And they are struggling with the requirements, too. They recover only the first stage right now because it's an order of magnitude easier to pull off... and it still took them 15 years from forming the company to reflying a recovered first stage. If we're lucky, we might see an experimental second stage return attempt with the Falcon Heavy demo flight this fall, but considering how much other important stuff SpaceX has on its plate, I wouldn't hold my breath.

So if I understand correctly, you're trying to say that this might actually cost more than the traditional use-up-and-ditch rockets, right?

That's quite understandable. After all, the Space Shuttle ran into problems with maintainance costs and was actually more expensive than normal rockets.

1 hour ago, sevenperforce said:

Oh, tremendous. See, I've been meaning to make precisely this same thread for a while now. I've got a pretty neat design I've been kicking around, so I'll run all the numbers and do a lineart render later today...still excited to see what everyone else comes up with, though!

Well that's great to hear! I'd love to see a sketch or blueprint too!

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39 minutes ago, XpertKerbalKSP said:

So if I understand correctly, you're trying to say that this might actually cost more than the traditional use-up-and-ditch rockets, right?

That's quite understandable. After all, the Space Shuttle ran into problems with maintainance costs and was actually more expensive than normal rockets.

Oh, it'll cost a lot more for sure! And it will generate less revenue when it flies, too. The challenge is saving so much through reusing the initially more expensive, less powerful vehicle that you end up paying less over many consecutive flights.

The Shuttle failed this challenge. The Falcon 9 is currently attempting it, results are not yet in. Let's keep our fingers crossed. :wink:

 

Edited by Streetwind

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1 hour ago, sevenperforce said:

The OP specified total reuse, which means the payload fairing either needs to be incorporated into the second stage or independently recoverable. One consideration is that for a man-rated RLV, you have a lot of additional constraints. If you want commonality between your manned and unmanned launches, you'll need to factor at least some of your man-rating designs into your unmanned design.

Note that SuperDracos are not necessary; if you have Raptor engines, you can use the pressurized intertanks to run a few of the hot-gas oxy+methane thrusters intended for the ITS Spaceship RCS/OMS system. They come in at around 86 kN SL thrust and 93 kN vacuum thrust; we can ballpark their TWR at around 190:1 for a dry mass of 45 kg each. Specific impulse is probably a little lower than Raptor.

SpaceX is currently working on the fairing reuse part. Though not sure if it would work with my idea as chutes don't scale well and a fairing for a 9 raptor vehicle would be much larger than the falcon 9 fairing.

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21 minutes ago, Streetwind said:

Oh, it'll cost a lot more for sure! And it will generate less revenue when it flies, too. The challenge is saving so much through reusing the initially more expensive, less powerful vehicle that you end up paying less over many consecutive flights.

The Shuttle failed this challenge. The Falcon 9 is currently attempting it, results are not yet in. Let's keep our fingers crossed. :wink:

This can't be stated enough.  Don't forget, there have been 31 new falcon9 boosters used/destroyed and one "flight proven" booster launched.  Expect it to take quite a while to pay for all the "extra tech" poured into earlier falcon launches.  I'd assume that spacex would add a large "R&D benefits for Raptor" bonus to falcon9's side of the ledger to get things to balance.  Of course, the falcon9 has some pretty cheap launch costs (especially to customers), so it isn't like they could be spending all that *much* on recovery (not like NASA did).

From the sound of it, block 5 is the "reusable falcon 9" and the 30+ earlier ones were essentially prototypes*.  No idea if they can recover all the costs to get to block 5.  Remember, even 1.0 could likely launch more [unrecoverable] mass than block 5 can with recovery.

* my definition of prototype is anything that requires an engineer's (or scientist's) job to get the thing working and out the door.  At least one [small volume] circuit board I was involved in was shipping in this state for *years*.

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7 minutes ago, wumpus said:

From the sound of it, block 5 is the "reusable falcon 9" and the 30+ earlier ones were essentially prototypes*.  No idea if they can recover all the costs to get to block 5.  Remember, even 1.0 could likely launch more [unrecoverable] mass than block 5 can with recovery.

While I have no argument with the rest of your analysis, I'm pretty sure this is incorrect. Falcon 9 v1.0 could only launch 4.5 tonnes to GTO, expendable. Falcon 9 FT sent SES-10 to GTO, with recovery, and that bird was over five tonnes.

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If SpaceX's plans for the Mars come to fruition, SpaceX would eventually want to transition Falcon 9 and Falcon Heavy payloads to a Raptor-based architecture, to enable access to space for payloads smaller than the full ITS/BFR/BFS capacity. On the flip side, if the Mars colonization plans never quite pan out, SpaceX will still need a use for their methalox engines. So either way, we need a Raptor-derived fully-reusable TSTO with payloads roughly equivalent to the Falcon family. It will need to be man-rated, too, since there will surely be a need for sending passengers to orbit in numbers lower than the 100+ capacity of the ITS/BFR/BFS system.

One problem with this is that a single-engine upper stage has a TWR too high to use for propulsive landing, even if it wasn't overexpanded at sea level. Thus, it needs auxiliary landing thrusters. Another problem is re-entry; an unmanned stage can come back using a heat shield on its nose, but that's not much fun for passengers, and I have a strong preference for a "true" TSTO where the crew cabin is integrated. You need biconic re-entry a la ITS. But if you're already using auxiliary thrusters and biconic re-entry, you don't necessarily have to align the aux thrusters with the main engine vector. That's where things get...interesting.

Here is a line drawing and a very very rough sketch of my concept:

Spoiler

 

Ascent.png

ascent2.png

 

Looks a bit like the ITS, doesn't it? Same basic principle (composite monocoque tanks, etc), except the diameter is only four meters, making it roughly the same size as the Falcon 9 but slightly wider. 

The first stage has two full-size SL Raptor engines for launch and boostback and six methalox hot gas SL thrusters for landing, along with four landing legs:

Spoiler

engine_cluster.png

The first stage has a dry mass of 17 tonnes and a propellant capacity of 421 tonnes; it delivers the upper stage at a notional staging velocity between 1.5 and 2.5 km/s and executes a boostback RTLS landing. Minimum initial TWR for the boostback burn is 2.7:1 with both Raptors at minimum throttle; maximum landing TWR on thrusters alone is 3:1 but it can easily hover. I've factored in the masses of the thrusters and everything else.

The upper stage is where it gets really interesting. Rather than using Raptor engines, which would be way oversized, it uses a pair of the Raptor Development engines (1,000 kN SL thrust) with vacuum nozzle extensions. I'm estimating their mass at 638 kg each. Total stage vacuum thrust is 2,292 kN. Dry mass is 6.6 tonnes and propellant capacity is 141 tonnes.

Because the vacuum engines cannot be used at sea level, I gave the upper stage eight SL-expanded methalox thrusters in addition to its vacuum-optimized RCS thrusters, with a combined SL thrust of 688 kN. But I didn't want to cluster them around the devRaptors in the tail, both for space considerations and because of damage to the engine bells.

See the wing extensions shown in the above line drawing? The landing thrusters are placed underneath the wing extensions, pointing down. For re-entry, the upper stage enters biconically, on its belly. It then glides/falls to the landing site before hydraulically-actuated panels open up underneath the wing extensions, both exposing the landing thrusters and providing rear "legs" for the vehicle to land on, so it lands vertically but in a horizontal attitude, eliminating the risk of tip-over. The landing would look like something out of Star Wars, because it drops, winglets open, and it lands on the wingtips with rocket propulsion.

Based on my simulations using this calculator, the launch system could deliver up to 6.8 tonnes to GTO with full reuse or up to 24 tonnes to LEO with full reuse. For LEO launches, the upper stage can also recover up to 30 tonnes of downmass. This is obviously plenty of margin to have a crewed version, which would use the same tank and body as the rest of the orbiter but have a crew cabin in place of the cargo bay. Payload capacity is high enough that the crew cabin could carry at least a dozen crew members plus unpressurized cargo and still have independent LES and re-entry capability (lifeboat).

Know what else is great? Due to the vertically-oriented thrusters, the upper stage could both land on and take off from the Moon or from the surface of Mars without needing a launch pad. On Mars, it would need to be refueled on the Martian surface; the lower gravity means that the thrusters have enough thrust to lift it off the ground so the main engines could be fired up. For lunar missions, simply being refueled once in LEO would give it ample dV to fly to the moon, land, SSTO, and return to LEO.

Edited by sevenperforce

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@sevenperforce that's pretty intense. Got any contacts at SpaceX? :wink: But seriously, if you were able to think that up in around 8 hours, what's taking SpaceX so long? It seems like a great idea, and they are already working on the required tech...

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35 minutes ago, Benjamin Kerman said:

@sevenperforce that's pretty intense. Got any contacts at SpaceX? :wink: But seriously, if you were able to think that up in around 8 hours, what's taking SpaceX so long? It seems like a great idea, and they are already working on the required tech...

Thanks!

Right now it would cannibalize Falcon 9 terribly, but it might be something they'd look at in the future. I'm really a sucker for horizontal, Star Wars style VTVL, so someone else might not have come up with something like this.

Here's an underside render so it makes a little more sense:

ascent3.png

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32 minutes ago, sevenperforce said:

Thanks!

Right now it would cannibalize Falcon 9 terribly, but it might be something they'd look at in the future. I'm really a sucker for horizontal, Star Wars style VTVL, so someone else might not have come up with something like this.

Here's an underside render so it makes a little more sense:

ascent3.png

Don't actually agree with landing it on its side as it would need a much larger heat shield, and so more weight. With the ITS it is probably needed due to the way mass scales faster than surface area, but for somthing smaller a nose or tail first approach and landing would be more efficient.

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22 minutes ago, Frozen_Heart said:

Don't actually agree with landing it on its side as it would need a much larger heat shield, and so more weight. With the ITS it is probably needed due to the way mass scales faster than surface area, but for somthing smaller a nose or tail first approach and landing would be more efficient.

Actually, the biconic entry is far, far more mass-efficient. A larger surface area will encounter drag at a greater altitude and will be able to use hypersonic lift, dramatically reducing g forces and peak heating. 

14 minutes ago, Benjamin Kerman said:

You could just spam parachutes for the second stage...

Fuel is cheaper, mass-wise.

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To the re-entry question, again....

IIRC, the Shuttle's external tank was so lightweight and yet so large that when it hit the atmosphere, it didn't immediately burn up. It would tumble, efficiently dissipating heat due to its high drag coefficient and low mass, until aerodynamic forces broke it up, and then the pieces would burn up.

The larger the surface area you can expose, the better. Going in nose-first would produce far more heating and a much nastier deceleration.

EDIT: I kept the thrusters behind the landing panels to protect them from re-entry -- I can't imagine that plasma is healthy for engines -- but you never know. There might be a way to have them exposed but not directly in the plasma stream, which would allow them to be used for RCS control as well.

EDIT 2: Note that with the first stage, the landing thrusters can be used to give an extra kick to TWR off the pad. They won't have as good of specific impulse as the main engines, but for very large payloads it might be a good tradeoff.

EDIT 3: You can also use a larger cluster of dev Raptors on the first stage in place of the two full-size SL Raptors and maybe be able to land on the center one, but they won't have as good of a TWR or isp. I like the idea of using a smaller version of the same engine on the upper stage.

It should be noted that the dry mass of the first stage is actually significantly lower than the dry mass of the Falcon 9 first stage, due to the use of a slightly greater diameter to hold more propellant for less mass, and the use of composites. It would be very easy to move and transport and still within road-transport size.

Edited by sevenperforce

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8 hours ago, sevenperforce said:

Actually, the biconic entry is far, far more mass-efficient. A larger surface area will encounter drag at a greater altitude and will be able to use hypersonic lift, dramatically reducing g forces and peak heating. 

Fuel is cheaper, mass-wise.

Agree on sideways for reentry but why land on the side and not rear or even nose as we discussed, 

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5 hours ago, magnemoe said:

Agree on sideways for reentry but why land on the side and not rear or even nose as we discussed, 

Landing on the tail (e.g., ITS Spaceship/Tanker) isn't possible because the smaller vehicle only has the high-thrust vacuum-expanded engines in the tail. You could put thrusters in the tail, but there isn't a lot of space back there, and you run the risk of damage to the engine bells, either from plume impingement or from debris being kicked up. The tail-first landing also requires very large fold-out landing legs to clear the engine bells.

Landing on the nose might work a little better, but it isn't suitable for manned launches for several reasons. First, emergency abort during landing isn't possible if you're coming down cabin/capsule-first. Next, there is an increased tip-over risk. Finally, there's no seating arrangement which can support rear g-forces during launch and forward g-forces during landing.

Coming down like a modern-scifi spaceship is by far the safest and most stable landing mode. 

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On 21/4/2017 at 11:23 PM, sevenperforce said:

If SpaceX's plans for the Mars come to fruition, SpaceX would eventually want to transition Falcon 9 and Falcon Heavy payloads to a Raptor-based architecture, to enable access to space for payloads smaller than the full ITS/BFR/BFS capacity. On the flip side, if the Mars colonization plans never quite pan out, SpaceX will still need a use for their methalox engines. So either way, we need a Raptor-derived fully-reusable TSTO with payloads roughly equivalent to the Falcon family. It will need to be man-rated, too, since there will surely be a need for sending passengers to orbit in numbers lower than the 100+ capacity of the ITS/BFR/BFS system.

One problem with this is that a single-engine upper stage has a TWR too high to use for propulsive landing, even if it wasn't overexpanded at sea level. Thus, it needs auxiliary landing thrusters. Another problem is re-entry; an unmanned stage can come back using a heat shield on its nose, but that's not much fun for passengers, and I have a strong preference for a "true" TSTO where the crew cabin is integrated. You need biconic re-entry a la ITS. But if you're already using auxiliary thrusters and biconic re-entry, you don't necessarily have to align the aux thrusters with the main engine vector. That's where things get...interesting.

Here is a line drawing and a very very rough sketch of my concept:

  Hide contents

 

Ascent.png

ascent2.png

 

Looks a bit like the ITS, doesn't it? Same basic principle (composite monocoque tanks, etc), except the diameter is only four meters, making it roughly the same size as the Falcon 9 but slightly wider. 

The first stage has two full-size SL Raptor engines for launch and boostback and six methalox hot gas SL thrusters for landing, along with four landing legs:

  Hide contents

engine_cluster.png

The first stage has a dry mass of 17 tonnes and a propellant capacity of 421 tonnes; it delivers the upper stage at a notional staging velocity between 1.5 and 2.5 km/s and executes a boostback RTLS landing. Minimum initial TWR for the boostback burn is 2.7:1 with both Raptors at minimum throttle; maximum landing TWR on thrusters alone is 3:1 but it can easily hover. I've factored in the masses of the thrusters and everything else.

The upper stage is where it gets really interesting. Rather than using Raptor engines, which would be way oversized, it uses a pair of the Raptor Development engines (1,000 kN SL thrust) with vacuum nozzle extensions. I'm estimating their mass at 638 kg each. Total stage vacuum thrust is 2,292 kN. Dry mass is 6.6 tonnes and propellant capacity is 141 tonnes.

Because the vacuum engines cannot be used at sea level, I gave the upper stage eight SL-expanded methalox thrusters in addition to its vacuum-optimized RCS thrusters, with a combined SL thrust of 688 kN. But I didn't want to cluster them around the devRaptors in the tail, both for space considerations and because of damage to the engine bells.

See the wing extensions shown in the above line drawing? The landing thrusters are placed underneath the wing extensions, pointing down. For re-entry, the upper stage enters biconically, on its belly. It then glides/falls to the landing site before hydraulically-actuated panels open up underneath the wing extensions, both exposing the landing thrusters and providing rear "legs" for the vehicle to land on, so it lands vertically but in a horizontal attitude, eliminating the risk of tip-over. The landing would look like something out of Star Wars, because it drops, winglets open, and it lands on the wingtips with rocket propulsion.

Based on my simulations using this calculator, the launch system could deliver up to 6.8 tonnes to GTO with full reuse or up to 24 tonnes to LEO with full reuse. For LEO launches, the upper stage can also recover up to 30 tonnes of downmass. This is obviously plenty of margin to have a crewed version, which would use the same tank and body as the rest of the orbiter but have a crew cabin in place of the cargo bay. Payload capacity is high enough that the crew cabin could carry at least a dozen crew members plus unpressurized cargo and still have independent LES and re-entry capability (lifeboat).

Know what else is great? Due to the vertically-oriented thrusters, the upper stage could both land on and take off from the Moon or from the surface of Mars without needing a launch pad. On Mars, it would need to be refueled on the Martian surface; the lower gravity means that the thrusters have enough thrust to lift it off the ground so the main engines could be fired up. For lunar missions, simply being refueled once in LEO would give it ample dV to fly to the moon, land, SSTO, and return to LEO.

I like the idea, but stability. It's the one thing that also irks me about ITS, how the heck do they control the attitude during reentry. The CG/CP positions must be very tightly controlled for such a thing (forget forcing things propulsively if you are trying to brake aerodynamically), and you have, at the very least, a cargo bay that might or might not be full. You need something ballast-y on the nose to balance the engines, but you need that to stay put and maintain its weight. Oh, and you need to account for varying fuel levels, or a tank at the CoM. And last but not least, it would be nice if the thing more or less stays rigid during reentry, which means it is capable of maintaining shape when empty and decelerating laterally at some 3Gs or more. Or with a full payload bay and doing the same, which would be more impressive, if you want downmass.

I mean, if you can work it out without too many structural efficiency sacrifices, yeah, great concept. But I would like a full structural/thermal/aerodynamic load analysis before assigning a mass fraction to that stage, unless I use a "fudge factor" of about 2. So you'll excuse me if I take your numbers with a grain of salt the size of some metaphorical houses.

Oh, and the thing about single-staging to the Moon and back from LEO... you are aware that is 6km/s each way, right? At least nine for the roundtrip, and that's with a high-velocity reentry directly from the Moon, several times harder than any LEO reentry. How much dV do you want to pack in this stage, again?

On 22/4/2017 at 0:50 AM, Frozen_Heart said:

Don't actually agree with landing it on its side as it would need a much larger heat shield, and so more weight. With the ITS it is probably needed due to the way mass scales faster than surface area, but for somthing smaller a nose or tail first approach and landing would be more efficient.

Actually, as has been said already, high surface area is nice, because it lowers peak heating. A big ballistic coefficient is always nice on reentry. And if you can use "only" ceramics and high-temperature metallic alloys, you might have a reusable non-ablative heatshield like the shuttle. Which will be much safer because it sits on top of its first stage, not at the side and facing it.

 

Rune. That being said, I believe reusable TSTO is doable, even in an economical way, at least for launches to LEO.

Edited by Rune

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22 hours ago, sevenperforce said:

Landing on the tail (e.g., ITS Spaceship/Tanker) isn't possible because the smaller vehicle only has the high-thrust vacuum-expanded engines in the tail. You could put thrusters in the tail, but there isn't a lot of space back there, and you run the risk of damage to the engine bells, either from plume impingement or from debris being kicked up. The tail-first landing also requires very large fold-out landing legs to clear the engine bells.

Landing on the nose might work a little better, but it isn't suitable for manned launches for several reasons. First, emergency abort during landing isn't possible if you're coming down cabin/capsule-first. Next, there is an increased tip-over risk. Finally, there's no seating arrangement which can support rear g-forces during launch and forward g-forces during landing.

Coming down like a modern-scifi spaceship is by far the safest and most stable landing mode. 

See it adds some benefits as you say, nose don't work manned.
And I admit its an cool way to land, you would also use the large drag to get an lower landing speed saving fuel. 
You would obviously fire up the landing engine earlier but idle them until closer to ground. 

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7 hours ago, Rune said:

I like the idea, but stability. It's the one thing that also irks me about ITS, how the heck do they control the attitude during reentry. The CG/CP positions must be very tightly controlled for such a thing (forget forcing things propulsively if you are trying to brake aerodynamically), and you have, at the very least, a cargo bay that might or might not be full. You need something ballast-y on the nose to balance the engines, but you need that to stay put and maintain its weight. Oh, and you need to account for varying fuel levels, or a tank at the CoM. And last but not least, it would be nice if the thing more or less stays rigid during reentry, which means it is capable of maintaining shape when empty and decelerating laterally at some 3Gs or more. Or with a full payload bay and doing the same, which would be more impressive, if you want downmass.

Elon said that the ITS Spaceship and Tanker will both use split body flaps to control roll and pitch during re-entry. It will have plenty of yaw authority from its auxiliary thrusters. One nice thing about using a biconic re-entry is that you do get substantial body lift, which is AOA-dependent. So there's an element of pitch self-correction in the overall design: if your COM is farther back than it should be and it kicks your nose up, your lift increases, which raises your altitude and decreases your drag, allowing you to pitch back forward. Split flaps are best at giving roll authority, perhaps aided by auxiliary thrusters, so hypersonic attitude control shouldn't be too difficult to manage.

With my design, it might even be possible to actuate the four underside panels differentially to provide the same sort of attitude control. If not, split flaps on the tail would do the job well enough. The overall aerodynamic design would most likely be tuned to passive re-entry with a crewed-version mass distribution, since that's the one version you are most concerned about re-entering safely. The cargo version would rely more heavily on the split flaps or actuated panels.

7 hours ago, Rune said:

I mean, if you can work it out without too many structural efficiency sacrifices, yeah, great concept. But I would like a full structural/thermal/aerodynamic load analysis before assigning a mass fraction to that stage, unless I use a "fudge factor" of about 2. So you'll excuse me if I take your numbers with a grain of salt the size of some metaphorical houses.

My mass-fraction numbers may be slightly optimistic, but I don't think so. At least, they're no more optimistic than Elon's structural mass numbers for the ITS system. I filled up about three Excel spreadsheets making sure all the math came out right. For structural mass, I took middle-of-the-road estimates for dry mass on the Raptor engines, adjusted based on TWR for the Vacuum Raptors, deducted total engine mass from the quoted ITS system dry masses, and used that as the structural/tankage mass. I took appropriate square-cube reductions and I added in my engines and my auxiliary thrusters on top of the calculated dry mass for the smaller vehicle.

7 hours ago, Rune said:

Oh, and the thing about single-staging to the Moon and back from LEO... you are aware that is 6km/s each way, right? At least nine for the roundtrip, and that's with a high-velocity reentry directly from the Moon, several times harder than any LEO reentry. How much dV do you want to pack in this stage, again?

Just shy of 12 km/s. No, really. With the structure alone as the payload and a full tank in LEO, it would have a whopping 11.678 km/s of dV. Of course, actually having positive payload is important. With LEO refueling it could deliver 19.2 tonnes to the lunar surface one-way or a more modest 7.6 tonnes round-trip. That 381 seconds of ISP, along with the really good tankage ratio of composites, can do a lot.

I mentioned this before, but the placement of the auxiliary thrusters within the wings means that the ship can land on unprepared surfaces with relative safety, since the thruster wash doesn't impinge on the ground nearly as harshly as with more conventional landing systems.

6 hours ago, magnemoe said:

See it adds some benefits as you say, nose don't work manned.
And I admit its an cool way to land, you would also use the large drag to get an lower landing speed saving fuel. 
You would obviously fire up the landing engine earlier but idle them until closer to ground. 

I try not to succumb to Rule of Cool too often, but I couldn't resist this. Mostly because it does genuinely offer some real advantages. And, good grief, who doesn't want to see a sleek spaceship rise straight off the lunar pad on thrusters, rotate gently to orient properly, and then fire up its big engines in the back to blast into orbit? It's exactly how the Millennium Falcon takes off.

I set the total auxiliary engine thrust high enough to allow the same vertical takeoff on Mars, which (coincidentally) is precisely what you need for a nice tight landing on Earth.

One thing I'm unsure of is the pressurization issues for the auxiliary thrusters. They are autogenously pressurized off the main Raptors, so I'm not sure how much dV they can push before they start to lose tank pressure.

Edited by sevenperforce

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1 hour ago, DECQ said:

10565790.jpg

There's a splendid blending of graceful awe and stunning complexity with a liquid-flyback-booster stack. Though I think the payload-to-dry-mass ratio is...not great.

How would you choose engines/body/fuel types/etc.?

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