Speculation thread: intermediate-lift Raptor-derived RLV

[AncientU]

Not "no matter what." If you refuel a BFS at near escape, then you have enough delta-V even after the departure burn and releasing the payload to flip around and do a burn to stay in Earth vicinity.

Probably doable. But at speeds for deep space, outer planet probes it will be a lot of propellant needed and that means a number of tankers going out there, all refueled in orbit. Is it worth it?

Yeah. ITS tankers are like $3.6 million per launch and $130 million to build. So it'd take 35 tanker launches to equal the cost of expending one of them (less since an expendable tanker would cost less to make, but still). Scaled for this smaller ITS, of course. So even if it takes like a dozen tanker flights, it's still worth it.

I think max performance plus cost would be to take payload plus expendable solid (or liquid like ACES) to HEO and release it there.  A fully-fueled ACES from EML-2 could beat most anything out there.

This could also be done with multiple ACES, of course.

[dror]

The unsolved problem for full reuse deals mostly with second stage recovery, since SpaceX seems to have first stage recovery down and is unlikely to change anything there. Recovering the second stage is tricky. With ITS, the plan is to include SL engines on the second stage (which help with TWR right after launch) and land it in much the same way as the booster, albeit after a biconic lifting-body re-entry. That might work for a monster like the ITS Spaceship, but the design doesn't really scale down well.

SpaceX is, however, planning to develop hot-gas methane-oxy thrusters for RCS on the ITS. In a Reddit AMA, they were confirmed at 10 metric tonnes vacuum thrust each. So if we need auxiliary landing thrusters, these might work.

It would also be nice to be able to use the upper stage as an SSTO for Martian missions or lunar missions, so that's another consideration.

I mentioned in the OP that I had my own idea for a fully-reusable RLV, so here goes:



Looks a little like the ITS, right? But it would be 4 meters wide with the same booster height as Falcon 9, so the booster would still be road-transportable.

Two full-size SL Raptor engines on the first stage, plus six auxiliary thrusters; four landing legs:



The upper stage is where it gets really interesting. It's the same monocoque design as the ITS Spaceship/Tanker, but it uses two vacuum-optimized 1000-kN development Raptors, and it has two winglet extensions instead of three:



Notional staging velocity is 1.5-2.5 km/s, depending on payload. Booster carries enough propellant for RTLS on every mission, and lands on its auxiliary thrusters.

The second stage re-enters on its belly using split flaps for attitude control, like the ITS Spaceship and Tanker. However, it has no landing legs near its tail, and no tail auxiliary thrusters. Instead, panels on the underside open up, both serving as landing skids and exposing auxiliary landing thrusters for a propulsive belly-first landing:



I've worked out all the specs...filled up about three Excel spreadsheets doing so, too.

First stage dry mass: 17 tonnes
First stage propellant: 421 tonnes
First stage mass ratio: 24:1
Thrust at launch: 6,266 kN

Second stage dry mass: 6.6 tonnes
Second stage propellant: 141 tonnes
Total vacuum thrust: 2,292 kN

Reusable payload to LEO: 24 tonnes
Reusable payload to GTO: 6.8 tonnes
Downmass from LEO: 22 tonnes

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).

Upper stage landing would look like something out of Star Wars, because it drops straight down, winglets open, and it lands on the wingtips with rocket propulsion.

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 kept the thrusters behind the landing panels to protect them from re-entry -- I can't imagine that plasma is healthy for engines -- but 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. 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.

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 with a good bit of downmass.

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.

With an adapted interstage, this upper stage could be used with Falcon Heavy as well.

Hello Seven,
Do you think there's merit in your design for the 9m ITS?

[livingjw]

Did this winter of 2016. Similar operations, but central payload bay with two sets of tanks on either side. Vacuum Raptors ended up smaller than shown on this picture. Obviously a reentry shell goes around the bottom with plenty of space on top for solar arrays and radiators. Basic aim was to reduce ballistic coefficient and provide more convenient payload access. Large payload access this through rear. Engines pivot outboard for rear access.