I figured out that ten SuperDraco engines have about the same thrust as a Merlin 1D. So if we replace Falcon 9's second stage with a Dragon V2 with 2 additional SuperDracos ( and replace some seats with fuel tanks,) can it plus a Falcon first stage propel itself into orbit?(or, at least, sub-orbit.) Plus, Dragon V2 is reusable, while a Falcon second stage is not. So I think replace a Falcon 9 second stage with a Dragon is pretty cost-saving, huh?
The thrust of the engines (for any launcher or spacecraft, really) is nowhere near as important as the amount of fuel feeding them.
It takes about 9400-10000m/s of ∆v, depending on trajectory, to reach stable orbit from Earth's surface.
The second stage of a Falcon 9 1.1, depending on payload, is responsible for the majority of that speed: 6200-6700 m/s. That gets 10-13 tons into LEO. It achieves this by carrying 92 tons of propellants.
According to the numbers filed with the EPA, Dragon V2 carries 450 gallons of hydrazine and NTO propellants, roughly 2 tons. With the dry mass of 4.2 tons, that's about 900 m/s delta-V assuming zero payload. With 3 tons of payload you've got about 565 m/s.
On the up side, losing the second stage means the first stage is carrying much less weight, so it gets you going a lot faster, yielding an extra 3200m/s of ∆v, but that leaves you 2000 m/s short of orbit.
Replacing the Dragon payload with fuel tankage could get the total system ∆v to 8800m/s, still 1000m/s short of orbit with no useful payload.
The critical number for rocket engines is their specific impulse (Isp, a measure of how efficient the engine is).
- The Merlin has an Isp of 311 s.
- The SuperDraco Isp is estimated to be around 225 s (which is in line with other hydrazine engines).
This means an upper stage that uses SuperDracos would need to be much heavier than a Merlin-powered upper stage, because it'd need more fuel to achieve the same $\Delta V$. So you'd end up replacing the second stage with a Dragon that's larger and heavier than the second stage (and as a consequence, will be harder to land intact).