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What difficulties will be faced making modifications for the Falcon 9 FT rocket, so in this way to make smoother, better and easier landings of the first stage from a GTO mission?

For example increasing a little the rocket size in height or diameter so to have a little bit more fuel to burn up 5 more seconds longer or even less than 5 seconds, just to give a better chance for successful landings. Of course modifying in that way that the aerodynamic pressure will be still ok. Another option could be a better engine performance so how much should be increased the performance, the thrust for example since the specific impulse usually is difficult to increase. This could increase the weight of the engines but engines weight is to small compared to the entire fueled rocket weight. So increasing rocket engines thrust in that mass that the new performance gives the rocket 5 more seconds thrust ( 5 seconds is taken as reference just for example ) at the final burn before landing. Another option could be a 4 or 5 engine burn more shortly than 3 engine burn before the final burn with 1 engine at the center of octaweb. Maybe this would not be a good choice but just i am supposing as option that could be better in the same way that SpaceX changed for GTO mission from a single engine burn to a 3 engine burn technique. What difficulties will be for this options which i took as examples or even for other options that could be as possible variants. These modifications or tests will have a cost and probably increase a little bit the cost of Falcon 9 FT but the SpaceX prices are still the lowest in the market and will be lower if a first stage lands successfully much more often and easier going toward perfection and 100% successful landings.

All this description was as an idea or possible suggestions.

So in a simple question what would be the challenges for SpaceX team to make some kind of modifications which will give more seconds to burn up longer the engines and landing easier and safer the first stage?

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If you think about it, if it was easy, SpaceX would be doing it. After all, a used first stage is potentially worth $40 million dollars, so not a minor thing to recover vs lose a stage.

Better engine performance would help, but of course, the Merlin engine has been under constant upgrades throughout it's life time.

The Merlin 1A was used on the Falcon 1. That was actually upgraded to the Merlin 1B on the last two Falcon 1 flights with better performance. The Merlin 1C started the first five Falcon 9 flights, then on to the Merlin 1D for the remainders on the Falcon 9 v1.1 and Full Thrust. Without changing the name, they upgraded the Merlin 1D from 85% to full thrust, and are rumoured to have yet ANOTHER thrust upgrade coming. So they are basically already doing this in the real world, and have been all along.

In terms of fuel, they have already stretched the first and second stage Falcon 1.0 to 1.1. Then they stretched the second stage a fair bit and the first stage a little bit in the F9 Full Thrust (since SpaceX refuses to call it 1.2). In addition they are sub-cooling the fuel and oxidizer to increase the density to stuff more in the same tanks. They cannot increase the girth of the stage, since it needs to be roadable, and they are limited by standard bridge heights. (Have to drive Hawthorne, CA -> McGregor, TX -> CCAFS, FL, and possibly Boca Chica, TX in the future).

They are limited in stretching the first stage length much more since they are approaching bending limits. If it is too long, you might bend the tube (ever done that with a paper towel roll?) when it is in flight and under stress.

The one thing they could really do to help is upgrade to a better engine on the second stage. The first stage's job is to get the second stage high enough and fast enough that the second stage can get the payload where it needs to go.

Thus a better second stage, perhaps using a Raptor which has better Isp (Measure of performance) since the Merlin 1D-Vac is an ok engine, but not a great upper stage performer. But that is anything but a cheap/easy change. Not just from a new stage design issue, but also a switch to Liquid Methane as a fuel, which means new stuff at the launch site, on the TEL, etc.

One of the nice things about the approach SpaceX is taking is that everyone is paying for an expendable launch right now. So they are selling the first stage at a cost they can live on. Then they get a free experiment, full scale, where they can try the variants of ideas their engineers have.

We saw this when they changed from a single engine landing burn, to a three-one engine burn, and then to a 1-3-1 landing burn approach. They are probing around the edges, seeing what they can do to both find where the edges are, and how close they can get to them.

At this point in the year, they potentially have 8 to 13 launches remaining in 2016, each of which they will try to recover the stage. Musk has said, he expects only 70% recovery rate as they try to find those edges in their test program. That they had 4 successes was amazing. Perfection is down the road, but first they need some experiments to get there.

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    $\begingroup$ In addition, they are approaching the limits of what you can move over the US Interstate System. $\endgroup$ – PearsonArtPhoto Jun 21 '16 at 11:38
  • $\begingroup$ I was just wondering for the increasing diameter part. Making taller could face problems with aerodynamic pressure but making wider increasing diameter, even cuple of cm or half a meter would increase a lot the performance. The Atlas V booster 3.81 m, Delta IV booster 5m or the Shuttle fuel tank, all these are transported by the sea. Mabye SpaceX could do the same. I don't know how much expensive is compared by the land transportation. No matter how much could be, it will rise a little bit the cost but still they will win a lot by having successful landings. $\endgroup$ – Mark777 Jul 12 '16 at 21:49
  • $\begingroup$ @MarkBoghdani Consider - a stage is manufactured near LA in Hawthorne, CA. It then travels via truck to McGregor, TX for testing. It might need to go back to the factory if issues found. Else it goes by truck to Cape Canaveral, FL. While LA and FL have ports, it would require a traverse of the Panama Canal, and McGregor is not on a port. So they would need a new test site. Also, at the rate they are building, testing, and launching, they would need a fleet of barges to carry them all, much slower than trucks over longer distances. $\endgroup$ – geoffc Jul 13 '16 at 0:22
  • $\begingroup$ Yes through Panama Canal was transported also the Shuttle fuel tank, but i see that this testing part is an issue. Also even at the current rate they are building they are delaying some missions. Transporting by the sea would cause more delays and for a longer time. So i see that by the sea for now is not an option. $\endgroup$ – Mark777 Jul 13 '16 at 1:20
  • $\begingroup$ Shuttle ETs went through the Panama Canal? I thought they only went from Alabama to Florida. $\endgroup$ – Fred Larson Jul 15 '16 at 2:15
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As geoffc has explained in his answer SpaceX is working for new modifications or solutions by finding the limits of what could be done.

Another possible option for GTO mission landings in theory could be, including the solid rocket booster and not modifying Falcon 9 FT at all. Using two small solid rocket booster only in GTO missions would improve the performance of Falcon 9 FT and would make possible to save more fuel at the first stage, rising a lot the chances for successful landings. It will rise the cost but providing successful landings and reusability, would drop the price alot and customers will still be satisfied. For example two small solid booster at the same category with GEM-40,GEM-46 ($2.5 million one) of Delta II could work for missions similar to those it has made so far. For greater payloads would help solid booster as powerful as GEM-60 of Delta IV, or AJ-60A of Atlas V. So in the future for some type of missions that Falcon 9 FT couldn't make, not necessarily should be used Falcon Heavy (their heaviest version) but a smaller version than it and less costly.

Their Merlin 1D FT is a great engine and they have done a great job. Projecting and producing a solid booster would not be difficult for SpaceX since they are far less complicated and cheaper. Also they could make them even reusable, since they are trying with payload fairings why not they can do this even with their solid boosters, dropping the cost of using them. Even if they won't reuse them still the price will be ok. Until they produce their solid booster they need to buy from Aerojet Rocketdyne or Orbital ATK. It looks difficult to happen since these companies are rivals with SpaceX. And SpaceX has shown ambitions that wants to make everything by themselves in a original way. But could be as option by thinking that they would make reality the reusability of all their first stages.

Of course maybe SpaceX will not need solid boosters, they just will make modifications, improvements or changing procedures that they follow for Falcon 9 FT first stage. They have made a great success with their landings and probably will have 100% success rate in perfection in a couple of months. But remembering the SES-9 GTO mission which was with the heaviest payload until now, SpaceX could have missions with payloads a little heavier or much more heavier than that. It would need a Falcon Heavy, but why using it if a smaller one than it and with more capabilities than Falcon 9 FT, a middle class, could make that mission.

I consider these as option because is a similar way that Delta IV family rockets operate, or as the project of Atlas V family where was planned even a heavy HLV version with three RD-180 rocket engine boosters. In these way they will resolve the difficulties in GTO missions without forcing, pushing to the limits everything in their rocket, and also their costumers don't need to pay more moneys for a heavier rocket (Falcon Heavy) if another version smaller and cheaper could make it.

Most important reason it is not to use solid booster configuration as a replacement for Falcon Heavy in some kind of missions. The most important one is to bring back safely 1st stages from a GTO mission. They are attepmting this with the current Falcon 9 FT and in the next year they expect 100% success. But if they continue to face problems and will not make it 100% success, than buying small solid boosters (or producing depends which option is better for them), could save their Falcon 9 FT 1st stage. Total launch price of a reused Falcon 9 FT is around 43 million. Including the solid boosters, the cost for a new GTO mission still will be lower than the current price 62 million, so no problems for customers they will be still satisfied. SpaceX will have some costs to adjust solid booster with Falcon 9 FT and to make changes in the launchpad, but still they have to many GTO missions upcoming. In this way this costs will not be a problem for a company like SpaceX, it will win a lot more moneys in the upcoming GTO missions. What is worthy for them it is that they could learn from tests and soft landings from GTO missions where we have high velocities. These difficult landings would help them with useful data for Mars landings which are also difficult and which is their goal also. So they could have the chances to study GTO missions (which are many missions) 1st stages by making them reusable. With 100% success the cost will drop more quickly for reusability, and also most important taking useful data for difficult missions in Mars. SpaceX could do further upgrades in Falcon 9 FT and could make possible 100% success in the next year or sooner. If not maybe this could be as option.

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  • $\begingroup$ SpaceX don't like solid boosters, so this would be an unlikely path. Solids are also harder to recover, so will be more expensive to reuse than the F9 first stage. If they need more delta-V, just switching all heavier payloads to Falcon Heavy seems much more likely than introducing a third variant between F9 and FH. $\endgroup$ – Hobbes Jul 14 '16 at 14:56
  • $\begingroup$ Do you have any idea how could be the price of a reused Falcon Heavy? If not then we could not be so sure if is more or less expensive.Maybe it could their policy not using but sometimes it is not about what do you wan't but what you can do and what could be successful or profitable at the moment.Until know price of a reused Falcon 9 is $42.8 million. I cant speak for other solids but in the answer of this question:space.stackexchange.com/questions/13388/… is 2.5M each,and i aspect that GEM-60(better performance) is not far from this price. $\endgroup$ – Mark777 Jul 14 '16 at 15:48
  • $\begingroup$ So in case that using a reusable solid booster have costs of refurbishment higher than a simple single use, than let use a low cost single use solid boosters. If they buy solid boosters of Orbital ATK, with these prices the total cost will be 47.8 million Dollars. Remember they are buying, if they produce by themselve total cost will be even lower. I am suposing that if they build advanced more powerful boosters cost could be 5 million each, so the total 52.8 million. SpaceX declares Falcon Heavy price 90 million Dollars (up to 8 t payyload GTO). $\endgroup$ – Mark777 Jul 14 '16 at 16:11
  • $\begingroup$ I am not sure if a reused Falcon Heavy will have a cost lower than 47.8 million Dollars or than 52.8 million Dollars that I supposed. $\endgroup$ – Mark777 Jul 14 '16 at 16:12
  • $\begingroup$ 47 million is the cost of a new stage. A reused stage would be much cheaper, basically they are trying to drive reuse cost down to the cost of the fuel. Also remember, part of the reason they're landing these stages is to learn about landing, refueling, and taking off from Mars, where you can manufacture liquid propellants, but solids would be difficult to manufacture. $\endgroup$ – Hobbes Jul 14 '16 at 16:23
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One thing SpaceX is working on that I haven't seen mentioned here yet is the landing struts. After their last failed attempt at a barge landing there were some comments about already having new landing struts in the works that would be able to survive the impact of the harder landing and would also be deploy-able earlier to provide more drag for passive speed reduction.

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